Stopping the prop to improve glide ratio
Lee Devlin
As some of you may remember, there was a 'Stopping the Prop' string on
rec.aviation quite some time ago that went on and on and on.... The
original poster was speculating that during an engine failure that
slowing the aircraft down to just above the stall speed and stopping the
prop might extend your glide ratio.
I've often heard that a windmilling prop causes a drag equivalent to a
disk the same size as the prop. This sounds like a rather ridiculous
assertion to me. After all, if the wind spins the prop in the same
direction that the engine turns it, then the prop also produces a thrust
component along with its drag component. A stopped prop can only
produce drag. My thinking leads me to believe that a windmilling prop
should produce no more drag than one that's stationary (no feathering,
of course).
Well, I finally got up the nerve to do a few experiments myself and
found that stopping the prop actually gave me a *worse* glide ratio at
the published best glide ratio speed of 80 mph. The most discouraging
thing I found was how _long_ it takes to stop the prop. After pulling
the mixture the plane must be flown at just above stall speed for more
than a minute. In the several times I tried it, it took all of my
concentration to maintain an MCA airspeed without stalling, which would
otherwise drop the nose and get the prop spinning faster. This is
valuable time that would be much better spent looking for a suitable
landing site. I couldn't get the prop stopped without losing at least
800 feet of altitude. And remember, my glide ratio was absolutely
terrible during this time because of my slow speed.
I found that I had to significantly re-trim the plane after the prop
stopped to get back up to 80 mph which gave me a descent rate between
900 and 1000 fpm. With the prop windmilling, I was able to get descent
rates between 800 and 900 fpm at this speed! In both cases, I had the
mixture pulled so the engine wasn't developing any power on its own. If
the myth of the equivalent 'disc drag' were true, I should have had
lower descent rate at all airspeeds with the prop stopped.
Incidentally, I performed all of these experiments over a private airstrip
that was always within gliding range.
My conclusions are:
1. A windmilling prop does not produce drag equivalent to a disk of the
same diameter.
2. Attempting to 'stop the prop' to extend the glide ratio is about the
most stupid thing a pilot can attempt during an engine failure.
--
Lee Devlin | HP Little Falls Site | phone: (302) 633-8697
Piper Colt N4986Z | 2850 Centerville Rd. | email:
"Spirit of rec.aviation"| Wilmington, DE 19808 | dev...@lf.hp.com
Jui Tien
You should try this in a plane with a constant speed prop.
Pushing the prop forward will definitely give you deceleration you can
feel in the plane.
The standard procedure for high performance airplane is to feather the props
ie. turn the prop blades into the slipstream. The same goes for multi-engine
planes.
I usually use the prop to slow me down from cruise when I enter ARSA/ATC just
by puttung props full forward; it also helps me run the engine at a higher MP
to aviod shock cooling when I start my descent.
Lars-Henrik Eriksson
I usually use the prop to slow me down from cruise when I enter ARSA/ATC just
by puttung props full forward; it also helps me run the engine at a higher MP
to aviod shock cooling when I start my descent.
How can increasing engine speed help you to run the engine at a higher
MP? To get the same power, you should reduce the MP if you increase
engine speed.
--
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 Fax: +46 8 751 72 30
Lee Devlin
: The standard procedure for high performance airplane is to feather the props
: ie. turn the prop blades into the slipstream. The same goes for multi-engine
: planes.
I guess I should mention that I wasn't discussing adjustable pitch
props. Let's try to keep the discussion limited to fixed pitch props
for now since you don't have to bother with slowing the plane down and
flying at MCA to stop the prop if you have full feathering props.
Reece R. Pollack
In article <C74oB...@news.cso.uiuc.edu>, jtg...@uxa.cso.uiuc.edu (Jui Tien) writes:
|>The standard procedure for high performance airplane is to feather the props
|>ie. turn the prop blades into the slipstream. The same goes for multi-engine
|>planes.
While the props on multi-engine aircraft can be feathered, I'm not aware
of any single with a featherable prop. You can certainly set the thing
to its lowest RPM setting, but that's not a true feather.
