Best Glide Speed and Headwinds/Tailwinds
Alan Foonberg
DE: What's the best glide speed for a 152?
Me: 60 knots
DE: With what kind of wind?
Me: (looking at the chart in the POH) Zero wind.
DE: What is the best glide speed if you have a 20 knot headwind?
Me: Not sure, but I'd still say 60 knots.
DE: No, you want to be at 80 knots so you have 60 knots relative to the ground
to get your best glide ratio.
Here's where I felt my idea of reality was being shattered. Here's the
question: when your engine fails in a 152, and the POH says the best glide
speed is 60 KIAS (I believe it said 'indicated'), and you have a 20 knot
headwind, what should you do?
My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
Another instructor said in a real emergency you wouldn't be measuring the wind,
you'd be trying to land, and you wouldn't have time to figure this out. He
said it was just a mind game for an oral exam, and he wasn't real sure of the
answer.
Another instructor says to always go for 60 KIAS no matter what the winds.
My first reaction was that you want to plane to be at a given pitch angle to
obtain the best glide ratio, so you should always shoot for 60 KIAS, no matter
what the wind, since you want to be at 60 knots relative to the mass of air
around you, no matter which way and how fast it's moving.
Then, I tried to understand what the DE told me. He said to take the extreme
case: you have a 60 knot headwind. What happens? If you maintain 60 KIAS,
you'll drop straight down. So, if you pitch down for 120 KIAS (really,
anything > 60 KIAS), you'll have some forward movement. I can see in this case
that you will get more forward movement per altitude lost if you pitch for 120
KIAS than if you pitch for 60 KIAS. But, what about the in between cases, with
less than 60 knot headwinds?
Does the best glide speed minimize the altitude lost per time? In which case
pitching for 60 KIAS in a 60 knot headwind will let me drop the slowest
(although, I'll drop straight down)? Or, does best glide speed maximize the
forward movement per altitude lost? In which case, I do want to pitch for a
speed greater than 60 KIAS in a 60 knot headwind.
I'm sure the net will have some good fun with this discussion. It's one of
those questions, where the phrase "not always right, but never in doubt"
applies.
Have at it.
---
Alan Foonberg
foon...@aero.org
Hal Kempthorne
>Here's where I felt my idea of reality was being shattered. Here's the
>question: when your engine fails in a 152, and the POH says the best glide
>speed is 60 KIAS (I believe it said 'indicated'), and you have a 20 knot
>headwind, what should you do?
>
>My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
>
Your DE is generating dangerous confusion! He should urge you to turn
so you have a tailwind! After all, the procedure is not a game but a
means for saving lives in an emergency!
Figure it out with a tailwind. You'll go further at best glide speed
than any other speed, in this case 60KIAS. If you are 10,000 ft above
the terrain, you'll glide about 10 miles in about 10 minutes with no
wind. With a 60 knot tail wind you'll go 20 miles - probably to an
airport! Your silly DE might go a mile or two into the wind.
Terrain may suggest something better than a turn directly away from the
wind but even then, best glide speed is still, as you understand it.
Tell your DE I said he should be ashamed of himself!
Hal Kempthorne
Debonair N6134V
Steve Pennypacker
> In article <1992Jan31.1...@aero.org> foon...@aero.org writes:
>
> >Here's where I felt my idea of reality was being shattered. Here's the
> >question: when your engine fails in a 152, and the POH says the best glide
> >speed is 60 KIAS (I believe it said 'indicated'), and you have a 20 knot
> >headwind, what should you do?
> >
> >My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
> >
> Your DE is generating dangerous confusion! He should urge you to turn
> so you have a tailwind! After all, the procedure is not a game but a
> means for saving lives in an emergency!
DISCLAIMER: I am not a CFI. This is only my opinion. I expect anyone reading
this to reach their own conclusions about what to do in the event of
an emergency.
I disagree with this recommendation of turning downwind, and I don't think the
case of 60 kt. winds is very realistic for a light single (in most cases).
Let's start with a simple case. No wind. Lets say you're at 10,000 feet in a
C-150. No matter which way you fly, if your engine quits and you fly at best
glide, you're going to land somewhere within about a 15 mile (radius) circle on
the ground. Now let's add a headwind. At best glide, you're still going to fly
through 15 miles of air. The only difference is that now that your groundspeed is
affected by the wind. The effect is that the 15 mile circle that you can land in
has shifted downwind. You can't land quite as far ahead (into the wind) as you
could have without the wind, but now you can land further behind you than you
could have before. Assuming that you still fly at 60 kts., your options haven't
increased or decreased, they've just shifted a bit. The more the wind, the more
the 15 mile circle shifts downwind.
Now I've left out one variable: your glide speed. To explain this, let's go to the
60 kt. headwind example (yup, the one that I already said I didn't like :-) ).
You're flying directly upwind when the engine quits, and there's a field one mile
directly ahead. If you fly at best glide, 60 kts, you'll never make the field.
Your 15 mile radius circle begins directly under you and ends 30 miles behind you.
However, if you increase your glide speed, say to 90 kts, you'll make the field
because now you've got a 30 kt. ground speed. You'll also sink faster, but in this
case you're not too concerned with rate of sink, you're interested in ground track.
By increasing your speed to compensate for the wind, you've actually *increased*
the area in which it's possible to land.
Of course in the real world, you probably won't be flying very often in low
altitude winds of 60 kts. At lower wind speeds, you'll want to increase your glide
speed by a smaller amount.
Steve Pennypacker
BSS_...@vd.seqeb.gov.au
> Article-I.D.: aero.1992Jan31.161051.27109
> Sender: ne...@aero.org
> Reply-To: foon...@aero.org
> Organization: The Aerospace Corporation
> Lines: 58
Best glide speed can be determined graphically from the aeroplanes's
polar curve, ( descent rate versus forward speed ). This looks like an
inverted parabola in the lower left quadrant. The best glide angle is
determined by the slope of the line from origin that just touches the
curve. The speed/descent rate being given by the point of intercept.
A headwind/tailwind has the effect of translating the curve horizontally
left/right by the amount of the wind. Lift/Sink has the effect of translating
the curve up/down.
Different weights have the effect of scaling the original curve around the
orgin by the square root of the ratio of the different weights
Donovan Hammer;685-2499;60-850;;sptekwv1
>
> Me: Not sure, but I'd still say 60 knots.
>
> DE: No, you want to be at 80 knots so you have 60 knots relative to the ground
> to get your best glide ratio.
The problem here is that the DE thinks he's a glider pilot but he got the information
wrong.
This is a very, very common computation that a glider pilot makes. For the glide angles
of most aircraft, you can assume that your your glide slope is the ground speed (horizontal) divided by your virtical speed. If your heading into a wind, this reduces your ground speed,
but if you maintain best glide speed (60kt), then your virtical airspeed remains the same.
