AN AUTOROTATIONAL PARADOX
roto...@fox.nstn.ca
I read with great interest the Oct 96 rotor & wing article by R.W.
Prouty with the above title. In the article, he states (and explains)
that the rate of descent in autorotation is inversely proportional to
the gross weight. In his words
"a helicopter comes down slower in autorotaion
when it is heavy than when it is light."
I must admit that I was somewhat skeptical, but by the time I had
finished his article he had me convinced. But, being the inquisitive
type and working in Test and Eval, I just had to try it out. Today, I
flew a CH-124A (H-3, or S-61B) in autorotation at two points during
the flight and discovered the following results.
Temp = -2 Deg Celcius
Pressure Altitiude = 1000 feet
Speed = 80 KIAS
Rate of Descent @ 19,000 lbs = 2450 fpm
Rate of Descent @ 17,000 lbs = 2250 fpm
Hmmmm.... The test data does NOT fit the theory. Granted I only have
two data points, and that is not exactly statistically significant,
but the results surprised me. My gut feel on this issue turned out to
be right, but now that it doesn't fit the theory, I wonder why?
Any comments of explanations offered? Free beer at TGIF in the 'O'
club for DETAILED explanations in person. 8-)
Thanks
\|/
(. .)
+----------------------.o0O--(_)--O0o.----------------------+
|Mark E. Chapman | http://ccn.cs.dal.ca/~ad392 |
|aka "RotorHead" | "To Fly is Heavenly, To Hover is DIVINE!"|
| "Flying of Course - what the heck other hobby is there?"|
+-----------------------------------------------------------+
Krish Krothapalli
roto...@fox.nstn.ca wrote:
>
> I read with great interest the Oct 96 rotor & wing article by R.W.
> Prouty with the above title. In the article, he states (and explains)
> that the rate of descent in autorotation is inversely proportional to
> the gross weight. In his words
>
> "a helicopter comes down slower in autorotaion
> when it is heavy than when it is light."
>
If Ray Prouty said it, I believe it.
Maybe he was talking about ideal autorotation.
Autorotation descent speed should vary with
the position of your collective stick.
It is possible that you were not close enough
to ideal autorotation in your flight test(s).
Krish
Gina and Carlos Rogers
Hi to all,
Missed the artical; however, I would tend to agree with your findings.
Let's take this to the extreme. Say your bird became magically over loaded
in flight, just before autoing. You can imagine that this would be a fast
flight to the bottom. If you look at it from a simple (Newtonian) physics
point of view, the potenial energy from the mass at altitude is much
greater than the ability to blead off engery (ie drag from the ship and the
rotors). Therefore, the velocity would increase until the two energies
(drag and mass*accel due to gravety) became equal. This would be a greater
velocity with more mass. The other side, if the ship lost 80% of the
weight, the potenial would be less = slower decent.
I guess that I shouldn't think out loud! But, I am not taking any of the
aerodynamics of the flight into account. They should not count in this;
however, I may be missing something.
What do you thinkl? At 19,000 lbs you fell at 2450 fpm. How fast would
it come down if you loaded to 40,000 lbs? (not including the fact that the
machine would break up..)
Carlos Rogers
Mechanical Engineer
:-)
>
Doug Friend
roto...@fox.nstn.ca wrote:
>
> I read with great interest the Oct 96 rotor & wing article by R.W.
> Prouty with the above title. In the article, he states (and explains)
> that the rate of descent in autorotation is inversely proportional to
> the gross weight. In his words
>
> "a helicopter comes down slower in autorotaion
> when it is heavy than when it is light."
>
I have seen -1 charts for two different models of the H-53.
On the low drag model, Prouty's statement is correct.
On the high drag model, the opposite effects take place.
I dug out the article to read and he does repeatedly use 'generally'
to quality his comments.
twood
Low Drag H-53??? Now there's an oxymoron for you!
Stan Gosnell
roto...@fox.nstn.ca wrote:
>I read with great interest the Oct 96 rotor & wing article by R.W.
>Prouty with the above title. In the article, he states (and explains)
>that the rate of descent in autorotation is inversely proportional to
>the gross weight. In his words
>
> "a helicopter comes down slower in autorotaion
> when it is heavy than when it is light."
>
>I must admit that I was somewhat skeptical, but by the time I had
>finished his article he had me convinced. But, being the inquisitive
>type and working in Test and Eval, I just had to try it out. Today, I
>flew a CH-124A (H-3, or S-61B) in autorotation at two points during
>the flight and discovered the following results.
>
>Temp = -2 Deg Celcius
>Pressure Altitiude = 1000 feet
>Speed = 80 KIAS
>Rate of Descent @ 19,000 lbs = 2450 fpm
>Rate of Descent @ 17,000 lbs = 2250 fpm
>
>Hmmmm.... The test data does NOT fit the theory. Granted I only have
>two data points, and that is not exactly statistically significant,
>but the results surprised me. My gut feel on this issue turned out to
>be right, but now that it doesn't fit the theory, I wonder why?