--
Reece R. Pollack
PP-ASEL-IA -- Octopus Flying Club (based GAI)
Jon Thackray
Well, I finally got up the nerve to do a few experiments myself and
found that stopping the prop actually gave me a *worse* glide ratio at
the published best glide ratio speed of 80 mph. The most discouraging
thing I found was how _long_ it takes to stop the prop. After pulling
the mixture the plane must be flown at just above stall speed for more
than a minute. In the several times I tried it, it took all of my
concentration to maintain an MCA airspeed without stalling, which would
otherwise drop the nose and get the prop spinning faster. This is
I'm not sure this is the bset way to stop the prop. My instructor
once made me do a dead stick landing from overhead our training field.
He continuously stalled the plane to get the prop stopped. The nose
was never allowed to drop far enough to increase the speed while he
was stopping the prop. The plane was a Beagle Pup, which is a two seat
low wing single engine trainer, with a fixed pitch prop, related to
the RAF Bulldog. As I recall, it took about 10 seconds to get the prop
stopped. I guess we probably lost about 400 - 500 feet during this
exercise.
valuable time that would be much better spent looking for a suitable
landing site. I couldn't get the prop stopped without losing at least
800 feet of altitude. And remember, my glide ratio was absolutely
terrible during this time because of my slow speed.
I found that I had to significantly re-trim the plane after the prop
stopped to get back up to 80 mph which gave me a descent rate between
I don't remember having a trim problem, but then the Beagle Pup is
very forgiving wrt trim, for instance going from zero flap to 40
degrees flap requires no trim change!
900 and 1000 fpm. With the prop windmilling, I was able to get descent
rates between 800 and 900 fpm at this speed! In both cases, I had the
mixture pulled so the engine wasn't developing any power on its own. If
the myth of the equivalent 'disc drag' were true, I should have had
lower descent rate at all airspeeds with the prop stopped.
Incidentally, I performed all of these experiments over a private airstrip
that was always within gliding range.
Very wise! I would also have started at 5000 feet plus if I was going
to do it on my own!
My conclusions are:
1. A windmilling prop does not produce drag equivalent to a disk of the
same diameter.
2. Attempting to 'stop the prop' to extend the glide ratio is about the
most stupid thing a pilot can attempt during an engine failure.
Absolutely agreed! In particular, you'd have to do it as almost the
first thing following the failure, and it would ruin your chances of
getting a restart if the problem did turn out to be carb icing,
failing to change fuel tanks, accidentally turning off the magnetos or
any of the other common pilot error ways of producing an engine
failure.
--
Lee Devlin | HP Little Falls Site | phone: (302) 633-8697
Piper Colt N4986Z | 2850 Centerville Rd. | email:
"Spirit of rec.aviation"| Wilmington, DE 19808 | dev...@lf.hp.com
--
Jon Thackray jo...@harlqn.co.uk 44 223 872522 (voice)
Harlequin Ltd. jg...@phx.cam.ac.uk 44 223 872519 (fax)
Barrington Hall
Barrington
Cambridge CB2 5RG
England
Jer/ Eberhard
> While the props on multi-engine aircraft can be feathered, I'm not aware
> of any single with a featherable prop. You can certainly set the thing
> to its lowest RPM setting, but that's not a true feather.
A Grob-109B can be licensed as an airplane (dual ignition system)
or as a glider (single ignition system). Its prop has 3 positions:
"climb", "cruise" and "feather". Yes, it dies the full feather.
--
Jer/ (Slash) Eberhard, j...@fc.hp.com, Jer_Eb...@fc.hp.com
Hewlett-Packard SWT, 3404 East Harmony Road MS-298, Ft Collins, CO 80525-9599
Phone 303 229-2861, FAX 303 229-3598, 6UR6, Incoming 40 44.1N x 105 33.0W
NOFZD, Civil Air Patrol, Pikes Peak 218, MSN Check Pilot, CFI Airplane & Glider
Mike Ciholas
>Is it not true that using the props for braking purposes throws the counter-
>weights out of position thus causing excessive wear on them?
Lycoming specifically states for the IO-540D4A5 engine in my Comanche
that it is equiped with a dynamic counter weight system and high rpm,
low manifold pressure operation should be avoided. They never specify
what those numbers are, however. In general, I've never let the
engine go below 15 inches at 2700 rpm, 10 inches at 2400 rpm. Is that
good enough or not?