The result is that your glide slope will be steepened. However, you can speed up your airplane
to regain your ground speed. THE PROBLEM IS THAT YOU WILL ALSO INCREASE YOUR VIRTICAL SPEED
AS SOON AS YOU LOWER YOUR NOSE TO PICK UP GROUND SPEED.
There is a complicated way to calculate the best glide into a head wind, but a simple rule of
thumb is to add half of the head wind to your best glide speed (e.g. 20kts/2= 10kts. so best
glide into this head wind is 60kts+10kts= 70kts.)
If you lose an engine however, you will go any direction that gets you to a safe landing.
Of course, if you can find a spot down wind, you will have the best chance of getting there with
the least altitude lost. But if the best landing can be made upwind, it is important to get the
best distance for your altitude and the method described will help.
BTW: The best glide speed for down wind is the same as best glide in dead air (60kts for the
C-150).
Don Hammer
Tektronix
/Computer Graphics Group
Rob Ballantyne
>
>DE: What's the best glide speed for a 152?
>
>Me: 60 knots
>
>DE: With what kind of wind?
>
>Me: (looking at the chart in the POH) Zero wind.
>
>DE: What is the best glide speed if you have a 20 knot headwind?
>
>Me: Not sure, but I'd still say 60 knots.
>
>DE: No, you want to be at 80 knots so you have 60 knots relative to the ground
> to get your best glide ratio.
>
>Here's where I felt my idea of reality was being shattered. Here's the
>question: when your engine fails in a 152, and the POH says the best glide
>speed is 60 KIAS (I believe it said 'indicated'), and you have a 20 knot
>headwind, what should you do?
The best answer I can give would be something in the middle, say 70kts.
>My first reaction was that you want to plane to be at a given pitch angle to
>obtain the best glide ratio, so you should always shoot for 60 KIAS, no matter
>what the wind, since you want to be at 60 knots relative to the mass of air
>around you, no matter which way and how fast it's moving.
>
>Then, I tried to understand what the DE told me. He said to take the extreme
>case: you have a 60 knot headwind. What happens? If you maintain 60 KIAS,
>you'll drop straight down. So, if you pitch down for 120 KIAS (really,
>anything > 60 KIAS), you'll have some forward movement. I can see in this case
>that you will get more forward movement per altitude lost if you pitch for 120
>KIAS than if you pitch for 60 KIAS. But, what about the in between cases, with
>less than 60 knot headwinds?
A great example why not to maintain 60kts.
>Does the best glide speed minimize the altitude lost per time? In which case
--------------------------------------------------------------
No, best glide speed means: the speed to fly to obtain the largest forward
distance from a given height -- how to go the farthest.
Minimum sink speed is what gives minimum rate of sink and therfore minimum
altitude lost per time.
>pitching for 60 KIAS in a 60 knot headwind will let me drop the slowest
>(although, I'll drop straight down)? Or, does best glide speed maximize the
>forward movement per altitude lost? In which case, I do want to pitch for a
>speed greater than 60 KIAS in a 60 knot headwind.
>
>I'm sure the net will have some good fun with this discussion. It's one of
>those questions, where the phrase "not always right, but never in doubt"
>applies.
>
>Have at it.
>---
>Alan Foonberg
>foon...@aero.org
The reason I can't give a definitive answer about what speed to fly in
a given headwind is that I don't have the C152 polar curve. The polar
is a graph of sink rate vrs forward speed. For every given headwind,
from this curve, you can calculate what your forward speed should be
to cover the most ground. On the other hand, your minimum sink speed
is always the same.
This stuff (min sink speed/best L/D speed, and best L/D speed with given
headwind) is well understood by any cross country glider pilot. If you
can find one he'll draw some pictures of polars and be able to explain
exactly what speed to fly for any given headwind.
--
--------------------------------------------------------------------------
| Rob Ballantyne | |
| email: ball...@cs.sfu.ca | |
| | |
Robert Herndon
Steve Peltz, so I feel compelled put my $0.02 in.
Unfortunately, I've never seen the glide speed vs. sink rate
"polar chart" that folks keep referencing for any plane I've
yet flown. One could certainly build such a chart given some
time at altitude, but [don nomex suit] a good rule of thumb is:
1) if flying into the wind, add 1/2 the wind velocity or so
to best glide speed to get a speed to maximize your distance
into the wind.
2) if flying into a tail wind, subtract 1/2 the wind velocity
from best glide speed to get your glide speed, but do not fly
below min sink velocity.
If you do not know min sink for your plane, do not fly below Vx.
This latter will increase your danger of stall should you not
have a good handle on keeping constant speed, so any error should
be on the fast side. Drag increases very rapidly *below* min
sink, so again, if in doubt, fly on the faster side.
------
At min sink, your plane will stay in the air as long as possible,
and since the wind is carrying you the direction you want to go,
you'll get as far as possible.
Unfortunately, not all of the "big four" airspeeds for planes
usually show up in the POHs of most planes. They are:
Vx, Vy, best glide, and min sink.
Min sink is most often missing. Ideally, for prop planes, Vx
should be very near min sink, and Vy near best glide, but due
to prop inefficiencies, airframe effects, etc., there are often
significant differences (10 knots or more) between Vx and min
sink (occasionally) or Vy and best glide (commonly). Anybody
know a good source for all of these numbers for GA singles?
------
Also, while flying downwind may maximize the potential landing
areas, landing downwind is a *VERY BAD* idea. Impact energy
increases with the square of the impact (ground) speed. Fatality
rates increase even faster -- for autos, they approximately double
for every 10 MPH above 50 MPH. If there's any wind at all out,
you want to land into it.
-------
Finally, remember that of GA single engine aircraft flown "in
control" until impact after engine out, approximately 97% do not
result in fatalities. Statistics are *MUCH* worse for aircraft
stalled before touchdown.
+rh
--
Robert Herndon -- not speaking officially for Cray Computer.
Ask about our designer dishwashers to match your washing machines!
Jim Chandler
Fly at 60 KIAS. Here are some of the reasons that I would do this:
1. It will keep you airborne the longest. 60 is L over D max I think in a
150. Best glide will give you the maximum lift for the given drag and will
keep you airborne longer.
2. With one airspeed in mind, you don't have to think about it.
3. You are assuming that you know what the wind is at the altitude that
you are at. I have seen 180 degree wind changes at 1000 AGL before. That
makes flying Simulated flameout patterns very interesting.
4. With an engine out, I am more interested in finding a suitable place to
put the plane down then going anywhere. Sure with a 60 knot wind you could
fly with a ground speed of about 120 KIAS but will you get to a suitable
field whether airfield or cow pasture?
