The P-Factor Factor

[swampyankee]

Sounds like engineer talk for: "Any amount of sideslip with a windmilling prop produces a tendency to the yaw-roll syndrome Frank G mentioned in an earlier post."
So here's an engineer question: Does that mean a twin engine pusher type a/c wouldn't be subject to yaw-roll syndrome?
Cheers,
Wes

I'm a former aeronautical engineer; I'm in recovery but I still backslide.

Do you mean roll-yaw coupling of the sort that caused quite a few problems with the F-100 Super Saber and some other jets of that era?

 

[XBe02Drvr]

I'm a former aeronautical engineer; I'm in recovery but I still backslide.

Do you mean roll-yaw coupling of the sort that caused quite a few problems with the F-100 Super Saber and some other jets of that era?

Well the first step in recovery is acknowledging the disease. Have you checked out your local chapter of EA?
I've never understood why sweptwings were considered more susceptible to roll-yaw coupling than straights. Is it due to the angle of the leading/trailing edges to the relative wind, or the relatively short overall span to the aspect ratio/wing area? I've noticed that an AA-1 Yankee with its stubby wings seems to have more yaw-roll reaction than a Schweitzer 2-32.
Actually, my comment was in reference to a post Frank G made earlier in the thread regarding a windmilling prop on a twin engine plane at speeds at or below VMC.
Cheers,
Wes

PS: Frank G and Swampyankee, look up an older thread called "Air Florida Flight 90, remember this?" Reply #7.

 

[GregP]

As they swept the wings and as wings got smaller, the percentage of mass concentration in the fuselage grew. The logical extreme was the F-104, but the original small-fin F-100 also had a LOT of fuselage-concentrated mass versus the fin and rudder area. Almost all the F-104 mass was in the fuselage. When it started to yaw, the small fin-rudder was overpowered and the small wings couldn't stop it either.

Roll-yaw coupling was due in part to adverse yaw when ailerons are applied in almost any conventional aircraft, coupled with the inability of the smallish fin and rudder to overcome the adverse yaw created. At high enough speeds, when adverse yaw happened, the fin snapped off, eliminating any recovery possibility. That was a definite possibility in the B-58 Hustler at high speeds if the engines suffered a cone-related thrust failure (unstart). When that happened at supersonic speeds, and all the cones were operated by the same sensor, the plane tuned sideways (since the power loss was never symmetric) enough to snap off the fin, and the rest disintegrated in-flight.

In propeller planes, when prop diameter or blade area was added or side area aft of the center of lift was lost, it had to be compensated for with a larger fin and rudder, or yaw stability was lost. Enter the dorsal fin extension for the P-47 / P-51 when the bubble canopy was introduced. Both lost turtledeck side area and needed more fin. The P-51 also got an additional blade and more prop area when it went to a Merlin. The P-47 got more prop area when it went to the paddle-bladed prop. Notice all the Griffon Spitfires have a large fin and rudder, too after they got a power bump and another blade versus the Merlin Spits. The AR-240 never DID get enough fin and ruder area to be stable, and neither did the Me 210.

 

[MIflyer]

The fin extension added to the P-51B and C primarily was to tie the horizontal stabilizers more robustly to the fuselage and prevent some the failures that had been occurring in that area. I guess adding some more fin area didn't hurt stability, either.

 

[XBe02Drvr]

when prop diameter or blade area was added or side area aft of the center of lift was lost, it had to be compensated for

Avia S-199??

 

[GregP]

Hi Miflyer,

Not quite. You might want to check that. The loss of turtledeck area coupled with the addition of power and a 4th blade caused loss of stability, particularly yaw, and it needed the dorsal ... and that didn't quite fix it, but fighters are not supposed to be strongly stable anyway. But you didn't see P-51Ds for very long without the dorsal fin extension.

 

[J.A.W.]

Frank G's poignant description of being 'hands-on' to a fatal crash of a Tempest really got me.

Having been in a similar situation with car wrecks, I always keep a knife in my own, just in case..

Anyhow, I'd thought that no Tempest had flown for ~60 years, & only one, ever in North America
( a Tempest F.6 sent to the RCAF for cold weather trials) so I did a bit of checking.

The Fury/Sea Fury does however, bear a fairly close family resemblance to the Tempest F.2,
& is much more common, (with many recovered & brought 'Stateside' - from Iraq, back in the `70s)
yeah so , Frank's 'Tempest' - turns out to be a Sea Fury, an ex-German two-seat T.20 trainer..

Kermit Weeks does have two Tempests,(both very rare), a Sabre-powered Mk V, & a prototype Mk II,
Centaurus radial-powered, like the Sea Fury, & has them both under resto, presently.

 

[MIflyer]

Stunt pilot Charlie Hillard was killed in a Sea Fury at Sun and Fun several years ago. He had a Sea Fury modified with an R-3350 engine and with F-80 brakes. He finished his acro routine and after landing the airplane went over on its back. He had removed the armor plate turnover protection and the airplane forced his head down against the ground so tightly that he could not breathe.

The combination of the better brakes, different engine, and reduction of aft weight led to the airplane flipping over.

 

[IdahoRenegade]

Sit on a BMW flat twin motorcycle, rev engine stopped at stoplight. Feel bike try to lean to one side. Torque showing effect. Engine trying to turn one way while turning the flywheel the other way, even if flywheel isn't connected to anything.

Motorcycle engines with crankshafts parallel to axles don't show this anywhere near as well due to longer moment arms of resistance. trying to lift and depress the front/rear suspensions.

Try it on a Triumph Rocket III, very noticeable. Can't imagine the impact with a Boss Hoss with a V-8.