>
>Any comments of explanations offered? Free beer at TGIF in the 'O'
>club for DETAILED explanations in person. 8-)
1. 2000 pounds is a very small percentage change.
2. How accurate did you hold your airspeed?
3. How accurate is the VSI?
4. How long did you hold the autorotation?
These points all contribute, & would have a significant effect on the
outcome.
I still believe Prouty. Physics is physics.
George Schneider
Gina and Carlos Rogers wrote:
>
> Hi to all,
>
> Missed the artical; however, I would tend to agree with your findings.
> Let's take this to the extreme. Say your bird became magically over loaded
> in flight, just before autoing. You can imagine that this would be a fast
> flight to the bottom. If you look at it from a simple (Newtonian) physics
> point of view, the potenial energy from the mass at altitude is much
> greater than the ability to blead off engery (ie drag from the ship and the
> rotors). Therefore, the velocity would increase until the two energies
> (drag and mass*accel due to gravety) became equal. This would be a greater
> velocity with more mass. The other side, if the ship lost 80% of the
> weight, the potenial would be less = slower decent.
please jump in and correct me where I am wrong!!! the whole effort of
autorotation is to ensure that the energy in the rotor remains. it can
only increase to a certain degree as the rotor mass will remain constant
and the rotor speed can only be at so high an RPM. the big trick is to
ensure that you have only sufficient rotor RPM...not too much, and
definitely not too little. if your rotor system is insufficient to keep
the aircraft in a stable descent once it has reached this state because
the craft is overloaded, the aircraft will probably accelerate toward
the ground (converse to losing a whole lot of weight, where the aircraft
will establish a slower descent rate and shallower angle of descent).
no real paradox here...you just have to realize where you want to put
the energy and manage it...the rotor has limits that must be maintained.
one way of measuring the autorotative capability of a rotorcraft is to
compare the energy of the craft (disc loading...weight of the craft
distributed over the size of the rotor) to the inertia of the rotor
(simple physics). consider the relatively high inertia of an Enstrom
rotor...makes for a real nice autorotation, but if you put that head on
an H-53, it would drop like a rock on Jupiter. similarly, if you put
the 53 head on a Robinson (much tighter ratio between rotor and aircraft
inertias, so more of an E-ticket ride in an autorotation than an
Enstrom), it would probably coast across North America with no
difficulty.
George
David L. Wasylenko
Gina and Carlos Rogers wrote:
>
> Hi to all,
>
> Missed the artical; however, I would tend to agree with your findings.
> Let's take this to the extreme. Say your bird became magically over loaded
> in flight, just before autoing. You can imagine that this would be a fast
> flight to the bottom. If you look at it from a simple (Newtonian) physics
> point of view, the potenial energy from the mass at altitude is much
> greater than the ability to blead off engery (ie drag from the ship and the
> rotors). Therefore, the velocity would increase until the two energies
> (drag and mass*accel due to gravety) became equal. This would be a greater
> velocity with more mass. The other side, if the ship lost 80% of the
> weight, the potenial would be less = slower decent.
>
> aerodynamics of the flight into account. They should not count in this;
> however, I may be missing something.
>
> What do you thinkl? At 19,000 lbs you fell at 2450 fpm. How fast would
> it come down if you loaded to 40,000 lbs? (not including the fact that the
> machine would break up..)
>
=
I missed the artical too... but physics is physics. ALL OBJECTS ACCELERATE
AT THE SAME RATE 32 FEET PER SECOND PER SECOND. The rate an object falls
has NOTHING TO DO with its weight.
-
So... weight only has an affect on the energy available. You must convert your
energy (altitude) to rotor speed. To keep this extra (heaver) energy from spinning
the head right off the mast, you pull pitch. This slows the aircrafts decent.
--
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GuessWho
On 17 Jan 1997 07:38:51 GMT, twood <woo...@aloha.com> wrote:
>douglas...@gtri.gatech.edu (Doug Friend) wrote:
>>roto...@fox.nstn.ca wrote:
>>>
>>> I read with great interest the Oct 96 rotor & wing article by R.W.
>>> Prouty with the above title. In the article, he states (and explains)
>>> that the rate of descent in autorotation is inversely proportional to
>>> the gross weight. In his words
>>>
>>> "a helicopter comes down slower in autorotaion
>>> when it is heavy than when it is light."
>>>
>>
>>
>>I have seen -1 charts for two different models of the H-53.
>>On the low drag model, Prouty's statement is correct.
>>On the high drag model, the opposite effects take place.
>>
>>I dug out the article to read and he does repeatedly use 'generally'
>>to quality his comments.
>
>Low Drag H-53??? Now there's an oxymoron for you!
>
Flight is accomplished by lift overcoming gravity!