5. If I did find a wind indicator, smoke, flag, etc. I would put the
plane into the wind prior to landing. I would rather have a zero knot gs
than a 120 knot gs when I landed in a field.
The point is fly at 60 which will give you the longest time to make
decisions, find a place and land.
BTW, if my instructor told me to add/subtract the wind from the published
glide speed, I would be looking for another instructor. Also, take
instruction from a few instructors anyway. You will get a few perspectives
and techniques that may work for you. Limiting yourself to one viewpoint
of flying will only narrow your view of flying. Some instructors may have
a way of doing things that works better for you than another. Have fun and
fly safe.
--
Jim Chandler ames!infopro!beagle!chandler
chan...@netcom.com j...@sactoh0.sac.ca.us <--- best chance
Bruno Pape
>question: when your engine fails in a 152, and the POH says the best glide
>speed is 60 KIAS (I believe it said 'indicated'), and you have a 20 knot
>headwind, what should you do?
>
Find a place to land. :-)
>My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
Find a closer place to land. :-)
Paul Raveling
>
> In article <1992Jan31.1...@aero.org> foon...@aero.org writes:
[about whether to fly 60 knots or 80 into a 20-knot headwind
for best glide ratio]
> >DE: No, you want to be at 80 knots so you have 60 knots relative to the ground
> > to get your best glide ratio.
>
> Hold the phone (or headset boom):
>
> The best glide speed is the AIRSPEED at which you will get the maximum
> forward distance gain per altitude loss.
That gives you best glide ratio relative to the airmass that
you're flying in. If the airmass is moving at 20 knots, it
won't be best glide ratio relative to the ground.
It sounded as if the DE thought that best glide ratio was
obtained at a given ground speed, independent of wind, and
that's wrong. But it's also wrong that best glide relative
to the ground is at a given airspeed, unless the air is still.
Others have suggested adding half the headwind to obtain
best glide speed. This is a reasonable approximation for most
light GA aircraft, but it helps to know your particular aircraft's
traits.
> 2. Consider a plane with these number and the time/distance to drop 5,000 feet:
> 60kts - 500fpm loss;
> 70kts - 625fpm loss; { of course your milage may vary }
> 80kts - 750fpm loss;
>
> 0 wind:
>
> KIAS Time GS Hours Distance
> 60kts: 10 minutes: 60 x 10/60 = 10 Nm
> 70kts: 8 min: 70 x 8/60 = 9.3 Nm
> 80kts: 6.7 min: 80 x 6.6/60 = 8.8 Nm
>
> Refigure with a 20 kt headwind (wich simply reduces ground speed):
> (Also note that vertical speed is constant with airspeed):
>
> 60kts: 10 minutes: 40 x 10/60 = 6.6 Nm
> 70kts: 8 min: 50 x 8/60 = 6.6 Nm
> 80kts: 6.7 min: 60 x 6.6/60 = 6.6 Nm :-o
Consider also a 40-knot headwind:
60kts: 10 minutes: 20 x 10/60 = 3.3 Nm
70kts: 8 min: 30 x 8/60 = 4 Nm
80kts: 6.7 min: 50 x 6.6/60 = 5.5 Nm
The extra 2.2 miles might help. Also note that actual numbers
are worse than these; this method of computing glide distance
assumes the glide path is early flat. As headwind increases
that's less true; in a relatively extreme case I've flown
about a 45-degree glide path (1:1 glide ratio) on part of
one approach, starting into a 40-knot headwind at about
55 or 60 knots in a high performance sailplane (a 1-35, still-air
glide ratio probably at least 36:1 with the gear down).
Consider also a different aircraft. I no longer recall actual
numbers for a draggy Scwheizer 2-33 glider, but they might be
something like this at minimum wing loading:
45 knots -- 200 fpm loss
50 knots -- 250 fpm loss
60 knots -- 350 fpm loss
70 knots -- 600 fpm loss
80 knots -- 900 fpm loss
With this sort of performance it pays to accelerate A LOT
into a headwind a lot, but not to the point where the glide path
is turning into a dive. If you your aircraft has struts and
draggy gear, its parasite drag goes up awfully fast after
some point, and you can't take advantage of higher airspeeds
to penetrate into a headwind. If you have a clean airframe
you CAN penetrate at a much higher speed and a much flatter
glide relative to the ground.
------------------
Paul Raveling
Rave...@Unify.com
darrell tinker
Anytime there is a strong wind blowing near the ground there will be a
decreasing wind velocity close to the ground. What this means is that
if you are landing into a strong wind, as you approach the ground your
airspeed will tend to decrease while your groundspeed stays the same.
This can be counteracted by diving to regain speed, but obviously you
don't want to dive very much very close to the ground. :^) For this
reason most glider pilots are taught to add half of the wind speed to
there normal LANDING speed for the landing pattern.
This is not completely relevant to the speed you fly to GET somewhere
if your engine quits, but it is a good thing to remember for any landing
into a strong wind, with or without your engine running.
-Darrell ross.com!darrellt or ross!darr...@cs.utexas.edu
-
Paul Raveling
> In article <10...@beagle.UUCP> chan...@beagle.UUCP (Jim Chandler) writes:
> >In article <1992Jan31.1...@aero.org>, foon...@Aero.org (Alan Foonberg) writes:
> >> Another instructor says to always go for 60 KIAS no matter what the winds.
>
>
> >1. It will keep you airborne the longest. 60 is L over D max I think in a
> >150. Best glide will give you the maximum lift for the given drag and will
> >keep you airborne longer.
>
> endurance will keep you in the air longest. This speed is at the bottom
> of the hook shaped Power required vs velocity curve I mentioned in my
> post on this subject. Thus the velocity for minimum sink will always
> be less than the velocity for L/D max.
This is a point worth another thought -- Flying max L/D
gives you maximum gliding range, but you'll reach the ground
a little faster than flying some other speed. Flying minimum
sink will keep you aloft longer, but you won't be able to
go as far.
If being able to reach a safe landing place appears possible
but not guaranteed, or if you want some reserve altitude before
entering a "normal" pattern for landing, you want to start by
stretching your glide. Max L/D is the baseline speed for this.
If you have a good landing spot below you, you may want to loiter
as much as possible to try to restart the engine or improve your
3D navigation for hitting a reasonable pattern entry point.
Minimum sink is what you'd want for this case.
If the air isn't still you often need to modify these baseline
speeds. Fly faster than max L/D to penetrate a headwind or sink,
minimizing your energy loss & therefore your altitude loss.
Fly slower, typically at minimum sink speed, to take advantage
of a tailwind or lift and maximize your energy gain from it.
------------------
Paul Raveling
Rave...@Unify.com
Donovan Hammer;685-2499;60-850;;sptekwv1
> from best glide speed to get your glide speed, but do not fly
> below min sink velocity.