Mike Ciholas
mi...@lcs.mit.edu
jack pines
>by puttung props full forward; it also helps me run the engine at a higher MP
>to aviod shock cooling when I start my descent.
Is it not true that using the props for braking purposes throws the counter-
weights out of position thus causing excessive wear on them?
jack p...@netcom.com
Brian Horn
[deleted]
>Absolutely agreed! In particular, you'd have to do it as almost the
>first thing following the failure, and it would ruin your chances of
>getting a restart if the problem did turn out to be carb icing,
>failing to change fuel tanks, accidentally turning off the magnetos or
>any of the other common pilot error ways of producing an engine
>failure.
Wait a sec. This is a "common" pilot error? I've never heard of
that happening. What's the net concensus? (The other two I'll go along
with.)
Jerry Samsen
>ie. turn the prop blades into the slipstream. The same goes for multi-engine
>planes.
What high performance airplanes (assuming singles given that mult was
called out separate), have full feathering props?
Jerry Samsen
San Diego, CA
Kerry Kurasaki
>
>What high performance airplanes (assuming singles given that mult was
>called out separate), have full feathering props?
>
The Cessna 208 (Caravan) comes to mind. I've seen the blades "feathered"
on the ramp. The TBM-700 is probably in this category along with the
Pilatus.
I can't think of any stock, SE production aircraft, non-kerosene burning
that fit this category however.
Michael Corvin
Several motorgliders have feathering props (eg Grob 109) to reduce drag
during soaring flight. The high performance, self-launching gliders
usually have some form of engine/prop retraction that completely removes
them from the slipstream.
-----------------------------------------------------------------------------
Michael Corvin PP-ASEL, PP-G zw...@starfighter.den.mmc.com
just another spaced rocket scientist at Martin Marietta Astronautics Group
-----------------------------------------------------------------------------
=============== My views, not Martin Marietta's ========================
-----------------------------------------------------------------------------
Gopal Ramachandran
>
>My conclusions are:
>
>1. A windmilling prop does not produce drag equivalent to a disk of the
> same diameter.
>
>2. Attempting to 'stop the prop' to extend the glide ratio is about the
> most stupid thing a pilot can attempt during an engine failure.
Several years ago, I did some similar experiments (at a safe altitude,
over an airport) in a Beech Baron B55. I observed the descent rate
with the aircraft at best glide speed with both engines windmilling,
one engine feathered (and the other windmilling), and both engines
feathered, along with different flap and gear configurations.
With both engines feathered, the airplane's descent rate
decreased from about 1500fpm to 800fpm at the same speed and
configuration. I was impressed with how much drag got done away with
by stopping the props.
Gopal
ATP-ASMEL
--
Gopal Ramachandran <go...@cirrus.com>
UUCP: uunet!cirrus.com!gopal
Cirrus Logic Inc, Fremont CA PH: (510)-226-2138 FAX: (510)-226-2160
Marshal Airborne Perlman
> With both engines feathered, the airplane's descent rate
>decreased from about 1500fpm to 800fpm at the same speed and
>configuration. I was impressed with how much drag got done away with
>by stopping the props.
Plus, it is much quieter... sort of odd to hear how quiet it is for once.
I was up in a M20J, and was doing sort of the same thing you were, trying
to see what the best glide is etc.... and when you pull the prob all the way
(heck...I couldn't even actually feather the thing and it was QUIET) you think
you are in a glider...
Later,
Marshal
P.S. I have a question. Recently I flew in a Bonanza V tail (fun plane!), and
noticed that when you bring in flaps, the plane seems to want to climb a bit,
but in the Mooney M20J, you use full flaps and that thing wants to head for the
ground like a lawn dart.... why is this?
{please reply via email}
cc: gopal
--
|o| Marshal Perlman Internet: per...@cs.fit.edu |o|
|o| Florida Institute of Technology IRC: Squawk |o|
|o| Melbourne, Florida Private Pilot, ASEL |o|
|o| 407/768-8000 x8435 Goodyear Blimp Club Member |o|
Lee Devlin
: With both engines feathered, the airplane's descent rate
: decreased from about 1500fpm to 800fpm at the same speed and
: configuration. I was impressed with how much drag got done away with
: by stopping the props.
