Winter / Focus / Fuel/Electric.

[Martin X. Moleski, SJ]

--On Wednesday, December 03, 2003 10:16 PM -0600 Adam Glatt <adam.g at sasktel.net> wrote:

> John, experimentation or theory, though I have yet to hear anyone qualify
> the statement that a slow turning prop causes more drag than a stopped one
> with an explanation.

I saw a web site with a theory about angle-of-attack and
the vector of thrust that kind of half made sense to me
<http://www.auf.asn.au/groundschool/propeller.html>.

Several people seem to agree that "the drag caused by a
windmilling prop is the same as that caused by a flat plate
of the same diameter" <http://www.edgrahamcfi.com/Gliding.html>.
Flight tests with airplanes, sailors' experience with propellors
on sail boats, and drag tests through water all seem to confirm
this axiom.  The results seem to have nothing to do with
whether the propellor is driving an engine or is freewheeling.

One fellow said to think about gyrocopters.  All of the lift
comes from a freewheeling prop.  Take it up a few thousand
feet in your imagination.  Then put a brake on the prop and
stop it from spinning.  You will notice that you now have a
lot less lift than you had before.
<http://www.matronics.com/archive/archive-get.cgi?RV-Archive.digest.vol-im>

Helicopters that are lucky enough to have engine failure
at a high enough altitude can land without power.  This is
called "autorotation."  I've done it successfully on flight
simulator perhaps five times out of a thousand tries.  :o(
I believe that the rotor freewheels in autorotation.  Lift
comes from the effects of the air striking the blades and
not from any resistance put up by the engine.

To see what's happening with the airplane, we have to mentally
rotate the gyrocopter or helicopter rotor 90 degrees and
pin them to the nose of the airplane.  In this mental experiment,
we have to be careful to point the underside of the rotor
toward the oncoming air, just as it is when the gyrocoptor
or helicoptor are descending.

If you have attached the rotor in the right orientation, you
will now FEEL the fact that the effect of the air spinning the
blades (rather than vice-versa) induces a thrust vector pointed
toward the nose of the plane, slowing it down.  Stop the
rotation and you will decrease the drag significantly.

I imagine that the angle of attack of the propellor affects
how much drag it causes, but someone who is out of power
on purpose or by accident doesn't have a huge degree of
freedom.  One way or another, the nose is probably going
to be pointing somewhat downward.  In a successful
autorotation, you get to convert energy stored in the
rotating blades into lift by hauling up on the collective
at the right moment.  That might be another way to think
of things--the air is storing energy in the rotating
mass, which robs energy from the rest of the system.
The static propellor only steals a limited amount of
energy through drag.

No matter what theory is used to explain this, it seems to
be a well-established fact:

	Barry Schiff wrote a very good article addressing this subject (AOPA
      Pilot  Jan 95  p71).  In summary, using a Cessna 182 as a test vehicle:
      1. There was essentially no difference in glide ratio with the engine
      2. Glide ratio with the prop at high pitch was 9.2% better than with
          prop at low pitch (prop control full forward).
      3. With the engine shut down, the glide ratio was IMPROVED 20%
          by stopping the prop (compared to prop windmilling at low pitch)

      The glide ratio  numbers he got (all at L / D Max) were:
        Engine Idle, Prop Low pitch windmilling   = 9.28 to 1
        (Engine shut down, low pitch windmilling = essentially the same)
        Engine idle, Prop High Pitch windmilling  =10.12 to 1
        Engine shut down, Prop stopped = 11.12 to 1
<http://www.matronics.com/archive/archive-get.cgi?RV-Archive.digest.vol-im>

"Aerodynamics For
Naval Aviators" page 148:

 "At smaller blade angles near the flat pitch position, the drag
added by the propeller is very large. At these small blade angles,
the propeller windmilling at high RPM can create such a tremendous
amount of drag that the airplane may be uncontrollable. The
propeller windmilling at high speed in the low range of blade angles
can produce an increase in parasite 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 autorotation. 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. The drag and
yawing moment caused by loss of power at high engine-propeller speed
is considerable, and the transient yawing displacement of the
aircraft may produce critical loads for the vertical tail. For this
reason, automatic feathering may be a necessity rather than a
luxury."
<http://pulsar.westmont.edu/aeronca/digest/restart/0035.html>

					Marty
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