I've never heard this rule of thumb applied to a tail wind situation, but never say never :-).
I think that it is more frequently suggested to just fly the best glide speed for tail winds.
This might be due to the following reasons:
1. Most piots have no idea of what the polar looks like anyway, which is why we have this rule
of thumb in the first place. Therefore, most pilots don't even know what the min. sink
airspeed is for a power aircraft.
2. As the polar is moved away from the origin, the best glide interception of the polar will
move up off (slower airspeed) the best L/D intercept point, BUT the curve is flatter at
this region of the polar and the benifit (decreased glide angle) is much less dramatic.
I agree that technically the best glide for a tail wind situation would be some airspeed
between dead air best glide (best L/D) and min. sink. It just that I've never heard of the
1/2 wind component applied to the tailwind situation. But then again, I might just be reading
the wrong books and hanging around the wrong crowd :-).
Dick Harrigill
>DE: What's the best glide speed for a 152?
>Me: 60 knots
>DE: With what kind of wind?
>Me: (looking at the chart in the POH) Zero wind.
>DE: What is the best glide speed if you have a 20 knot headwind?
>Me: Not sure, but I'd still say 60 knots.
>DE: No, you want to be at 80 knots so you have 60 knots relative to the ground
> to get your best glide ratio.
Hold the phone (or headset boom):
The best glide speed is the AIRSPEED at which you will get the maximum
forward distance gain per altitude loss. It is an AIRSPEED. The maximum
distance you can glide will be attained at this (or nearly - see below
calculations) airspeed.
>My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
I'd check this guy's credentials.
>Another instructor said in a real emergency you wouldn't be measuring the wind,
>you'd be trying to land, and you wouldn't have time to figure this out. He
>said it was just a mind game for an oral exam, and he wasn't real sure of the
>answer.
If he's not sure, I'd get another instuctor. In an emergency landing you'd
better be thinking about the wind.
>Another instructor says to always go for 60 KIAS no matter what the winds.
Get this guy.
>Then, I tried to understand what the DE told me. He said to take the extreme
>case: you have a 60 knot headwind. What happens? If you maintain 60 KIAS,
>you'll drop straight down. So, if you pitch down for 120 KIAS (really,
>anything > 60 KIAS), you'll have some forward movement....
1. I'd rather "crash land" with 0 forward ground velocity and a minimal
vertical velocity, than ~60 knots horizontal and almost as much vertical.
I wonder what kind of vertical speed you would get with a gliding 120kts
in a C152. A bit too much for this roller coaster enthusiast! In fact,
I would argue that a gliding C152 is not capable of obtaining 120kts of
horizontal airspeed. It would surpass Vne.
[Also - see this month's AOPA mag for good article on descent speeds.]
2. Consider a plane with these number and the time/distance to drop 5,000 feet:
60kts - 500fpm loss;
70kts - 625fpm loss; { of course your milage may vary }
80kts - 750fpm loss;
0 wind:
KIAS Time GS Hours Distance
60kts: 10 minutes: 60 x 10/60 = 10 Nm
70kts: 8 min: 70 x 8/60 = 9.3 Nm
80kts: 6.7 min: 80 x 6.6/60 = 8.8 Nm
Refigure with a 20 kt headwind (wich simply reduces ground speed):
(Also note that vertical speed is constant with airspeed):
60kts: 10 minutes: 40 x 10/60 = 6.6 Nm
70kts: 8 min: 50 x 8/60 = 6.6 Nm
80kts: 6.7 min: 60 x 6.6/60 = 6.6 Nm :-o
Now I know that the above calculation just blew a hole in my argument.
However, go ahead and find out the numbers (glide horiz vs vertical speed)
for your plane and do them yourself. In the above example, I'd rather stick
with the slower speed in either case because it gives you more time to
consider your options. The difference gap closed in the headwind scenario,
but the 60kts gained us a a whole lot with no wind.
I think you will find that the exact "best glide" AIRSPEED will not deviate
a great amount except where the headwind is significant with respect to the
published best glide speed.
--
Dick Harrigill, an independent voice from: Boeing Commercial Airplanes
M/S 9R-49 PO BOX 3707 Renton Avionics/Flight Systems
Seattle, WA 91824 Computing Support
(206) 393-9539 rfh...@galileo.rtn.ca.boeing.com PP-ASEL
Prof. David F. Rogers
>> Another instructor says to always go for 60 KIAS no matter what the winds.
>Fly at 60 KIAS. Here are some of the reasons that I would do this:
>1. It will keep you airborne the longest. 60 is L over D max I think in a
>150. Best glide will give you the maximum lift for the given drag and will
>keep you airborne longer.
Unfortunately this is wrong. The minimum sink speed or speed for maximum
endurance will keep you in the air longest. This speed is at the bottom
of the hook shaped Power required vs velocity curve I mentioned in my
post on this subject. Thus the velocity for minimum sink will always
be less than the velocity for L/D max.
Almost every decent set of study material I have seen for the Private
Pilot talks about this topic. Please go look it up.
Dave Rogers
Paul Raveling
> > 2) if flying into a tail wind, subtract 1/2 the wind velocity
> > from best glide speed to get your glide speed, but do not fly
> > below min sink velocity.
>
>
> I've never heard this rule of thumb applied to a tail wind situation, but never say never :-).
I haven't heard precisely that rule, but did do best-glide
calculations long ago for a sailplane long ago. They showed
that the speed for flattest glide (ground-relative) QUICKLY
dropped toward minimum sink as tailwind increased, with max L/D
at about 50 knots & minimum sink at about 40. I think the conclusion
was that best glide-stretching speed was minimum sink once
the tailwind got to about 10 knots or more.
Best glide relative to the ground would approach min sink
more slowly for power planes or for other aircraft with higher
ratios of speeds such as max L/D to tailwind.
------------------
Paul Raveling
Rave...@Unify.com
Fred Black
>
>My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
>
>Another instructor said in a real emergency you wouldn't be measuring the wind,
>you'd be trying to land, and you wouldn't have time to figure this out. He
>said it was just a mind game for an oral exam, and he wasn't real sure of the
>answer.
>
>Another instructor says to always go for 60 KIAS no matter what the winds.
>
The two instructors have the best point of view (fly 60 kt and find somewhere
to land, preferably in one piece).
If you consider the technicalities, the published best glide speed of 60 kts will
get you the most horizontal distance per altitude lost in zero wind. If you have
a headwind, your glide ratio will suffer, no matter what speed you fly at. If
you fly a little faster than best glide into a headwind, the increase in your
rate of descent may be less (relatively speaking) than the increase in your air-
speed, thus the decrease in your glide ratio would be a bit less. This is
definitely true in a sailplane, but I wouldn't bother playing around in an
engine out single.
Find a good field, land the plane (no dents), find a phone, call the airport
and complain.
