Different speeds for best climb, best glide
Per Lindberg
This weekend I got my third type rating: the Aerospatiale Tobago TB10.
It resembles an Archer, plus a constant-speed propeller. Nice plane!
One thing that confused me somewhat during the technical course was
finding Vy in the manual. It finally dawned on me that the speeds for
best climb and best glide are *different*, 73 and 86 knots.
I have previously thought that this was alway the same, called Vy, and
magically linked to L/D. I had expected *small* differences, but 73/86
is a lot. Anyone care to comment on this?
--
Per Lindberg (p...@front.se) ! __!__
Front Capital Systems, Box 5727, ! _____(_)_____ Ceci n'est pas une Piper
11487 Stockholm, Sweden. 8-6611510 ! ! ! !
Steven Philipson
> [on the Tobago] the speeds for
> best climb and best glide are *different*, 73 and 86 knots.
> I have previously thought that this was alway the same, called Vy, and
> magically linked to L/D. I had expected *small* differences, but 73/86
> is a lot. Anyone care to comment on this?
Sure. Best glide and best rate speeds have little to do with each
other.
Best glide (L/Dmax) is the speed for lowest drag in a power-off glide.
This is the minimum point on the DRAG curve for the aircraft. (This is
(very close to the curve for THRUST required for level flight.)
Vy (speed for max rate of climb) is the speed that produces the maximum
excess POWER. Power required and power available (from the engine) can be
plotted on a single graph. Vy is the speed for which the difference
between these two curves is maximal.
The difference between the two is that drag or thrust is a force, but
power is force times velocity. Drag and thrust are measured in pounds.
Power is measured in units like horsepower. One horsepower is 33,000
foot-pounds per minute -- a force, pounds, times a velocity, feet per
miunte, multiplied by a constant, 33,000.
Other speeds that are very useful are the speed for minimum sink
rate (minimum POWER required), and best climb angle (Vx, maximum excess
thrust). For some aircraft, Vy is very close or identical to the L/Dmax
speed, and Vx is very close to min sink speed. They are not the same
though, and the ordering of the four speeds varies among aircraft models.
An additional fine point is that the speeds for minimum power required
and L/Dmax are different for power-off and power-on flight regimes. This
is because the drag of the prop is significant. The increased drag of a
windmilling prop decreases both speeds. Thus the speed for maximum
endurance (power-on) is higher than the speed for minimum sink, and
the speed for max cruise range is higher than the speed for max glide
distance.
There are even more considerations in choosing speeds for max
performance, but the above covers primary factors. In summary,
min sink speed == minimum power required
L/Dmax (best glide speed) == minimum drag
Vx == max excess thrust
Vy == max excess power
Steve
(the certified flying fanatic)
ste...@decwrl.dec.com
Kerry Kurasaki
>
>One thing that confused me somewhat during the technical course was
>finding Vy in the manual. It finally dawned on me that the speeds for
>best climb and best glide are *different*, 73 and 86 knots.
>
>I have previously thought that this was alway the same, called Vy, and
>magically linked to L/D. I had expected *small* differences, but 73/86
>is a lot. Anyone care to comment on this?
>
and goes something like this:
If you were to consider the drag effects of the airframe
alone, then the best rate of climb and minimum sink speed
should be equal. This is the point of minimum drag. Best
glide is slightly faster than minimum sink since you are
trying to maximize distance covered instead of descent time.
Now the additional factor that is often overlooked, is the
drag of the windmilling propellor. In the published climb,
the propellor is providing thrust, in the best glide, it is
adding to the drag component. Since drag goes up as the square
of the airspeed, then a minor reduction in airspeed, substantially
reduces the overall drag. (Until you get back to Max L/D)
One thing that helps, and is recommended in the Piper Arrow POH,
is to adjust the propellor control to high pitch (low RPM) to
help reduce the propellor drag.
Kerry
Barney Lum
>It resembles an Archer, plus a constant-speed propeller. Nice plane!