Yes, but the point is moot. It's *easy* to stop the props on a plane
with feathering props. Move one lever and the engine pretty much stops
itself. And your drag reduction isn't caused by the props standing
still as much as it is with their much reduced angle of attack with the
relative wind.
In my original posting, I specifically wrote:
>My thinking leads me to believe that a windmilling prop
>should produce no more drag than one that's stationary (no feathering,
>of course). ^^^^^^^^^^^^^^^
^^^^^^^^^^^^
Almost immediately, it started up into a discussion about feathering adjustable
pitch props.
So I thought I'd head everyone off at the pass with to following posting:
>I guess I should mention that I wasn't discussing adjustable pitch
>props. Let's try to keep the discussion limited to fixed pitch props
>for now since you don't have to bother with slowing the plane down and
>flying at MCA to stop the prop if you have full feathering props.
Even after that nearly a dozen more responses came along almost all
dealing with side discussions about *adjustable pitch props*!
Sheesh. I give up. No more flight testing for you guys :-).
Steven H Philipson
>And your drag reduction isn't caused by the props standing
>still as much as it is with their much reduced angle of attack with the
>relative wind.
In _Aerodynamics for Naval Aviators_ (1965 edition) There is a
diagram on page 149 showing propellor drag contribution as functions
of blade angle and whether or not the prop is windmilling. For blade
angles through about 15 degrees, a windmilling prop has up to about
three time the drag as a stationary prop at the same angle. The
difference decreases to parity at about 20 degrees, at which point
the stationary prop has more drag then the windmilling prop. In this
angle range, both conditions have 1/4th to 1/6th the drag of the
winmilling prop at a low to flat (zero) blade angle. At angles
above about 50 degrees, there is essentially no difference between
windmilling and stationary props.
>In my original posting, I specifically wrote:
>
>>My thinking leads me to believe that a windmilling prop
>>should produce no more drag than one that's stationary (no feathering,
>>of course). ^^^^^^^^^^^^^^^
>^^^^^^^^^^^^
Here's a quote from _ANA_, page 148.
The propellor windmilling at high speed in the low range of
blade angles can produce an increase in parasitic drag which
may be as great as the parasite drag of the basic airplane.
An indication of this powerful drag is seen by the helicopter
in auto-rotation. The windmilling rotor is capable of
producing autorotation rates of descent which approach that
of a parachute canopy with the identical disc area loading.
Thus, the propeller windmilling at high speed and small
blade angle can produce an effective drag coefficient of the
disc area which compares with that of a parachute canopy.
Steve
(the certified flying fanatic)
ste...@shell.portal.com
Kevin R. Kirtley (SVER)
>
> Here's a quote from _ANA_, page 148.
>
> The propellor windmilling at high speed in the low range of
> blade angles can produce an increase in parasitic drag which
> may be as great as the parasite drag of the basic airplane.
> An indication of this powerful drag is seen by the helicopter
> in auto-rotation. The windmilling rotor is capable of
> producing autorotation rates of descent which approach that
> of a parachute canopy with the identical disc area loading.
> Thus, the propeller windmilling at high speed and small
> blade angle can produce an effective drag coefficient of the
> disc area which compares with that of a parachute canopy.
>
Is this whole paragraph a quote? Me thinks the autorotation analogy
to a windmilling prop is wrong since the relative wind is coming
from UNDER the rotor in autorotation and so true LIFT is created
whereas for the windmilling prop the relative wind is in the opposite
direction, i.e., from the from the front, producing pure drag.
Kevin Kirtley
Short Wing Piper Lover
Lars-Henrik Eriksson
In article <C7qB6...@unix.portal.com>, ste...@shell.portal.com (Steven H Philipson) writes...
to a windmilling prop is wrong since the relative wind is coming
from UNDER the rotor in autorotation and so true LIFT is created
whereas for the windmilling prop the relative wind is in the opposite
direction, i.e., from the from the front, producing pure drag.