--
Fred G. Black E-mail: cr...@bnr.ca Bell-Northern Research
PP-ASEL,G P.O. Box 3511 StationC
My opinions only. Ottawa, Ontario
Canada K1Y 4H7
a...@raven.aosg.gsf.dec.com
>would do this:
>
>. It will keep you airborne the longest. 60 is L over D max I think in a
>keep you airborne longer.
Both of these poins are, unfortunately, VERY wrong. As a previous poster has
pointed out, If you're flying into a 60 kt head wind, flying at 60kt airspeed gets
you nowhere but down. Also, best L/D speed is NEVER equal to the minimum sink
speed. If we ignore wind milling props, minimum sink speed is about 75% of the
best L/D speed.
Incidentally, choosing the correct airspeed for a certain task in various wind
conditions is part of the great challange of soaring. This calculation is only
trivial on a dead calm day. If you're trying to go from point A to point B, fly
at max L/D speed. If you're climbing in a thermal you slow down to minimum sink
speed. Of course since min sink is just a couple of knots above stall you might
want to add a few knots of the thermal is turbulent. And, of course, if point B
is a suspected thermal and you think that it may die soon you'll want to add a
few knots above best L/D to get there faster..... Forget what I said about
trivial. And, when you add a wind, it becomes much more complicated. Now, when
you soar ridges the rules change again.....
Tony verhulst
PP - ASEL, Glider
Steve Peltz
Paul Raveling has pretty much covered everything, but I have a few comments:
In article <7...@galileo.rtn.ca.boeing.com> rfh...@galileo.rtn.ca.boeing.com (Dick Harrigill) writes:
>In article <1992Jan31.1...@aero.org> foon...@aero.org writes:
>>My DE says pitch down for 80 KIAS so you have 60 knots ground speed.
>
>I'd check this guy's credentials.
I agree with that. It's hard to believe a DE doesn't understand the situation
any better than that.
>>Another instructor said in a real emergency you wouldn't be measuring the wind,
>>you'd be trying to land, and you wouldn't have time to figure this out. He
>>said it was just a mind game for an oral exam, and he wasn't real sure of the
>>answer.
>
>If he's not sure, I'd get another instuctor. In an emergency landing you'd
>better be thinking about the wind.
This instructor is pretty close to right-on, though I wouldn't characterize
the question as a mind game, and I hope the "wasn't real sure" was simply
referring to the exact numeric value, rather than the basic principles
behind it (i.e. I hope he knows that you DO go faster into a headwind if
you want to go the furthest distance, even if he doesn't know exactly how
much faster, and could maybe figure it out using the polar for the aircraft).
He is right that the last thing you need to be doing is calculating wind
velocity (though of course you want to know wind direction, which should
be much easier).
>>Another instructor says to always go for 60 KIAS no matter what the winds.
>
>Get this guy.
Well, yeah, get this guy a book on aerodynamics. I prefer this answer slightly
to the DE's answer, though it is a rather simplistic one.
>>Then, I tried to understand what the DE told me. He said to take the extreme
>>case: you have a 60 knot headwind. What happens? If you maintain 60 KIAS,
>>you'll drop straight down. So, if you pitch down for 120 KIAS (really,
>>anything > 60 KIAS), you'll have some forward movement....
The DE is at least indicating an understanding of the principle here, but
fails in the more detailed analysis.
> I would argue that a gliding C152 is not capable of obtaining 120kts of
> horizontal airspeed. It would surpass Vne.
The speeds discussed here are slant-angle airspeeds, NOT horizontal airspeeds.
If your best L/D is 1:1 at 200 kts (hmm, can a human body achieve 1:1?),
that's talking about your speed down that 45 degree angle. However, the L/D
ratio IS talking horizontal to vertical distances. Factor in the angle when
figuring out sink rates at a particular airspeed and L/D. Properly speaking,
a polar should be labeled with horizontal velocity, with the slant velocity
(airspeed) being slightly non-linear, and slightly different for each different
polar (often more than one is plotted on the same graph).
However, for most angles achieved by GA aircraft, the difference isn't
significant, even for achieving 120kts horizontal airspeed in an unpowered
C152.
>2. Consider a plane with these number and the time/distance to drop 5,000 feet:
> 60kts - 500fpm loss;
> 70kts - 625fpm loss; { of course your milage may vary }
> 80kts - 750fpm loss;
>
> 0 wind:
>
> KIAS Time GS Hours Distance
> 60kts: 10 minutes: 60 x 10/60 = 10 Nm
> 70kts: 8 min: 70 x 8/60 = 9.3 Nm
> 80kts: 6.7 min: 80 x 6.6/60 = 8.8 Nm
>
> Refigure with a 20 kt headwind (wich simply reduces ground speed):
> (Also note that vertical speed is constant with airspeed):
>
> 60kts: 10 minutes: 40 x 10/60 = 6.6 Nm
> 70kts: 8 min: 50 x 8/60 = 6.6 Nm
> 80kts: 6.7 min: 60 x 6.6/60 = 6.6 Nm :-o
First of all, you'll see a bigger increase in sink rate between 70 and 80
than between 60 and 70. Second, you chose the one headwind that, combined
with your linear change in sink rate, yields a constant glide distance,
i.e. 125fpm / 10kts = 500fpm / (60kts - headwind) yields a headwind of 20kts.
Of course, in real life it isn't linear, so that doesn't work.
Some observations about all of this: the comments about going faster means
you don't have as much time left in the air are certainly true, and are
something to consider. If you can land on the field directly beneath you,
you don't have to worry about going the maximum possible distance. If you
don't know that there is a field upwind that you can make, it might be a
better idea to go downwind instead, since that will give you quite a bit
more territory to find a good field AND you can do that while going slower
and staying in the air longer. However, IF the best field is upwind, you
can go farther by going faster than your Max L/D airspeed. A little bit
too fast is better than a little bit too slow, and the 1/2 headwind rule
of thumb is pretty close. The comments about wind shear are also relevant,
and definitely do indicate you should have a higher airspeed as you head
upwind while descending. And finally, during an emergency I'd rather be
concentrating on taking care of things, so it is much safer to have a nice
single airspeed to concentrate on getting to, so until you DO have a field
in sight, stick to the best glide speed. Then, if conditions warrant, you
may want to speed up some.
Oh yeah, regarding maximizing glide distance going downwind, I definitely
teach my students that going downwind at a safe altitude you should drop
back to min sink or slightly above if you want to go as far as possible.
Emphasis on SAFE ALTITUDE. However, since thermals drift with the wind
also, that isn't the best way to reach that cu popping in the distance.
It also doesn't apply when you're going through sink.
--
Steve Peltz
Internet: pe...@cerl.uiuc.edu PLATO/NovaNET: peltz/s/cerl
Ron
> to get your best glide ratio.