The heavier aircraft does cause more air to flow up through the
rotor. Why, because it falls faster. More gravity for lift (min
pitch on rotor). This speeds up the head. Requiring pitch to
applied. Creating more lift. Slowing rate of descent.
At the bottom....
At 100% Nr you only have X amount of energy available. Right.
X amount will have to slow the descent of both aircraft. Do you want
to be heavy or light.
You should be at the same rate of descent regardless of weight.
The flare cancels out the weight factor.
UH-1N aircraft commander, Functional test pilot.
roto...@fox.nstn.ca
st...@hal-pc.org (Stan Gosnell) wrote:
>1. 2000 pounds is a very small percentage change.
10%
>
>2. How accurate did you hold your airspeed?
EXACTLY
>3. How accurate is the VSI?
Matters not the exact accuracy as long as the measurement is taken
with the same instrument and the results are repeatable. The
important value is the delta not the absolute figure.
>4. How long did you hold the autorotation?
2000'
>These points all contribute, & would have a significant effect on the
>outcome.
Yes they would and that was taken into account
>I still believe Prouty. Physics is physics.
Yes... I belive it was once said that it was "physically impossible
for humans to survive a velocity of 60 miles per hour" (Steam engine
Tom Thumb - England circa 1800) ;-)
The fact is.. (I checked again with a different airframe) this theory
does NOT work on the S-61. Lighter weight gives slower rate of
descent.
EGELSONE
From Rotorhead: and others;
>>1. 2000 pounds is a very small percentage change.
>10%
>>
>>2. How accurate did you hold your airspeed?
>EXACTLY
>>3. How accurate is the VSI?
>Matters not the exact accuracy as long as the measurement is taken
>with the same instrument and the results are repeatable. The
>important value is the delta not the absolute figure.
>>4. How long did you hold the autorotation?
>2000'
>>These points all contribute, & would have a significant effect on the
>>outcome.
>Yes they would and that was taken into account
>>I still believe Prouty. Physics is physics.
>Yes... I belive it was once said that it was "physically impossible
>for humans to survive a velocity of 60 miles per hour" (Steam engine
>Tom Thumb - England circa 1800) ;-)
>The fact is.. (I checked again with a different airframe) this theory
>does NOT work on the S-61. Lighter weight gives slower rate of
>descent.
Hey fellas, this is a good thread. Mike, did you maintain constant rotor
RPM in both descents with both aircraft? That would make a significant
difference. The higher gross weight would force you to make an adjustment
in aurorotational RPM by adding pitch to maintain the same RPM. This
would in normal circumstances cause a reduction in rate of descent.
Considering the higher gross weight it would be logical to assume that
increased pitch and increased weight would have a cancelling effect. Was
the aircraft at the most effecient autorotational descent airspeed? Is
this airspeed dependent upon gross weight? If so you must repeat the
experiment adjusting airspeed, autorotational RPM, trim and all other
parameters to have "perfect" conditions under each aircraft condition.
As for the landing, it WILL be a little harder for ther higher gross
weight helicopter, not because of any factor other than the increased
gross weight. Since most autorotational landings use the excess energy
stored in the rotor system to cushion the landing, the lighter aircraft
will have more energy vs the maproducable in any given situation. i..e. if
a system produced x lift under ideal or max gross conditions (higher gross
would produce more potential energy up to the ability of the rotor system
to store it). the helicopter would require more lift to arrset the descent
due to higher gross weight. All things considred a lighter helicopter
(below it's design max gross) will be easier and safer to autorotate.
Ed
This is DEFINETLEY just an opinion, I need to get out some references and
resaercg this. Could someone post the article or if they can't e-mail me
and I will give you a fax number.
Angus Ross-Thomson
Yes, I read the article as well, and although Mr Prouty discussed
the paradox, he did not give an explanation.
I have now lost the article, but I think I can remember the
details. He discussed the paradox by using the power required
in level flight (after all an aircraft in steady state
autorotation still requires power, extracted from the upcoming
air). The effect only worked over a limited range of of weights,
and when the increase in power required for a given increase in
AUM was proportionally less than at other masses (ie when the
lines on a Power vs Speed at various AUM graph are closely
spaced).
I think the reason is that at that point, the blades are working
at their optimum Cl/Cd ratio. Anyone else have any thoughts on
my private theory?
Regards,
Angus Ross-Thomson
10161...@compuserve.com
--
**************************************************
* Angus Ross-Thomson 10161...@compuserve.com *
**************************************************
David L. Wasylenko
Tim Butler wrote:
>
> David L Wasylenko writes:
>
> > I missed the artical too... but physics is physics. ALL OBJECTS ACCELERATE
> > AT THE SAME RATE 32 FEET PER SECOND PER SECOND. The rate an object falls
> > has NOTHING TO DO with its weight.