It is the same in both cases: the force generated is in the same
direction as the airflow. In the propeller case, the airflow is
horizontal, so we think of the force as drag, in the rotor case, the
airflow is vertical, so we think of the force as lift. Except for the
way we label the forces, the situation is exactly the same.
William J Levenson
>Me thinks the autorotation analogy
>to a windmilling prop is wrong since the relative wind is coming
>from UNDER the rotor in autorotation and so true LIFT is created
>whereas for the windmilling prop the relative wind is in the opposite
>direction, i.e., from the from the front, producing pure drag.
>
The analogy between a windmilling prop and an autorotating
Helicopter rotor is valid. It really is irrelevant whether
the relative wind is coming from below or in front of the
rotor/prop. The significant thing is the airflow is causing
the rotor to turn which, in reaction to the flow, creates a
force. Whether you call the force lift or drag, the force is
due to the aerodynamics of the rotating airfoils.
In an earlier post, the statement was made that a rotating
prop generates forces roughly equivalent to a disk of equal
diameter (or something to that effect). This is also roughly
true. I'll try and look up the details this weekend, but a rotor
(as for a helicopter) in forward flight behaves like a disk
at an angle of attack. It becomes more apparent if you think
of an autogyro which is powered by a forward facing prop and the
lift is provided by an unpowered rotor at an angle of attack.
The geometry is somewhat different for a helicopter becase the
both the forward thrust and the lift are provided by a powered
rotor but the aerodynamics of the lift is similar.
The windmilling prop is the same thing as the rotor on a autogyro
or an autorotating helicopter except the "angle of attack" of the
windmilling prop is near 90 degrees as opposed to a more reasonable
number (for an airfoil) for an autoratation rotor. By angle of attack
I am refering to the relative angle of the airflow with respect to the
disk of the rotor/prop.
I hesitate to say anymore until I do some research, but I believe the
analogy of a rotating rotor/prop to a solid disk is valid, at least
for relatively high rpm's. ONce the rpm drops, the rotor/disk starts
to behave more like a stopped prop/rotor.
BTW, one thing which helps a helicopter make a soft touchdown after an
autorotation is the kinetic energy stored in the spinning rotor. As the
pilot "flairs" or pulls up the collective, the energy is traded for extra
lift which can actually hover some helicopters. At Bell Helicopter, they
experimented with a high intertia rotor scheme which allows the test
pilot to autorotate to a soft landing, lift-off again and land, etc. for
several cycles, without engine power.
If anyone disagrees with this, let us know...I'll take a look at afew
of my notes this weekend.
Bill Levenson
PP-ASEL-AI
(former Bell engineer- V-22 Osprey)
Steven H Philipson
>>
>> Here's a quote from _ANA_, page 148.
>Is this whole paragraph a quote? [...]
Yes, it was.
Lee Devlin
: It is the same in both cases: the force generated is in the same
: direction as the airflow. In the propeller case, the airflow is
: horizontal, so we think of the force as drag, in the rotor case, the
: airflow is vertical, so we think of the force as lift. Except for the
: way we label the forces, the situation is exactly the same.
Oh contraire. I believe Kevin Kirtley (Ph.D. Aerospace Engineering,
SWPL:-) is correct on this one. A helicopter falls in the *opposite*
direction of the airflow that its rotor normally generates when the
engine fails. THIS IS THE EXACT OPPOSITE OF AN AIRPLANE! I SAID:
*** THIS IS THE EXACT OPPOSITE OF AN AIRPLANE! ****
In order to make the relative wind spin the rotor, it is necessary to
reverse the rotor's pitch. Otherwise, the relative wind would attempt
to spin the rotor in the wrong direction. During an autorotation, the
energy is stored in this rotating airfoil and then recovered at the
precise moment by *reversing the pitch again* to allow a slow decent for
the last few meters before what would otherwise be a very sudden impact.
Without an adjustable collective pitch rotor, a helicopter with a failed
engine would fall just like a rock, and maybe a little faster.
It just goes to show that even 30 year old government textbooks can
contain flawed analogies.