What a coincidence... i too got checked out in our club's TB10 this
past weekend! I had flown it once a year ago, but for various reasons
(money, and the aircraft being down for several months) didn't finish
the checkout until Saturday and Sunday.
With its controls being really tight and responsive, it will either
stay on its path or diverge rapidly if you're not paying attn.
It also makes for building up of my left arm muscles :).
Funny how they make sports cars like "airplanes", and this airplane like
a "sports car" :)
-------------------------------------------------------------------------------
>|
>| Bar...@usc.edu Bar...@USCVM.Bitnet
--> --> --> | ======= --- --- --- --- --- --- --- ---
>| Permanent Student Pilot, On the Numbers
>| ToBAYgo, ToBAHgo :)
-------------------------------------------------------------------------------
Paul Cantrell
>finding Vy in the manual. It finally dawned on me that the speeds for
>best climb and best glide are *different*, 73 and 86 knots.
>
>I have previously thought that this was alway the same, called Vy, and
>magically linked to L/D. I had expected *small* differences, but 73/86
>is a lot. Anyone care to comment on this?
Well, the two airspeeds are used to get two different results. One of them
minimizes total drag (Vy - also minimum sink rate), the other maximizes
distance over the ground.
Remember that at slow airspeeds profile and parasite drag are low, but
induced drag is high. As airspeed goes up, profile and parasite drag
increase and induced drag decreases. If you sum the three curves you
usually get an upside down bell shape.
At the lowest point, total drag is minimized. At this airspeed, it takes
the least energy to stay aloft. If the engine is producing full power, you
will go up the fastest because the excess energy is being turned into
altitude - potential energy. If the engine is producing no power, you get
the energy to combat drag from a loss in altitude. Since drag is minimized,
loss in altitude per unit time is also minimized. This minimum sink rate may
be influenced in airplanes by windmilling propellors - I'm not sure. But in
a glider Vy and minimum sink should be the same number.
Best rate of climb (Vy) is a *very* useful airspeed to know. Not only will
it produce the fastest rate of climb, it will also give you the maximum
time aloft. If you want to go up and take pictures of one spot for as long
as possible, orbit at Vy. Same goes for IFR holds. If you've got to wait
for 20 minutes, doing it at Vy will conserve the most fuel.
If you lose your engine and you have a terrific tailwind, slow to Vy. That
will maximize your time aloft, and thus the distance the wind can help you
cover.
If you lose your engine and you want maximum time aloft to say your prayers,
say them at Vy!
Best glide will allow you to cover the most ground for a given loss in
altitude. The best glide quoted in the POH assumes a slight headwind. In
reality, the best glide speed can change considerably depending on the
wind.
The best glide speed can be determined by plotting the aircraft total drag
curve, and drawing a tangent line whose origin is at the speed of the
head/tail wind. This is because at the tangent, the ratio (slope) of speed
to drag is at the highest it can reach.
An easy example of how wind effects best glide: if the POH states that the
best glide speed is 65 knots, but you know you have a 65 knot headwind,
you know you will have 0 groundspeed. It's clear in this case that you
need to increase airspeed to make any progress over the ground. (Or turn
around and make it a tailwind :-)
Best rate of climb, however, is not dependent on wind effects. If the POH
claims 55 knots Vy, this is true whether you have a tailwind, headwind, or
calm wind.
One question for Prof. Dave Rogers: (or others) Why is best range
different than best glide? My guess is that it adds another curve to the
energy usage curves, and the inefficiencies of the power plant skews the
curve slightly to the high side of best glide. A perfectly efficient power
plant would result in a best range speed identical to best glide. Am I on
the right track?
knowing how dangerous it is to post on aerodynamics in this group,
PC
--
uucp: pc@stardent
Lars-Henrik Eriksson
>Well, the two airspeeds are used to get two different results. One of them
>minimizes total drag (Vy - also minimum sink rate), the other maximizes
>distance over the ground.
>At the lowest point, total drag is minimized. At this airspeed, it takes
>the least energy to stay aloft. If the engine is producing full power, you
>will go up the fastest because the excess energy is being turned into
>altitude - potential energy.
