P-factor? Twin engine critical engine discussion

[polzin]

If a twin engine airplane was to be designed with counter rotating props, would it not be designed to have the descending blade of each engine on the inside next to the fuselage? Due to P-factor the descending blade produces more thrust therefore keeping it near the center of the fuselage would keep center of thrust nearest to the center of the aircraft. Right? Then why does the P-38 have the descending blades on the outside???????

I am now Y-2 compliant.

[GoneWest]

What is a P-38 ??? .... and is it relevant to flying instructors?

[Long Briefing]

I don't know what a P-38 is either but I think the post is relevant to flying instructors. With revalidations now 3 years apart it does no harm to sometimes get the mind turning and keep on top of the theory.

I certainly agree that the aircraft will be more controllable with the downgoing blades on the fuselage side. Why the P-38 designers chose otherwise I have no idea.

[CaptAirProx]

I have a theory, but it may be a load of twaddle....

I think the P-38 is a Lightning. A twin boom supercharged prop thing.

If the blades were to rotate the ideal P- factor way, then the prop wash would be forced into the middle of the aircraft. This would cause both prop washes to fight each other, perhaps causing lots of buffet. This also happens to be the area in between the booms where the air can fight quite freely. Perhaps they decided this casued too much buffet on the tailplane so turned the rotations around. Thus pushing the individual prop washes away from the boom area each side.

When you look at something say like the PA34 Senapod, I wonder if this theory is negated because you have the fuselage in the way of the two fighting propwashes for there to be much buffet.

Any takers??? Sounds plausible but could be utter cobblers.

[Noggin]

You are not far out. The P38 is indeed the Lightning with twin booms and a Rudder directly behind each engine.

In normal flight there will be no asymetric blade effect (P factor) and the slipstram effect on each rudder will balance out. If you lose an engine, the yaw caused by engine loss and the introduction of asymetric blade effect at a high angle of attack will be oposed by the slipstream effect on the rudder behind the live ingine. If the engines rotated the other way, the slipstream effect would add to and increase the yaw.

The title refers to critical engine. There is no critical engine in this case.

[ 04 January 2002: Message edited by: Noggin ]</p>

[CaptAirProx]

Sorry Noggin, but I don't understand.

Say the left engine failed, then with the prop on the right turning clockwise as suggested by the topic starter, then surely the slipstream will actually help yaw the aircraft further towards the failed engine. So why still have props in that manner as opposed to the other 'inward' turning props of other twins as suggested in the start. Have I got it wrong?

[Noggin]

Yes you're right, I drew it incorrectly. Having the rudder in line with the prop must help considerably.

[tacpot]

Stephen Gray's article on the P-38 in Pilot (June 1995), mentions that 'opposite' propeller rotation was tried in an effort to eliminate control problems at high (diving) speeds, due to compressibility.

However it is not clear whether this is 'opposite' in the sense opposite to the normal P-38 rotation as described at the start of the thread, or opposite to the 'normal' rotation on most twins. Regardless of the sense, the change didn't cure the control problems.

But, if they tried opposite sense prop rotation to cure the control problems and found that this had other benefits, even through they didn't cure the control problems, they may have left the engines in this 'odd' configuration.

Stephen's article doesn't give any hints as to the reason for the 'odd' rotational arrangement.

[polzin]

Thanks gentlemen. Many moons ago when I was a flight instructor I tossed out questions just to get students to remember that every airplane is a compromise. I have asked many people over the years and only one told me a story very close to the Capt. Air Prox reply. I have no factual evidence but I was told that originally the engines were mounted with the descending prop blades on the inside. There was buffeting of the tail surfaces. One engineer suggested switching the engines from one side to the other. And that was that.

[polzin]

Thanks gentlemen. Many moons ago when I was a flight instructor I tossed out questions just to get students to remember that every airplane is a compromise. I have asked many people over the years and only one told me a story very close to the Capt. Air Prox reply. I have no factual evidence but I was told that originally the engines were mounted with the descending prop blades on the inside. There was buffeting of the tail surfaces. One engineer suggested switching the engines from one side to the other. And that was that.