--
Lee Devlin, PE, SWPLII | HP Little Falls Site | phone: (302) 633-8697
Lars-Henrik Eriksson
Lars-Henrik Eriksson (l...@sics.se) wrote:
: It is the same in both cases: the force generated is in the same
: direction as the airflow. In the propeller case, the airflow is
: horizontal, so we think of the force as drag, in the rotor case, the
: airflow is vertical, so we think of the force as lift. Except for the
: way we label the forces, the situation is exactly the same.
Oh contraire. I believe Kevin Kirtley (Ph.D. Aerospace Engineering,
SWPL:-) is correct on this one. A helicopter falls in the *opposite*
direction of the airflow that its rotor normally generates when the
engine fails. THIS IS THE EXACT OPPOSITE OF AN AIRPLANE! I SAID:
*** THIS IS THE EXACT OPPOSITE OF AN AIRPLANE! ****
I noticed. When an aircraft or helicopter is operating under power, it
is indeed the exact opposite. When they are not, i.e. windmilling and
autorotating, it is the same.
In order to make the relative wind spin the rotor, it is necessary to
reverse the rotor's pitch. Otherwise, the relative wind would attempt
to spin the rotor in the wrong direction. During an autorotation, the
energy is stored in this rotating airfoil and then recovered at the
precise moment by *reversing the pitch again* to allow a slow decent for
the last few meters before what would otherwise be a very sudden impact.
Without an adjustable collective pitch rotor, a helicopter with a failed
engine would fall just like a rock, and maybe a little faster.
I don't know if the pitch of the rotor is actually reversed if by this
you mean relative to the horizontal plane - the angle of attack of the
rotor blades is always positive, indeed it would be the same in an
established autorotation as in normal flight. This doesn't matter
much, though. I fully agree that the pitch of the rotor blades is much
lower when autorotating then in normal flight and that the angle of
attack is increased to arrest the descent when the helicopter flares.
However, I don't see how this is relevant in this case. What I
understand we are discussing is the claim that if the aerodynamic
force generated by an autorotating helicopter rotor is of the same
order as a parachute of similar dimensions, then the aerodynamic force
of a windmilling propeller would be, too.
Since we are talking about windmilling propellers and autorotating
helicopter rotors, I don't see how the situation with power applied is
relevant.
It just goes to show that even 30 year old government textbooks can
contain flawed analogies.
No it doesn't. Of course a 30 year old "government book" can be wrong,
as can any book, and, in fact, Ph.Ds. The merit of an analogy depends
on facts and not on credentials.
Lee Devlin
: I noticed. When an aircraft or helicopter is operating under power, it
: is indeed the exact opposite. When they are not, i.e. windmilling and
: autorotating, it is the same.
I still think a prop is different. A windmilling prop is being rotated
by the air which impinges on its *airfoil* surface and spins it in a
direction that makes the air molecules prefer to go from the front to
the back of the plane (the direction of the relative wind).
An autogyro or helicopter in autogyration has its airfoils being rotated
by air that is impinging on the *flat* side of the airfoil. Please note
that the flat side must be pitched into the relative wind to get the
desired direction of spin. [I realize that some helicopter rotors have
symmetric airfoils but lets consider those that have flat bottoms (like
props) for this discussion.] Becuase the air strikes the blade on the
flat side, the airfoil side can be spun in a direction so it can
counteract the motion of relative wind. Thus you can get drag from the
rotor that approaches that of a parachute.
A fixed pitch prop can't do that because the relative wind spins it in
the wrong direction.
Lars-Henrik Eriksson
An autogyro or helicopter in autogyration has its airfoils being rotated
by air that is impinging on the *flat* side of the airfoil. Please note
that the flat side must be pitched into the relative wind to get the
desired direction of spin. [I realize that some helicopter rotors have
symmetric airfoils but lets consider those that have flat bottoms (like
props) for this discussion.] Becuase the air strikes the blade on the
flat side, the airfoil side can be spun in a direction so it can
counteract the motion of relative wind. Thus you can get drag from the
rotor that approaches that of a parachute.
The air does not strike an autorotating helicopter rotor blade from
the flat side. In an autorotating helicopter, the rotor blades are
"flying" just as they are when the helicopter is under power. As I
wrote previously, the angle of attack of the blades of an autorotating
rotor is the same as when the rotor is under power. The air strikes
the *rotor disc* from the "flat side", but since the rotor is rotating
at a fair speed, the individual blades meet the air head-on.