>
> atmosphere into outer space perhaps, but otherwise, no.
>
> The acceleration due to gravitational force is 32 ft/s^2, but that
> is not the only force on the object. The force due to aerodynamic drag
> is opposite the velocity direction, presumed down if the object is
> falling, and is proportional to the square of the velocity.
>
> Try an experiment:
>
> Crawl under a piece of furniture somewhere and dig out a clump
> of dust. (affectionately know as a dust bunny.)
> Hold said "dust bunny" out at arms length with one hand, and do the
> same with a coin in the other hand. Let them both go. I'll bet you'll
> see that the coin hits the ground first.
> The reason that this is so is that the gravitational force on the
> dust is much smaller than it is on the coin, while it has a much
> larger drag factor.
>
==
Uh, no, I am not totally incorrect. Weight has nothing to do with
acceleration of an object. Period. Nothing. Nada. No way. nope.
The acceleration is ALWAYS 32 ft/s^2 regardless of atmosphere. You are
mis-representing acceleration. An object doesn't have to "move faster"
to be affected by the accelerating forces of gravity. The acceleration
forces are just offset by friction and lift caused by the interaction of
the object and the air/particals it moves through; the acceleration
however remains constant.
-
Aerodynamic forces exerted on the aircraft are also the same, assuming
the same aircraft, same speed, attitude and pitch (etc etc etc).
-
With that said, the heavier aircraft would fall faster... but not if
the pilot is trying to keep the aircraft within the same autorotation
parameters: airspeed, rotor speed and the like.
-
and so, like i said in my original post:
Mahan
Drop that dust bunny and that coin in a jar from which air has been
evacuated, and they hit the bottom at the same time.
Fred in Florida
LghtngHelo
Drop that dust bunny and that coin in a jar from which air has been
evacuated, and they hit the bottom at the same time.
Fred in Florida
Then, put two helicopters (one small and one large) in a jar with all the
air evacuated and you will see that both helicopters reach the ground at
the same time!!!!!!!!
EGELSONE
From before:
>Fred in Florida
I think we are getting goofy here. The point was that the statement about
acceleration being 32 feet per second per second, is conditioned upon the
acceleration occuring in a vacume. This is true in a literal sense. The
comments from previous about the acceleration being the same appear to me
to be patently false. I do however believe that the energy of the
acceleration remains the same and that the atmospheric conditions of drag
both induced and parasitic cause the confusion.
I would contend that the reduction in rate of descent at higher gross
weights is limited by the design of the rotor ayatem. If the gross weight
exceeds the design limits of the rotor system, the dam thig will have too
much pitch in the rotor blades to maintain autorotation. Then we would
see something akin to the two helicopters in the jar. SPLAT. If the
angle of attack of the driving portion of the rotor system goes positive,
this region then starts to produce lift and nothing is driving the rotors.
The end result is predictable. Loss of rotor RPM, loss of lift and a
rapid termination of flight.
While it may be reasonable to expect that the rate of descent will be
reduced as we add collective to maintain rotor RPM, it is aldo reasonable
to assume that this is a fairley narrow range of gross weights. All
things being equal, a lighter helicopter is safer in autorotation. I am
mainly speaking of the termination assuming that the engine (s) have
failed. It might also be noted that the angle of descent is a factor more
likely to give one grief than the actual rate of descent. This can be
adjusted by judicious application of collective to adjust RPM and
adjustment of airspeed to either extreme of the best rate of descent
airspeed or the minimum rate of descent airspeed. I should look at some
charts, I will get back to you on that.
Ed
Excuse the poor typing and lack of proof reading.
Gene Shimko
the general momentum theory shows that the descent rate of a helicopter
in steady state autorotation is directly proportional to the square root
of the gross weight. Generally, if the weight increases, the rate of
descent increases. For example, increase the gross weight by 10% and the
descent rate will increase by just over 3%.
Minimum descent rate is also a function of how much power the rotor is
consuming in relation to the lift produced. This very much depends on
the blade section characteristics. The ratio of blade section profile
drag to lift coeficient should be minimum to achieve minimum descent
rate.
An inflight helicopter is in a very complex flow state. The result of
changing a parameter (such as gross weight) may at first seem susprising
if the results are not what we "intuitively" expect. If the descent rate
of a helicopter varies unexpectantly with gross weight it is because of
the complex play between all of the aerodynamic factors.
It is certain that a helicopter generally will descend faster with an
increase in weight. Does a parachute (cup shape)come down slower the
heavier the parachutist?
Thanks for reading this.
George Schneider
David L. Wasylenko wrote:
>
> Tim Butler wrote:
> >
> > David L Wasylenko writes:
> >
> > > I missed the artical too... but physics is physics. ALL OBJECTS ACCELERATE
> > > AT THE SAME RATE 32 FEET PER SECOND PER SECOND. The rate an object falls
> > > has NOTHING TO DO with its weight.
to restate: all objects are subject to the same gravitational force as a
function of their mass, resulting to the same accelerating force, but
read on...