He's full of it. One thing people keep forgetting is that until your
within touching distance of it, ground speed has no effect on the
plane. The best glide speed is the point that gives the plane the
maximum forward movement over descent ratio. If there's a head wind,
yes it ain't going to be 10:1, but putting the plane at some other
speed is going to bring you down even faster.
Keep 60 KIAS and remember that wind will shorten or lengthen your
actual ground track from the ideal 10:1.
Someone should make this turkey fly dead stick into a 20 knot
wind. He'd learn fast.
-ROn
Brian Smith
> Does the best glide speed minimize the altitude lost per time? In which case
> pitching for 60 KIAS in a 60 knot headwind will let me drop the slowest
> (although, I'll drop straight down)? Or, does best glide speed maximize the
> forward movement per altitude lost? In which case, I do want to pitch for a
> speed greater than 60 KIAS in a 60 knot headwind.
In a glider this is refered to as penetration. I got burned on a question
like this too. When asked what speed to fly to get back to the runway
if things are getting tight, I answered best L/D. This is not the case in
a headwind. The speed must be higher to penatrate into the wind. I am
training in a Schweizer 2-33. The manual recommends adding the headwind
speed to the best L/D when attempting to cover ground going upwind.
It is interesting to note that higher performance gliders i.e. better
L/D, fly faster. The 2-33 with a 20:1 L/D has a best glide of 45 MPH while
the high performace ships have a best L/D speed quite a bit faster (mid 60s
and above, I think). These high perfomance ships are also much better at
covering ground upwind.
It is also interesting to note that folks involved in competition soaring
add water ballast to help with penetration. Adding weight increases the
airspeed for roughly the same L/D, up to a point 8-)
I don't know if this helps much other than to confirm that there is
more to it than just best L/D and, the manual for the glider I fly
suggests adding the headwind velocity to the airspeed. Your mileage
may vary, and I do mean that literally.
- Brian -
Dick Harrigill
>Dick Harrigill writes:
>> I would argue that a gliding C152 is not capable of obtaining 120kts of
>> horizontal airspeed. It would surpass Vne.
>
>The speeds discussed here are slant-angle airspeeds, NOT horizontal airspeeds.
>If your best L/D is 1:1 at 200 kts (hmm, can a human body achieve 1:1?),
>that's talking about your speed down that 45 degree angle. However, the L/D
Yes, however the original example referred to 120kts KIAS yeilding 120kts
horizontal speed. Airspeed is in the direction of flight, not horizontal.
I am trying to say that to get to 120Kts in an unpowered C152 the angle
is significant. To yeild 120kts horizontal would require significantly
greater airspeed.
>However, for most angles achieved by GA aircraft, the difference isn't
>significant, even for achieving 120kts horizontal airspeed in an unpowered
>C152.
Agreed in most "normal" cases. But 120kt in an unpowered C152 IS significant.
Again what I was trying to say is that when you are calculating the
distance you will glide, you must take the vertical and horizontal velocities
into consideration. You are correct that for most cases the angle is
small enough so that the horizonatal speed is approximated by the IAS.
However, the faster your glide, the steeper the angle. For a C152 to
glide at 120 KIAS, the angle is significant. If, for example, the angle is
45 degrees, the horizontal speed will be 120/sqrt(2) or about 85kts. If
you are hoping for 120kts horizontal you have missed by 35kts which is very
significant. Again, using 45 degrees, to obtain 120kt horizontal you would
need an airspeed of 120Xsqrt(2) or about 168KIAS
Therefore, when doing calculations you should take the angle into
consideration, especailly at faster speeds.
Weekend Pilot
Jim replies
>Fly at 60 KIAS. Here are some of the reasons that I would do this:
>
>1. It will keep you airborne the longest. 60 is L over D max I think in a
>150. Best glide will give you the maximum lift for the given drag and will
>keep you airborne longer.
In unaccelerated straight gliding (or powered) flight the lift is
equal to the weight of the aircraft at any speed. The drag is
minimized at best L/D.
[...]
>
>5. If I did find a wind indicator, smoke, flag, etc. I would put the
>plane into the wind prior to landing.
Pay attention to your pattern and drift, it will tell you a lot about
wind direction and speed, too. This is why straight in off field
landings are *severely* frowned upon in glider training (among other
reasons).
>
>The point is fly at 60 which will give you the longest time to make
>decisions, find a place and land.
>
I think the point is to get to a safe landing location and execute
a safe off field landing. Using the best glide performance may aid
the former objective considerably.
>BTW, if my instructor told me to add/subtract the wind from the published
>glide speed, I would be looking for another instructor.
I guess we won't be seeing much of you at the local soaring operation.
Speaking as a 235 hour glider only pilot who has in the last week
started adding on a PPSEL in a c152 and yesterday afternoon executed
a simulated engine out emergency from 2500 agl in roughly 15 - 20
knot winds aloft I will *assure* you that you will notice a *useful*
increase in gliding range by increasing your speed while going upwind.
I used about 67 kts (after seeing how poorly we were making headway at
the book value of 60) and told my instructor what I was doing and why.
He agreed that it was useful in this case. Although the increased sink
rate I incurred by flying faster would indeed put me on the ground faster,
in this case I feel it was well worth it in that I arrived over my
chosen field higher, with that much more time and altitude to
look things over and fly a good pattern.
The glider pilot's most elementary guideline as it applies to this
situation is to use best glide speed plus one half the estimated
head wind component. This is based on fairly sound aerodynamic
reasoning and (I believe) applies fairly well to a engine out light
single like the 152. It will certainly be a lot better than straight
old hard and fast 60 kts. If you read an advanced soaring text
such as Reichmann, you will find that the glide ratio penalty is
actually worse for going 5 knots too slow than 5 knots too fast.
Things get even more fun when you consider vertical airmass movements
on optimum speed to fly, but that's getting a bit far afield for
rec.aviation. Consult your friendly local cross country soaring
pilot for details.
--
Evan Ludeman "I'll tell you why we do this,
ludeman%astroa...@cs.wisc.edu we do it so we can say 'Sierra Mike,
{...}!uwvax!astroatc!ludeman two minutes'".
-- John Seymour "SM"
Prof. David F. Rogers
!In article <2...@shaman.wv.tek.com!, dono...@orca.wv.tek.com (Donovan Hammer;685-2499;60-850;;sptekwv1) writes:
!! ! 2) if flying into a tail wind, subtract 1/2 the wind velocity
!! ! from best glide speed to get your glide speed, but do not fly
!! ! below min sink velocity.
!!
!!
!! I've never heard this rule of thumb applied to a tail wind situation, but never say never :-).
!