Robert Herndon
annoying statements made.... to pick an authoritative answer once
provided in the past that answers this question:
> From: dr...@athena.mit.edu (Mark Drela)
> Newsgroups: sci.aeronautics,rec.models.rc
> Subject: Re: Prop drag - stopped vs free-wheeling
>
> In article <1992Apr21....@csc.canterbury.ac.nz>,
> ele...@csc.canterbury.ac.nz writes:
|> A bit of a discussion has started in rec.models.rc about whether a propeller
|> has less drag stopped or free-wheeling. This assumes that the prop can't be
|> feathered or folded. In practice, rubber powered free flight models generally|> let the prop free-wheel (large diameter and area props designed for slow
|> rev's), while for electric powered gliders a stopped prop gives the best
|> performance (smaller higher reving prop).
|>
|> Can anyone give an intuitive explaination of what's happening here and which is
|> really best?
|>
|> Regarding the rubber powered models, some of these have prop diameters
|> approaching half the wing span, so stopping the prop would cause some
|> interesting trim problems...
>
> For an unpowered prop, the power needed to drive the prop blades through
> the air is extracted from the airplane via the drag on the prop. This
> holds true whether the prop is stopped or freewheeling. Integrating along
> the blade, we have
>
> / 1 2 2 2
> P = DV = B | - rho (V + w r ) Cd c dr
> / 2
>
>
> P = power needed to drive prop
> D = prop drag
> V = flight speed
> B = # of blades
> rho = air density
> w = prop rotational speed (= 0 if prop stopped)
> r = radial position
> Cd = local profile drag coefficient (small if freewheeling, large if stopped)
> c = local chord
>
> The main tradeoff is between w and Cd. For high pitch props such as on
> rubber models, (wr)^2 is comparable to V^2 inside the integral anyway, so
> getting a low Cd by freewheeling is best. For low pitch props, (wr)^2 would
> be much bigger than V^2 over most of the blade if the prop freewheeled. So
> it is best to set w = 0 by locking the prop and taking the hit in increased
> Cd.
So, there appear to be cases in which less drag is encountered by stopping
the prop, and there are cases where a free-wheeling prop has less drag.
Now I will readily admit that the question is an interesting one. But
if we're going to argue, let's remember some things:
1) A propeller is an airfoil. That its "lift" is called "thrust", or in
the case of a prop not generating thrust "drag", doesn't change the laws
of aerodynamics. It is still an airfoil.
2) As an airfoil, it can stall. This will change its L/D properties
significantly.
3) As a device for transfering rotational energy to linear air motion,
it couples in both directions -- it can transfer energy from engine
to air, and it can transfer energy from air to engine. If the prop
is turning the engine, that energy is coming directly from the energy
of the aircraft, and the aircraft's descent rate will be increased
accordingly.
4) The frontal area of most props is not that large, so their stopped drag
at the relatively low speeds typical of "best glide" is not huge.
If you're really determined to figure things out, you can go and measure.
A couple of thoughts present themselves.
1) Go out and fly your plane with a stopped prop and with a freewheeling
prop.
Advantages:
a) If done properly, this will authoritatively answer the question for
YOUR airplane, engine, and prop combination.
Disadvantages:
a) If you change your prop or engine, it may change the results;
b) The results are probably very dependent on airspeed and engine
drag characteristics which an engine failure would likely alter
(I.e., if your engine seizes, it won't matter anyhow);
c) Your 'best glide' speed may change depending on whether the
prop is stopped or not, and determining this and the new best
speed may take some work;
d) Performing this experiment is moderately dangerous.
2) Guesstimate for your plane. Figure out the approximate prop
rotation speed at best glide; determine how much drag the engine
exerts (make a simple guess, based on torque required to turn the
prop) and do a little arithmetic to figure how much energy the
prop is extracting from the airstream. Add 20% for prop inefficiency.
Compare with the throttle setting required to turn the prop at that
speed, and figure the approximate horsepower from the POH. Compare
these with the energy required to move a flat plate with the frontal
area of the prop at your best glide speed.