Sorry, but this is not correct. Vy is *not* the speed for minimal
sink. Vy is the speed where the *power* surplus is largest. I.e. where
the difference between the power required to maintain the speed, and
the power produced by the engine/propeller is largest.
This does not depend only on the airframe but also on the properties
of the engine and propeller. This also means that for different
amounts of engine/propeller power, you should use different Vy. In
particular, as you climb a normally aspirated airplane, air density
decreases, so power goes down and Vy changes. The POH of some aircraft
(e.g. Cessna 172) actually give you information about the different Vy
in the POH. Other (e.g. for Piper Cherokee) give only a single Vy.
Presumably Vy changes little in that aircraft.
The reason I write engine/propeller power is that the interesting
power is that which goes into moving the aircraft ("thrust
horsepower"), not the power actually produced by the engine ("brake
horsepower").
Since thrust horsepower varies with speed, you can't assume that the
speed for minimum drag is the speed where you will have the largest
power surplus. Indeed, since power is the product of force (i.e.
thrust or drag) and speed, the speed for minimum drag will not even be
the same as the speed for minimum power required.
--
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
Cantrell) writes:
>
> Well, the two airspeeds are used to get two different results. One of
them
> minimizes total drag (Vy - also minimum sink rate), the other
maximizes
> distance over the ground.
Actually minimum sink is probably closer to Vx for most
power planes. I think Steve summarized the differences
between the various speeds pretty well (disclaimer: I only
skimmed it), but there are a couple key points. One is
that Vy is most directly related to excess power rather
than just speed.
Another point is that the prop is a variable; its contribution
to drag differs a lot between normal operation (applying to Vx & Vy)
and a post-power-failure glide. In the latter cases it also
makes a differenct whether the prop is windmilling or stopped.
About the intersection of the induced & parasite drag curves...
> At the lowest point, total drag is minimized.
> ... Since drag is minimized,
> loss in altitude per unit time is also minimized.
Actually this is the speed for maximum lift to drag ratio,
and minimizes loss in altitude per unit distance rather than
per unit time. Getting minimum altitude lost per unit time
requires slowing down further (increasing Cl) to minimum sink
speed.
> If you lose your engine and you have a terrific tailwind, slow to Vy.
That
> will maximize your time aloft, and thus the distance the wind can help
you
> cover.
Again that should be minimum sink speed instead of Vy. On
the other hand, if you need to penetrate a roaring headwind
to reach a landing site you'll need to speed up a lot. Details
depend on wind speed and how draggy your airframe is; there's
no single magic number to pick.
> One question for Prof. Dave Rogers: (or others) Why is best range
> different than best glide? My guess is that it adds another curve to
the
> energy usage curves, and the inefficiencies of the power plant skews
the
> curve slightly to the high side of best glide.
I agree about powerplant efficiency skewing this speed. I haven't
studied power planes, but a good comparison is two cars: The
Saab 900 turbo that I used to drive got best milage around 70 to
75 mph because it was tuned for best breathing at relatively
high engine speeds. That factor overshadowed the growth of
drag (proportional to V**2) up to the 70ish speed range.
On the other hand the Taurus SHO that I drive now probably
would get best milage around 45-50 mph because its intake system
has "dual" tuning; one short set of intake runners opens only
above about 5000 rpm, the long set is always open and provides
an excellent low end torque curve. With good efficiency at a low
engine speed, the V**2 drag dictates best economy at the slowest
speed possible for driving in 5th gear without lugging.
Besides intake & exhaust tuning there can be other engine
efficiency factors, such as how much mass is the engine
accelerating on each stroke in pistons & rods, and how much
energy goes into compression on each stroke.
If you have a fixed-pitch prop, the prop pitch adds yet
another factor in determining the speed for best economy.
For light singles this probably overshadows differences
among engines.