! I haven't heard precisely that rule, but did do best-glide
! calculations long ago for a sailplane long ago. They showed
! that the speed for flattest glide (ground-relative) QUICKLY
! dropped toward minimum sink as tailwind increased, with max L/D
! at about 50 knots & minimum sink at about 40. I think the conclusion
! was that best glide-stretching speed was minimum sink once
! the tailwind got to about 10 knots or more.
Paul, technically best glide speed will ALWAYS be greater than best
rate of sink speed. Just look at the shape of the curve and the
tangent requirement. The tangent can never be horizontal (except for
infinite tailwind). Now practically, for gliders they may be so
close that it makes no difference--can you say less than 1-2 knots.
However, it is important to realize that best glide and best (mimimum)
rate of sink can never be equal.
! Best glide relative to the ground would approach min sink
! more slowly for power planes or for other aircraft with higher
! ratios of speeds such as max L/D to tailwind.
Dave Rogers
Prof. David F. Rogers
!!Dick Harrigill writes:
!!! I would argue that a gliding C152 is not capable of obtaining 120kts of
!!! horizontal airspeed. It would surpass Vne.
!!
!!The speeds discussed here are slant-angle airspeeds, NOT horizontal airspeeds.
!!If your best L/D is 1:1 at 200 kts (hmm, can a human body achieve 1:1?),
!!that's talking about your speed down that 45 degree angle. However, the L/D
!!ratio IS talking horizontal to vertical distances...
!Agreed in most "normal" cases. But 120kt in an unpowered C152 IS significant.
!Again what I was trying to say is that when you are calculating the
!distance you will glide, you must take the vertical and horizontal velocities
!into consideration. You are correct that for most cases the angle is
!small enough so that the horizonatal speed is approximated by the IAS.
!However, the faster your glide, the steeper the angle. For a C152 to
!glide at 120 KIAS, the angle is significant. If, for example, the angle is
!45 degrees, the horizontal speed will be 120/sqrt(2) or about 85kts. If
!you are hoping for 120kts horizontal you have missed by 35kts which is very
!significant. Again, using 45 degrees, to obtain 120kt horizontal you would
!need an airspeed of 120Xsqrt(2) or about 168KIAS
!
!Therefore, when doing calculations you should take the angle into
!consideration, especailly at faster speeds.
Ah... Let's say that the sink rate in a C150 at 120kts is 2000fpm.
The sink angle (glide angle) is then tan^-1 2000/(6080*@) or 9.34 degrees.
The slant velocity/range then differs from the horizontal velocity/range
by less than 2 per cent (cos 9.34 degrees).
So, it does not appear to be too significant.
Dave Rogers
Paul Raveling
> !
> ! I haven't heard precisely that rule, but did do best-glide
> ! calculations long ago for a sailplane long ago. They showed
> ! that the speed for flattest glide (ground-relative) QUICKLY
> ! dropped toward minimum sink as tailwind increased, with max L/D
> ! at about 50 knots & minimum sink at about 40. I think the conclusion
> ! was that best glide-stretching speed was minimum sink once
> ! the tailwind got to about 10 knots or more.
>
> Paul, technically best glide speed will ALWAYS be greater than best
> rate of sink speed. Just look at the shape of the curve and the
OK, I got sloppy about wording in the last sentence -- it
happens when your job description says "must be diligent
and adept at frying brain". In fact someone just mentioned
that I attributed lyrics by Crosby Stills & Nash lyrics
to the Moody Blues yesterday on rec.railroad!
The main thing I was trying to say is that (at least for
sailplanes) if you have enough tailwind to want to compensate
for it, then V(minimum sink) is a better reference speed than
V(max L/D). Best ground-relative glide speed does an asymptotic
approach torward V(min sink) as tailwind rises.
> The tangent can never be horizontal (except for
> infinite tailwind). Now practically, for gliders they may be so
> close that it makes no difference--can you say less than 1-2 knots.
Often MUCH less (see note below). Besides having V(min sink)
and V(max L/D) separated by only a few knots for many sailplanes,
their actual L/D at V(min sink) is often surprisingly close
to max L/D.
> However, it is important to realize that best glide and best (mimimum)
> rate of sink can never be equal.
Agreed. I started to do a numeric example, but quickly
ran into trouble with imprecise data. The only polar I have
on hand here is in the January Soaring's report by Dick
Johnson on measured performance of an old J-4 Javelin.
By eyeballing Johnson's polar plot it appears likely that
L/D at V(min sink) is almost as high as max L/D, perhaps
30:1 or 31:1 instead of 32:1. In this case the difference
in sink rate becomes virtually dominant in determining
downwind glide ratio.
For the benefit of anyone who appreciates perversity, here's
why my attempt at doing this numeric example fell apart:
The article quoted some numbers the factory supplied, and
the author (Johnson) quoted some measured numbers, but neither
the factory nor Johnson gave all the numbers needed to
plug into glide performance calculations. Trying to merge
the two seemed sensible, but it required two types of compensation:
-- Translate between 746 lb gross weight for Johnson's test
and 804 pounds for the factory quotes.
-- Allow for Johnson using CAS and the factory using IAS,
with about CAS about 2 knots higher than IAS in the
speed range involved.
-- Were the particular specimens really identical?
Did one have a less wavy gelcoat, better taping,
better construction tolerances,...?
-- How accurate WAS everyone's data was anyway?
For example, look at the scatter in data points in Johnson's
plot, especially in this speed range. How still
was the air during measurements? Were the factory
numbers measured values or predicted values?
Even if we just accept Johnson's curve fit, it's hard to read
points to enough accuracy. I tried running the graph through
a copier 3 times at 142% enlargement. Still couldn't discern
enough detail around V(min sink) & V(max L/D) to get reliable
numbers. After applying a calculator to several guesses some
of them showed a better L/D at V(min sink) than V(max L/D)!
Getting data off the plot might be easier if he hadn't plotted
the tangent line for max L/D, obliterating much of the polar
in this area.
Anyway, when there's a bit of time I'll rebuild a simple
model of gliding performance and will calibrate it against
some published sailplane polars. Then I'll try to apply it
to some power plane, though I might not have a polar for
a "real" power plane. For sailplanes the model gets a bit
tricky at or just below minimum-sink speed because it's getting
into the area where AOA is high enough...
(1) to approach or enter the nonlinear part of the Cl/AOA curve
(2) to require accounting for form drag as a function of AOA,
preferably modeling the fuselage and wing separately
because of the sailplane's relatively high angle of incidence
------------------
Paul Raveling
Rave...@Unify.com
John Gilbert
In article <1992Feb6.1...@ux1.cso.uiuc.edu> pe...@cerl.uiuc.edu (Steve Peltz) writes:
>....
>The speeds discussed here are slant-angle airspeeds, NOT horizontal airspeeds.