Advantages:
a) Gives you an answer without risking your plane and/or neck.
b) If your engine fails and the drag characteristics of the
engine change significantly, you can adjust your guesstimate
easily, since you should be able to guess new parameters.
(i.e., if your engine swallows a valve, and the new RPM is
half what the 'engine out' RPM was due to additional internal
drag, you might be able to make a WAG within spitting distance
of the right answer.)
c) If you change your prop or engine, you can refigure your
results easily.
Disadvantages:
a) It's only a guesstimate.
b) It takes some mathematics.
3) Ignore the whole thing, anyhow. If one has an engine out, one
probably has more important things to worry about than stopping the
prop. Doing so:
a) Will cost a lot of altitude, since slowing to just above stall
for a long time is usually required;
b) May cost significant distance, since (a) will be below best glide;
c) May limit opportunities for restart (due to such things as water
in the fuel, carb ice, ...)
d) Will provide at best a modest increase in glide distance (from
numbers some netters measured a while back in a typical airframe,
no gain for gliding altitudes of less than 5,000' AGL, and marginal
gains above that).
-r
Lee Devlin
: Can we please kill this thread off? There have been some incredibly
: annoying statements made....
As the culprit for *all* of the annoying statements let me just say:
I am hereby admitting that I was wrong, and the people disagreeing with
me are right. Furthermore, I'd like to thank them for having the
patience and persistence to help me better understand the problem.
In trying to figure out how an autogyration works, I began thinking that
if a prop had a symmetrical airfoil, then the helicopter/propellor
analogy would be valid. Then I began to realize that even an
asymmetrical airfoil could be flown 'upside down' making the air
following the flat side take the longer path. This is exactly what's
happening with a windmilling prop. The angle of attack of the blade
goes from positive to negative when the airspeed exceeds the no-slip
speed of the prop. I think it could potentially produce significant
drag if the prop was at all efficient in this upside down configuration.
In the case of my Colt, the engine spins at about 1500 rpm when gliding
at 80 mph. The prop's pitch is 48", and, at this RPM, it wants to move
forward at 68 mph assuming no slip. Thus, the windmilling prop is like
an asymmetric airfoil that is flying upside down. The Colt is renowned
for its spirited power-off sink rate and the drag produced in spinning
vs. stopped prop is not very noticeable because its high drag airframe.
I suppose if you had a very slippery airframe, the windmilling prop
drag effect could be much more dramatic.
I also talked with a helicopter pilot who felt that the stopping the
prop helps reduce drag but not significantly, since he's done it in his
plane too. His Defiant is on the cover of Kitplanes this month flying
with, you guessed it, the front prop stopped. Burt Rutan recommended
flying the Defiant with the front prop stopped should a pilot decide to
operate on only the rear engine. However, it must be flown very slowly
(80 kts) to get the prop to stop and cannot be flown faster than 110 kts
lest the engine begin windmilling again on its own.
So there, I suppose that kills this thread off and makes all those who
knew I was wrong *very* happy :-).
--
Mark Cousins
Take a Piper Arrow. Disable the backup gear extender (if installed). Cool
the engine slowly by reducing power in stages. Bring the power back to idle,
mixture rich, prop full hi rpm (forward). Establish a glide speed of 89 KIAS.
Now, bring the prop control ALL THE WAY BACK. The Arrow has enough prop pitch
control even at idle RPM/oil pressure to increase the blade pitch. You will
feel a rather sudden acceleration, and with the same trim setting your speed
will momentarily increase. When the plane finally settles on its trim speed
again, you will notice a reduced rate of descent and a somewhat flatter
gliding angle.
Push the prop control forward and watch what happens. It will feel like you
just deployed a parachute out the back!
To recover, prop forward. Confirm mixture rich. Bring power back on slowly.
Re-enable backup gear extender. Set power and mixture. Fly on!
You won't hurt the engine by bringing the prop control all the way back at
idle power. BMEP is quite low and not hazardous. Just don't forget to push
it up again before bringing the throttle up.
Mark
--
Mark Cousins Hewlett-Packard Co. m...@hpsemc.cup.hp.com
HP-UX VAB programs 19055 Pruneridge Ave., MS 46TU2
(408) 447-4659 Cupertino, CA 95014 FAX: (408) 447-4364