> > Uh, this is totally incorrect. If you managed to fling the earth's
> > atmosphere into outer space perhaps, but otherwise, no.
> >
> > The acceleration due to gravitational force is 32 ft/s^2, but that
> > is not the only force on the object. The force due to aerodynamic drag
> > is opposite the velocity direction, presumed down if the object is
> > falling, and is proportional to the square of the velocity.
> >
right up!!
> > Try an experiment:
> >
> > Crawl under a piece of furniture somewhere and dig out a clump
> > of dust. (affectionately know as a dust bunny.)
> > Hold said "dust bunny" out at arms length with one hand, and do the
> > same with a coin in the other hand. Let them both go. I'll bet you'll
> > see that the coin hits the ground first.
great imagery...I think da Vinci demonstrated the weight thing by
dropping two different sized objects off the Tower of Pisa. I forget
who tried a similar demonstration using a feather in a vacuum and a
similar-sized object, to null out the effects of drag (couldn't have
been da Vinci as they hadn't discovered air yet).
> > The reason that this is so is that the gravitational force on the
> > dust is much smaller than it is on the coin, while it has a much
> > larger drag factor.
once again, you can control the drag force on your rotor by adjusting
collective (and on the aircraft by adjusting airspeed???), but there is
a limit, which is set by the inertia of the rotor. autorotative
capability is set by the ratio of aircraft inertia (its mass) and the
inertia of the rotor (rotor speed, rotor mass)...perhaps someone could
talk to the ability of a UH-1 to do a full touchdown auto, pick up, and
move the aircraft to a different location, all with no engine power,
just using the residual energy in the rotor.
George
EGELSONE
GEORGE WROTE:
>perhaps someone could
>talk to the ability of a UH-1 to do a full touchdown auto, pick up, and
>move the aircraft to a different location, all with no engine power,
>just using the residual energy in the rotor.
>George
That is cool to do, however you don't get to move it far. have actually
been in a Huey that did a 50 foot AGL hovering auto. Scared the ----out
of me. The UH-1 is almost hard to screw up in. (auto wise) Cobra is about
the same except for the narrow tracked gear.
roto...@fox.nstn.ca
egel...@aol.com (EGELSONE) wrote:
>Hey fellas, this is a good thread. Mike, did you maintain constant rotor
>RPM in both descents with both aircraft?
No, I accepted the rotor RPM as it settled with the collective fully
bottomed.
>If so you must repeat the
>experiment adjusting airspeed, autorotational RPM, trim and all other
>parameters to have "perfect" conditions under each aircraft condition.
Yes, you have a point there... Soooooo.. I tried it again tonight.
Here are the data:
Test Point One:
GW 17,900 lbs
PA 2000'
Temp 0 Deg C.
IAS 80 Kts
Rotor RPM 100% Nf = 96% (A True Auto)
---------------------------------------
Rate of Descent = 2100 fpm
---------------------------------------
Test Point Two:
GW 15,900 lbs
PA 2000'
Temp 0 Deg C.
IAS 80 Kts
Rotor RPM 100% Nf = 96% (A True Auto)
---------------------------------------
Rate of Descent = 2600 fpm
---------------------------------------
Mr. Prouty, I salute you. The data does fit the theory after all.
The rate of descent in Autorotation is highest with a lighter gross
weight, assuming ALL other factors are constant ESP Rotor Speed.
>As for the landing, it WILL be a little harder for ther higher gross
>weight helicopter,
We always do a power recovery to 35' so I'm afraid I cannot comment on
the landing except to imagine that the CH-124 (H-3A, S-61B) goes in
with a bit of a "Thump" at the end of an auto. If I ever do one and
walk away smiling, it will have been a sucess. Regardless of the
condition of the helo.
Travis Peery
On Sun, 19 Jan 1997, George Schneider wrote:
> to restate: all objects are subject to the same gravitational force as a
> function of their mass, resulting to the same accelerating force, but
> read on...
This is wrong! F = m a is the law. If a is the same for both masses, each
mass being different, then the force acting on each of those masses is
also different, such that their ratio, i.e. a = F/m is equal. Mass has
NOTHING to do with weight. A larger mass requires a larger force, as per
above, in order to be accelerated the same. Thus gravity pulls with
greater force on more massive objects than it does with lighter ones such
that ALL objects fall at a = g = 9.8m/s. It takes a greater force to
accelerate a larger MASS (not weight). Weight is a force, not a mass.
Travis
Jeffhauck
Just some "gee-whiz" info to validate your experiment results. I have been
working Military helicopters for over 15 yrs. (Sikorsky HH-3E, Sikorsky
HH-53B/C, Sikorsky HH-60G, and Bell HH-1H). In all of are Tech Manuals for
all of these helicopters, we have a chart for calculated autorotation
rotor speed and rate of descent.