------------------
Paul Raveling
Rave...@Unify.com
Kerry Kurasaki
In article <16...@front.se> p...@front.se (Per Lindberg) writes:
>
>One thing that confused me somewhat during the technical course was
>finding Vy in the manual. It finally dawned on me that the speeds for
>best climb and best glide are *different*, 73 and 86 knots.
>
>I have previously thought that this was alway the same, called Vy, and
>magically linked to L/D. I had expected *small* differences, but 73/86
>is a lot. Anyone care to comment on this?
>
Steven Philipson
> best climb and best glide are *different*, 73 and 86 knots.
> I have previously thought that this was alway the same, called Vy, and
> magically linked to L/D. I had expected *small* differences, but 73/86
> is a lot. Anyone care to comment on this?
Sure. Best glide and best rate speeds have little to do with each
Lars-Henrik Eriksson
> Best glide (L/Dmax) is the speed for lowest drag in a power-off glide.
>This is the minimum point on the DRAG curve for the aircraft. (This is
>(very close to the curve for THRUST required for level flight.)
Actually, no. Best glide speed is the speed for highest *ratio*
between lift and drag. This *always* happens at a higher speed than
speed for least drag!
Lars-Henrik Eriksson
>In article <1991Jun16....@sics.se> l...@sics.se (Lars-Henrik Eriksson) writes:
>>In article <676362...@egsgate.FidoNet.Org>, Steven.Philipson@f98 (Steven Philipson) writes:
>>> Best glide (L/Dmax) is the speed for lowest drag in a power-off glide.
>>>This is the minimum point on the DRAG curve for the aircraft.
>>
>>between lift and drag. This *always* happens at a higher speed than
>>speed for least drag!
>
>flight, the the L part of L/D is going to remain constant. The maximum
>L/D will occur at the minimum D.
The lift is *not* constant. In a glide, the weight of the aircraft is
carried by the resultant of the lift and the drag, i.e. the square of
the weight is equal to the sum of the squares of the lift and the
drag. The lift depends on the glide ratio.
Consider a light aircraft that has a weight of about 1000 kg (2200
lbs) and a glide ratio of 1:8. In a glide, the lift will be about 990
kg and the drag about 125 kg.
Best glide speed is the speed where the lift/drag ratio is highest.
This means that the coefficient of lift/coefficient of drag ratio is
highest. For a given airfoil, this happens at a particular angle of
attack. If the weight of the aircraft is increased, the wing has the
optimal angle of attack at a higher airspeed, while the glide ratio
remains the same.
Glider pilots know this well, as they ballast their aircraft with
water to increase the best glide speed while maintaining the best
glide ratio.
(I have disregarded the effects of airframe drag here. It will cause
the best glide ratio to decrease when weight goes up. The basic idea
of a higher best glide speed at a higher weight still holds, however.)
Steve Peltz
>> Best glide (L/Dmax) is the speed for lowest drag in a power-off glide.
>>This is the minimum point on the DRAG curve for the aircraft.
>
>between lift and drag. This *always* happens at a higher speed than
>speed for least drag!
Uh, I think you're confusing min sink with min drag. In unaccelerated
flight, the the L part of L/D is going to remain constant. The maximum
L/D will occur at the minimum D.
Max L/D will occur at a higher speed than min sink, which is the point of
least power (drag * speed) required. The power comes from the weight (= lift),
and vertical velocity (lift * sink).
--
Steve Peltz
Internet: pe...@cerl.uiuc.edu PLATO/NovaNET: peltz/s/cerl
Lars-Henrik Eriksson
> [Most of a discussion of best glide speed deleted]
>Best glide speed is the speed where the lift/drag ratio is highest.
>This means that the coefficient of lift/coefficient of drag ratio is
>highest. For a given airfoil, this happens at a particular angle of
>attack. If the weight of the aircraft is increased, the wing has the
>optimal angle of attack at a higher airspeed, while the glide ratio
>remains the same.
I was confusing drag with coefficient of drag. Best lift/drag ratio
does happen at the speed for minimum drag (but NOT at the speed for
minimum Cd, which is what I was thinking about all along). Steve Pelz
and Steven Philipson are entirely correct.