Hi,

A nice easy question ...

In movies (namely the 'Aviator', leonardo dicaprio) when aircraft
engines stall, sometimes the propellers stop spinning.
I believe that the wind resistance would continue to turn the prop
even when the engine had stopped ?

I fly some remote control planes, and their props spin when the engine
cuts out, but i guess these little motors are probably nothing like
the real thing.

Anyway, it came down to a bet (for a pint) with a mate :)

Alex.

---

Clarification of Question by quacky-ga on 01 Apr 2005 05:49 PST

Just to clarify, as mentioned in the comments below, i incorrectly
used the term 'stall', i mean an engine cutout due to failure,
possibly mechanical or out of fuel situation :).

Alex.

Quacky ?

The answer is: it depends on the failure, the engine type and the
propeller/governor type.

It may amuse you to know that even experienced pilots argue over this:
I?ve watched the same issue being debated among experienced pilots on
a Mooney mail list (a Mooney is a high performance single-engine
aircraft built in Kerrville, TX).  But it?s an important issue ?
particularly if you ever face engine failure --because there is an
immediate impact.  An aircraft with the propeller stopped will glide
significantly farther than one in which the propeller is turning.  But
I?m getting ahead of the answer here . . .

---

AIRCRAFT PROPULSION
======================

A propeller-driven airplane has an engine, prop governor and
propeller.  Think of it the same way you would a car, with engine,
transmission and wheels.  And, just like a car can be pushed to a
start with the transmission in disengaged (in neutral), the prop
governor determines whether the propeller continues to spin.

The propeller governor in modern ?constant speed? propellers changes
the angle of the blades to change power, while the prop turns at a
constant speed.  The prop governor also determines whether or not the
blade can be ?feathered? ? that is whether or not the blade can be
turned parallel to the wind.  ?Feathering? the prop will stop the
wind?s force on the blade, stopping the prop ? but it?s a feature
usually only used on more-expensive commercial aircraft.

The ?feathering? is virtually always used with turbine aircraft.  But
even with feathering, turbine engines move so easily that you?ll see
propeller slings on some of the larger propeller-driven aircraft. 
These tie-downs keep them from spinning in the wind and creating wear:
Cee Bailey Aircraft Accessories
?Propeller Slngs?
http://www.ceebaileys.com/accessprod.htm

By contrast, in most piston-engine aircraft when the engine is off,
the prop governor links directly to the drive shaft and normal winds
will not move the prop ? which is why you don?t see the propellers
spinning in the wind on a Mooney or most Cessnas or Pipers.  There's
just too much force required to move that drive shaft and pistons for
the wind to be able to turn the prop.

Now let?s examine what happens in flight, as in the example from the
movie about Howard Hughes, ?The Aviator.?  Virtually all prop
governors rely upon engine oil to operate.  When not operating, the
governor will lock the prop in position on most piston-engine aircraft
as it?s directly linked to the drive shaft.

If the oil pump fails or an oil leak drains the oil (and there are
lots of ways these things can happen), the prop governor will fail
within about 5 minutes and the mechanism will seize.  It happened to
my partner in a Mooney 252 and his comment after running off the end
of the runway was, ?I can?t believe how far the airplane would glide
when the prop stopped.?

Below is an excellent diagram of the propeller governor from McCauley,
a major propeller manufacturer.  And an article by John Ruley
describing the operation of the propeller system:

Avweb.com
?Constant Speed Propellers? (Ruley, May 18, 2003)
http://www.avweb.com/news/maint/185020-1.html

Prop governor: McCauley
http://www.avweb.com/newspics/185000_constant-speed_prop.jpg

----

This article on ?Windmilling & Drag? has a Shockwave animation that
helps demonstrate the forces involved that keep a propeller turning:

Selkirk College Aviation Intranet
?Drag from Windmilling Propeller,? (undated)
http://142.26.194.131/aerodynamics1/Multi/Page2.html


And here?s a history and description of propeller designs, done by
John Deakin of  Avweb:
?Those Marvelous Props,? (Deakin, April 19, 1999)
http://www.avweb.com/news/columns/182082-1.html




The Google search strategy must be designed to avoid using the word
?stall,? which has a precise definition in aviation.  And it helps to
find the precise terms used here:
Aircraft propeller ?prop governor?
Aircraft propeller governor diagram
Aircraft propeller ?engine failure?
Aircraft propeller ?feathering?
Aircraft propeller ?variable pitch?
Aircraft propeller windmilling

So, perhaps the best outcome is for each of you to buy a pint for the
other.  But don?t forget the pilots? rule of thumb: ?Eight hours from
bottle to throttle.?

Best regards,

Omnivorous-GA

Okay, nobody wins then :)
Cheers.

I'm just guessing that when a propeller engine stalls, it doesn't spin
even with the wind because the brake of the propeller becomes engaged.

How could it spin with the dead machinery of an engine still
mechanically connected to it?

Well a car facing down a hill in a forward gear can roll with the
engine shutoff. Yeah it might take a little 'push', but a prop would
have the air pushing it.. and lots of it.
Im unconvinced so far :)

Yes, the prop will spin.

A stall has to do with the lift of the wings, see the following:
 DEFINITIONS.  A stall is a loss of lift and increase in drag
      that occurs when an aircraft is flown at an angle of attack
      greater than the angle for maximum lift.  If recovery from a
      stall is not effected in a timely and appropriate manner by
      reducing the angle of attack, a secondary stall and/or spin may
      result.  All spins are preceded by a stall on at least part of
         the wing.  The angle of the relative wind is determined primarily
      by the aircraft's airspeed.  Other factors are considered, such
      as aircraft weight, center of gravity, configuration, and the
      amount of acceleration used in a turn.  The speed at which the
      critical angle of the relative wind is exceeded is the stall
      speed.  Stall speeds are listed in the Airplane Flight Manual
      (AFM) or the Pilot Operating handbook (POH) and pertain to
      certain conditions or aircraft configurations, e.g., landing
      configuration.  Other specific operational speeds are calculated
      based upon the aircraft's stall speed in the landing
      configuration.  Airspeed values specified in the AFM or POH may
      vary under different circumstances.  Factors such as weight,
      center of gravity, altitude, temperature, turbulence, and the
      presence of snow, ice, or frost on the wings will affect an
      aircraft's stall speed.  To thoroughly understand the stall/spin
      phenomenon, some basic factors affecting aircraft aerodynamics
      and flight should be reviewed with particular emphasis on their
      relation to stall speeds.  (This advisory circular is principally
      concerned with and discusses airplanes.  However, much of the
      information also is applicable to gliders.)  The following terms
      are defined as they relate to stalls/spins.

(I would also venture to guess that to 'show' or interpret a stall,
the Aviator shows the prop stopping, for visual effect, rather than
showing a wing that has exceeded its lift....)

The answer is no, The propeller is connected to the engine as people
have pointed out, when the power is cut the propeller would come to a
stop.
Have you ever seen a grounded aircrafts propellers spinning in the wind?

Okay okay, not a 'stall' as such :) but an engine cutout. lol, good point though.

As for "Have you ever seen a grounded aircrafts propellers spinning in
the wind?" thats hardly relevant, the forces exerted on the prop from
a breeze on the ground are greatly different from when the plane is
falling out of the sky at >200mph.

Cant wait to hear the definitive answer on this one now :)

The propellers on powered gliders do not spin whilst the engine is off.