>If your best L/D is 1:1 at 200 kts (hmm, can a human body achieve 1:1?),
>that's talking about your speed down that 45 degree angle. However, the L/D
>ratio IS talking horizontal to vertical distances. Factor in the angle when
>figuring out sink rates at a particular airspeed and L/D. Properly speaking,
>a polar should be labeled with horizontal velocity, with the slant velocity
>(airspeed) being slightly non-linear, and slightly different for each different
>polar (often more than one is plotted on the same graph).
>
>However, for most angles achieved by GA aircraft, the difference isn't
>significant, even for achieving 120kts horizontal airspeed in an unpowered
>C152.
>
>....
>Steve Peltz
In my reading, renewed with a great gusto as this thread develops, I
haven't found anything to indicate other than -
L True airspeed
- = -------------
D sink rate
In Tom Knauff's "Glider Basics - From Solo to License", the polars are
labeled 'airspeed - knots'; in SGS 1-26 'V - mph'. This seems to
indicate that the slant speed is the speed measured. Knauff even talks
about how a polar gets made - measuring airspeed against sink rate.
Alan Foonberg (foon...@aero.org) asked in his original post:
> Then, I tried to understand what the DE told me. He said to take the
> extreme case: you have a 60 knot headwind. What happens? If you
> maintain 60 KIAS, you'll drop straight down.
^^^^^^^^^^^^^
REALLY? Here's a trivia question that I'll open up to all of rec.aviation.
I'll post my answer after one week or so, if anyone needs it!!!
Consider this hypothetical:
You are engine out in, say the Arctic, DIRECTLY over an island 100' x
100'. Best glide @ 0 wind = 60 knots with L/D = 10, headwind = 60 knots,
altitude = 5000'. Pretend the wind is uniform down to 10 feet. If
you make the island, there's some shelter from the wind. You and the
spamcan will survive.
1) If you fly 60 knots airspeed, will you make the island?
Prove your answer! (I shoulda been a math teacher!!)
2) If not, what speed do you need to fly to make the island?
3) BONUS QUESTION: How long is your rollout??
Have fun!!!!
John Gilbert (xcs...@fluke.com)
Lars-Henrik Eriksson
>This is a minor nit, until you have an L/D < 7 ...
>In my reading, renewed with a great gusto as this thread develops, I
>haven't found anything to indicate other than -
>
> L True airspeed
> - = -------------
> D sink rate
I don't think this is right. L=Lift, D=Drag. All aerodynamics books
I've read further state that L/D=CL/CD. In this case L/D is also equal
to horizontal speed (through the airmass) / sink rate. I.e. if you
proceed straight down in a 90 degree dive. The weight of the aircraft
will be supported by its drag and lift will be zero., so L/D=0 and not
1.
>In Tom Knauff's "Glider Basics - From Solo to License", the polars are
>labeled 'airspeed - knots'; in SGS 1-26 'V - mph'. This seems to
>indicate that the slant speed is the speed measured. Knauff even talks
>about how a polar gets made - measuring airspeed against sink rate.
This is not unreasonable. For the L/D ratios we are talking about in
the case of a glider, I would expect the measurement errors to be
greater that the errors you get from using slant speed instead of
horizontal speed.
>Alan Foonberg (foon...@aero.org) asked in his original post:
>
>> Then, I tried to understand what the DE told me. He said to take the
>> extreme case: you have a 60 knot headwind. What happens? If you
>> maintain 60 KIAS, you'll drop straight down.
This is clearly incorrect. For one thing, it should be true airspeed
and not indicated. It should also be horizontal speed and not slant
speed - which I guess is your point.
--
--
Lars-Henrik Eriksson Internet: l...@sics.se
Swedish Institute of Computer Science Phone (intn'l): +46 8 752 15 09
Box 1263 Telefon (nat'l): 08 - 752 15 09
S-164 28 KISTA, SWEDEN
Paul Raveling
> In article <1992Feb18.2...@tc.fluke.COM>, xcskier@tc (John Gilbert) writes:
> >This is a minor nit, until you have an L/D < 7 ...
>
> >In my reading, renewed with a great gusto as this thread develops, I
> >haven't found anything to indicate other than -
> >
> > L True airspeed
> > - = -------------
> > D sink rate
>
Yup, it's not right, though it's often tolerably close.
L/D = Cl/Cd = glide slope, but that depends on the definition
of lift as acting perpendicular to the flight path and drag
as acting in the flight path. Relating that to airspeed
and sink rate produces...
L/D = Cl/Cd = glide_slope = horizontal_speed / sink_speed
= airspeed * cos(glide_angle) / sink_speed
and glide_angle = arc sin(sink_speed / airspeed)
since cos(glide_angle) = sqrt(1 - sin^2(glide_angle)
L/D [etc] = (airspeed/sink_speed) * sqrt(1 - (airspeed/sink_speed)^2)
Let me know if I flubbed that simple derivation --
I'm hurrying because I wanna go home!
There's also the question of whether to use TAS, IAS, or CAS.
Subject to further discussion, if you read your sink rate from
well-calibrated pressure-sensing instruments (VSI, variometer,
altimeter + timer), the right choice for airspeed would be CAS.
TAS is fine if you can also measure or derive true altitude
instead of pressure altitude, or its time derivative.
------------------
Paul Raveling
Rave...@Unify.com
Dick Harrigill
In article <14...@usna.NAVY.MIL> d...@usna.NAVY.MIL (Prof. David F. Rogers) writes:
>Ah... Let's say that the sink rate in a C150 at 120kts is 2000fpm.
>The sink angle (glide angle) is then tan^-1 2000/(6080*@) or 9.34 degrees.
>The slant velocity/range then differs from the horizontal velocity/range
>by less than 2 per cent (cos 9.34 degrees). So, it does not appear to be too significant.
OK, I'm not going to beat this subject anymore because both Dave Rogers & I
are correct with our examples (45 degrees vs 2000fpm). The question is,
which example is most correct? My example was based on memories of experiences
in a C150, not on measured or published data.
So, I got out my C150 manual and tried to look up sink rates, etc. for
gliding airspeeds. However, such data is not to be found in the POH.
This leads me to a question. Where can you obtain this information without
actually going up there and trying it (since it isn't in the manuals)?
Paul Raveling
>
> Let me know if I flubbed that simple derivation --
> I'm hurrying because I wanna go home!
Yup, that turkey who posted the quickie math sure loused it up.
Missed a paren, wrote a fraction upside down... All I can say
is ouch! How embarassing! The bottom line should NOT have been:
> L/D [etc] = (airspeed/sink_speed) * sqrt(1 - (airspeed/sink_speed)^2)
but should be (with dumb flubs omitted):
L/D [etc] = (airspeed/sink_speed) * sqrt(1 - (sink_speed/airspeed)^2)
------------------
Paul Raveling
Rave...@Unify.com