I can say for certain, that in these charts the rate of descent is listed
as proportional to the gross weight of the helicopter i.e. the higher the
gross weight, the higher the rate of descent.
I have found this to be entirely true, as a crew chief, during functional
checkflights, this is a common ck if blade adjustments etc were made.
I don't know how this will fit into "Rotor & Wings" theory, but I believe
that your results are true of all helicopters. Jeff
jeff...@aol.com
da...@airstrip.demon.co.uk
David L. Wasylenko wrote:
> Uh, no, I am not totally incorrect. Weight has nothing to do with
> acceleration of an object. Period. Nothing. Nada. No way. nope.
> The acceleration is ALWAYS 32 ft/s^2 regardless of atmosphere. You are
> mis-representing acceleration. .....
Ummm, no he wasn't.
Acceleration is a measurement just like it looks - it is the rate at
which the speed of an object is changing. If two objects
start from rest and they are _not_ increasing in speed at exactly the same
rate, then they clearly do not have the same acceleration - this is
what acceleration _means_ 32 ft/s^2 is telling you that after 1 second
the object will be travelling at 32 fps, after 2 seconds at 64 fps,
after 3 seconds at 96 fps etc. The coin and the dust-bunny clearly
do _not_ accelerate at the same rate. The terms "g-force" and
"acceleration" are often confused. Sitting in this chair, my body is
being pulled by a "g-force" of 1G. I am definately not accelerating.
Why the confusion?
Gravity produces a 32ft/s^2 acceleration _only_ if there are no other
forces involved. This is the fact often glossed over in high-school physics.
My chair is exerting an opposite force of 1G, and so I do not accelerate.
It takes more force to accelerate a heavy object to 32 ft/sec/sec than it
does a light object. This is regardless of the direction of acceleration.
To get a train up to 5mph in 1 second needs a bigger engine (more force)
than to get a bicycle up to 5mph in 1 second. Both will experience
the same acceleration.
It so happens that the force of gravity is directly proportional to mass,
and also the required force needed to produce a given acceleration is also
directly proportional to mass. Therefore, _in the absence of any other
forces_ gravity will produce the same acceleration on any object of any mass.
But ... objects falling through the air are definately subjected to one
other very major force - aerodynamic drag (friction).
Aerodynamic drag is also a force. It is not proportional to mass, but to
size, shape & speed. Therefore, a heavy and a light object of the same size
& shape will experience the same aerodynamic drag (at a given speed).
Gravity will act with more force on the heavy object, because the heavy
object needs more force to accelerate it to 32 ft/s^2.
The total force acting upon each object will be gravitational force minus
aerodynamic drag. Aerodynamic drag is the same, and so will "cancel out"
a bigger percentage of the gravitational force on the lighter object, which
will therefore fall at a slower rate of acceleration.
Both objects _start_ to accelerate at 32 ft/s^2, because they start at
rest - no speed=no aerodynamic drag. As soon as they start to gain any
speed at all, the aerodynamic drag begins to slow down their acceleration -
more so for the light object. As the objects gain speed, aerodynamic drag
increases, which progressively reduces the acceleration. Eventually, both
objects will attain a speed where the aerodynamic force exactly equals the
gravitational force, and acceleration will become zero - i.e. the speed
will remain constant. This is called "terminal velocity", and will occur
sooner & slower for a light object than a heavy one. (Only if the
shape & size are the same, of course).
If my chair were to suddenly disappear, I would begin to accelerate downwards.
Eventually my bottom would reach the floor. The floor will need to
de-accelerate my body at a much greater rate than 32 ft/s^2, because it will
take less time to stop than it did to fall. The momentary "G" force
exerted by the floor will be surprisingly high - maybe (guessing) 25G or more.
After I fully stop, the floor will again exert an upwards force of 1G, but
I will have a very sore bottom :-)
==================
Dave Mould
==================
Greg Malcangi
Hi Ed,
<< All things being equal, a lighter helicopter is safer in
autorotation. >>
I might be out of my depth here but as I understand it, with my limited
experience, all things aren't equal. From the practical point of view of
entering autorotation and cushioning the landing, rotor head inertia
makes a big difference. The inertia gives you a lot longer before the
RPM decays which gives you more time to get into the auto and more lift
when it comes time to "hit the dirt". From what I've been told it is
much easier to land a heli in auto with a higher head inertia than a
heli with low head inertia. By definition, light helis have low head
inertia, particularly the R22 which I fly. Doesn't this mean a heavier
heli (with greater rotor head inertia) would be safer in auto rotation?
Regards,
Greg
**************************************
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C E N T R A L O F F I C E
----------------------------
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James Long
Greg Malcangi <gr...@evelyn.co.uk> wrote in article
<VA.00000005.00400b24@solaba76>...
> I might be out of my depth here but as I understand it, with my limited
> experience, all things aren't equal. From the practical point of view of
> entering autorotation and cushioning the landing, rotor head inertia
> makes a big difference. The inertia gives you a lot longer before the
> RPM decays which gives you more time to get into the auto and more lift
> when it comes time to "hit the dirt". From what I've been told it is
> much easier to land a heli in auto with a higher head inertia than a
> heli with low head inertia. By definition, light helis have low head
> inertia, particularly the R22 which I fly. Doesn't this mean a heavier
> heli (with greater rotor head inertia) would be safer in auto rotation?
>
> Regards,
>
> Greg
I see this as a mass to mass comparison. You would have to examine a
particular aircraft and compare its over all mass to its moving rotational
mass (rotor head).
Your previous statement could be correct in some circumstances. Say a R22
has rotor weight percentage of 1% of its total weight and a Bell 222 has a
rotor weight of 3% of its total weight. Then the Bell would be easier to
autorotate.
This totally depends on the head weight compared to the total weight. If
the percentages are the same the bird is going to autorotate close to the
same.(we are discussing physiscs here not actual flying characteristics)
Remember the more mass the more energy to start and stop motion.
James Long
(Hey...just my $.02)
George Schneider
James Long wrote:
>
> Greg Malcangi <gr...@evelyn.co.uk> wrote in article
> <VA.00000005.00400b24@solaba76>...
> > I might be out of my depth here but as I understand it, with my limited
> > experience, all things aren't equal. From the practical point of view of
> > entering autorotation and cushioning the landing, rotor head inertia
> > makes a big difference. The inertia gives you a lot longer before the
> > RPM decays which gives you more time to get into the auto and more lift
> > when it comes time to "hit the dirt". From what I've been told it is
> > much easier to land a heli in auto with a higher head inertia than a
> > heli with low head inertia. By definition, light helis have low head
> > inertia, particularly the R22 which I fly. Doesn't this mean a heavier
> > heli (with greater rotor head inertia) would be safer in auto rotation?
> >
> > Regards,
> >
> > Greg
>
as stated below, the real issue is a comparison of the rotor head
inertia to the aircraft inertia. you can't really change much about the
head inertia (except to change the speed, which is not recommended in
either direction...too fast and you may over-stress the rotor, too slow
and you will overstress your brain on impact, or worse, risk damaging
the aircraft...read on the Mass Police A-Star accident two years ago on
the Charles River in Boston...they pulled pitch at altitude and made a
near-vertical descent onto a boathouse...the wreckage was contained in a
few hundred square feet), but you can change the aircraft inertia
depending on how much fuel, baggage, personage you have on board.
> I see this as a mass to mass comparison. You would have to examine a
> particular aircraft and compare its over all mass to its moving rotational
> mass (rotor head).
>
> Your previous statement could be correct in some circumstances. Say a R22
> has rotor weight percentage of 1% of its total weight and a Bell 222 has a
> rotor weight of 3% of its total weight. Then the Bell would be easier to
> autorotate.
>
> This totally depends on the head weight compared to the total weight. If
> the percentages are the same the bird is going to autorotate close to the
> same.(we are discussing physiscs here not actual flying characteristics)
>
> Remember the more mass the more energy to start and stop motion.
>
> James Long
>
> (Hey...just my $.02)
and basically a good $.02...
again, you need to compare the inertia of the rotor to that of the whole
aircraft. of course, this ratio will change as a function of disc
loading (gross weight), and so the performance (and index) of the
aircraft will change over the range of its loading (CG also comes into
play here, as you will need enough forward cyclic stick authority to
overcome a large aft CG...probably not an issue in the R-22 with its
limited capacity for baggage and placement of passengers, but possibly
an issue for say, an R-44 or B206...once again falling into the pilot's
responsibility for proper pre-flight planning.
you can make a relatively simple chart of this to compare the
performance of many different aircraft at an assortment of loadings.
the chart I have seen has lines drawn through the origin to represent
ratios of 20 through 80 of the resulting non-dimensional parameters with
aircraft plotted according to their rotor inertia vs. aircraft inertia,
with the lower numbers implying poorer autorotative performance (less
forgiving of errors at the bottom) and the higher numbers implying a
mellower glide with more forgiveness at the bottom and possibly
remaining inertia once you get on the ground. as I recall, a heavily
loaded H-60 will be similar to a heavily loaded H-53, and be in the
lower range. Enstrom and B206 will be at the higher ratios (notice that
the points will not coincide as the basic non-dimensional parameters
will be different, just that their ratios will be similar.
George
George Schneider
da...@airstrip.demon.co.uk wrote:
>
> David L. Wasylenko wrote:
>
> > Uh, no, I am not totally incorrect. Weight has nothing to do with
The coin and the dust-bunny clearly
> do _not_ accelerate at the same rate. The terms "g-force" and
> "acceleration" are often confused. Sitting in this chair, my body is
> being pulled by a "g-force" of 1G. I am definately not accelerating.
>
had the opportunity to be in DC last week and managed a stop at the Air
and Space Museum. in the Theory of Flight area, they had a tube that
contained a feather on a disk that allowed it to be rotated. it also
was connected to a vacuum pump and a valve so you could add air up to 1
atm. or remove it to a -40 (???) psi vacuum, turn the disk, and watch
the feather's rate of descent. at atmospheric pressure, the feather
took a leisurly descent, but as you removed air you could see it speed
up.
George
Stan Gosnell
Greg Malcangi <gr...@evelyn.co.uk> wrote:
>Hi Ed,
>
><< All things being equal, a lighter helicopter is safer in
>autorotation. >>
>
>I might be out of my depth here but as I understand it, with my limited
>experience, all things aren't equal. From the practical point of view of
>entering autorotation and cushioning the landing, rotor head inertia
>makes a big difference. The inertia gives you a lot longer before the
>RPM decays which gives you more time to get into the auto and more lift
>when it comes time to "hit the dirt". From what I've been told it is
>much easier to land a heli in auto with a higher head inertia than a
>heli with low head inertia. By definition, light helis have low head
>inertia, particularly the R22 which I fly. Doesn't this mean a heavier
>heli (with greater rotor head inertia) would be safer in auto rotation?
Gross weight of the helicopter has nothing to do with blade inertia.
The inertia comes from mass and rotational speed. The helicopter with
a heavier blade turning faster has more inertia. One of the drawbacks
of this is that the tips of the blades are nearing supersonic, & thus
very loud. One of the engineering subjects manufacturers are working
hardest on is noise. Adding more blades, & allowing slower rotation,
decreases noise & increases speed, in most cases. You may have
noticed that even Bell has abandoned the 2-bladed teetering rotor
system. I have heard UH1's & 212's coming at a distance of close to
10 miles. Civilians on the ground don't like this, but air defense
gunners do.
G W Cumbey
Greetings, Stan,
High inertia blades Do significantly improve autorotational performance
(regardless of overall GW, as you mentioned). By high inertia, I mean
blade mass x moment arm (blade length). Depending upon blade design (esp.
wt. & blade center of gravity), longer but slower blades can actually have
more inertia than shorter, faster blade designs.
Ref. noise, blade tip speed is certainly a factor, esp. when near sonic.
However, blade tip design (e.g., swept tips found on the UH-60 & AH-64) can
dramatically reduce noise throughout the rotor RPM envelope. I think this
is partly because a big contributor to blade noise is wingtip vortices.
This might explain why you can make a Huey REALLY POP with subtle changes
in collective (often if reduced a bit, as for a descent)...
GWCumbey
Stan Gosnell <st...@hal-pc.org> wrote in article
<32f0e794...@news.hal-pc.org>...
Mike Rehbein
> Greg Malcangi <gr...@evelyn.co.uk> wrote:
>
> >Hi Ed,
> >
> ><< All things being equal, a lighter helicopter is safer in
> >autorotation. >>
> >
snip
> Gross weight of the helicopter has nothing to do with blade inertia.
> The inertia comes from mass and rotational speed. The helicopter with
> a heavier blade turning faster has more inertia.
snip
I think we are missing the mark here by only looking at blade inertia.
It is as was stated earlier, the ratio of blade inertia to the ship's
gross weight that determines how much energy is available for
AuToRoTaTiOn.
If you are looking at JUST the blade inertia, yes, of course the ship's
gw will not determine ANYTHING about blade inertia. The topic is
autorotation. And that must include the ship's gw, cg, blade inertia. Is
there anything else? :)
Mike
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Stan Gosnell
"G W Cumbey" <gwcu...@kih.net> wrote:
> High inertia blades Do significantly improve autorotational performance
>(regardless of overall GW, as you mentioned). By high inertia, I mean
>blade mass x moment arm (blade length). Depending upon blade design (esp.
>wt. & blade center of gravity), longer but slower blades can actually have
>more inertia than shorter, faster blade designs.
Inertia comes from mass X velocity, not arm. 2-blade systems must
turn faster than systems with more blades to provide the same amount
of lift. In general, they are heavier because each blade must be
larger in order to get the same area.
No modern design uses the 2-blade teetering system.
> Ref. noise, blade tip speed is certainly a factor, esp. when near sonic.
>However, blade tip design (e.g., swept tips found on the UH-60 & AH-64) can
>dramatically reduce noise throughout the rotor RPM envelope. I think this
>is partly because a big contributor to blade noise is wingtip vortices.
>This might explain why you can make a Huey REALLY POP with subtle changes
>in collective (often if reduced a bit, as for a descent)...
Swept tips reduce noise, but they are already turning slower than a
2-blade system, thus reducing noise even more.