avoiding undertorque

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Zacke
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avoiding undertorque

Post by Zacke »

Hey guys,

could you give me some advice or numbers to avoid undertorque before I take her to the skies first time? For the Spit Mk1 I remember boost 0/rpm 2600 is safe.

Thx,
Zacke

Zacke
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Re: avoiding undertorque

Post by Zacke »

While on approach somewhere around 150 mph (rpm full foward) throttle quite closed just before the gear warning horn comes on I heard some specific sound from the engine like metal parts hitting in the wind or so (hard to describe). Kind of repeating "clunk".

Could that be a sign of undertorque?

Maybe it's the same sound as here: viewtopic.php?f=82&t=32132&hilit=clunk. But I don't think that's backfiring. I know backfiring from the Texan as a loud bang. And I didn't make any rapid movements of the throttle.

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MkIV Hvd
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Re: avoiding undertorque

Post by MkIV Hvd »

Zacke wrote: 01 May 2020, 07:37 Could that be a sign of undertorque?
That's a term I've never heard. What exactly are you talking about? :?
Rob Wilkinson
A2A: Civilian Mustang, T-6, Bonanza, Comanche, Cub, C182, Spitfire, P-40, Cherokee, P-51 - VATSIM P4 and some other stuff...

Zacke
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Re: avoiding undertorque

Post by Zacke »

Hello Rob,

I'm talking about the situation when the prop drives the engine instead vice versa because of relative high speed and relative low power output.

I think I heard this term first time on the forums in conjunction with the Connie. Or do I mix something up here? :roll:

Edit: Just searched for the term "undertorque" in the forums but didn't get a result. I'm quite confused now. I was sure...

Tomas Linnet
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Re: avoiding undertorque

Post by Tomas Linnet »

I would call that wind milling, but I'm an armchair pilot only.
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Tomas

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Zacke
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Re: avoiding undertorque

Post by Zacke »

Ok, let's call it wind milling until I have sorted that what makes me call it undertorque. :wink:

Do have any ideas if the above mentioned sound could indicate wind milling?

Zacke
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Re: avoiding undertorque

Post by Zacke »

It seems I meant "negative torque" - sorry!

Tomas Linnet
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Re: avoiding undertorque

Post by Tomas Linnet »

well, sorry for being so fussy about this, but there is no such thing as negative torque, just like there is no negative speed ;-) I would still call it win milling :-) You are right about the Connie, I think there is something about using fine pitch on the props, as it will act as a brake if in an emergency decent.
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Tomas

Sim: FSX SE
Accu-Sim aircraft in my hangar:
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ratty
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Re: avoiding undertorque

Post by ratty »

When a piston engine is producing power in, say, level flight, it is pulling the airplane along, with appropriate stresses on the engine components like connecting rods and crankshaft. If you throttle back, like lifting your foot off the gas in a car and using the engine to slow the car, the airflow is now driving the propeller and turning the engine, and a lot of the engine stresses are reversed. This might be bad. I've read a lot on the subject, and I'm not convinced either way.
Going fine in pitch is like shifting down, and increases said "stresses". I suspect that how bad that is for the engine depends on the particular powerplant.
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Zacke
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Re: avoiding undertorque

Post by Zacke »

Hey guys,

just found the noise I mentioned above in a video of another guy: https://www.youtube.com/watch?v=2FuQmWrqm88. At 39:56 when he pulls back the throttle just the second before he touches down there's that specific noise like rattling. Is that the sound of wind milling or "negative torque" (that should be avoided in descents)?

Regards,
Zacke

AviationAtWar
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Re: avoiding undertorque

Post by AviationAtWar »

That's the engine popping when I closed the throttle, that's normal. Check out some videos of real fighters landing and you can hear it in some.


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Scott - A2A
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Re: avoiding undertorque

Post by Scott - A2A »

Accu-Sim models this as "negative thrust." There is a point at which the prop crosses over from providing thrust to providing braking. We model this and can actually develop popping in the sim as a result. This normally happens on short final when sometimes the pilot pulls the throttle all the way back.

Scott.
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ratty
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Re: avoiding undertorque

Post by ratty »

Scott - A2A wrote: 07 May 2020, 05:46 Accu-Sim models this as "negative thrust." There is a point at which the prop crosses over from providing thrust to providing braking. We model this and can actually develop popping in the sim as a result. This normally happens on short final when sometimes the pilot pulls the throttle all the way back.

Scott.
Nice touch. You guys are so great.
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Dogsbody55
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Re: avoiding undertorque

Post by Dogsbody55 »

As I understand the original question, we're talking about the engine condition in which RPM exceeds manifold pressure. IE; running at, say 2400RPM and 15in MAP and the possible engine damage caused by this called running undersquare. This is not a condition that applies to any inline engine, be it a V12 Allison or Merlin, or a Lycoming flat 4 or 6 aero engine.

However, it is a condition to be avoided except on final approach in radial engine aircraft. This is due to the design of the engine in which there is only one big end bearing for that row of cylinders, and the bearing is only lubricated once in each revolution. Running a radial engine undersquare will reduce the life of the big end bearing due to the increased pressure of the propeller driving the engine in this condition where, say, 9 cylinders are exerting a braking pressure on one big end bearing, which is only being lubricated once a revolution. This is the difference to inline engines, which have one big end bearing to one cylinder and one shot of lubrication per revolution, so running undersquare doesn't worry them. Most airlines of the day absolutely forbade running undersquare because of the increased maintenance costs and reliability issues, but of course, it is unavoidable in the landing phase. During descent, or any other low power phase of flight, the engines had to be kept at square (eg 2100rpm, 21in MAP) or slightly over square (eg 2000rpm, 21in MAP) to improve engine life, and typically a plane would slow before beginning descent. There is a well known story of an Air Atlantique pilot flying a DC-6B over the Alps and being directed by ATC to descend to 8000ft and slow to 160kn. His response was that he could either descend or slow down - which does ATC want?? In normal operations with planes such as the Connie, the engine power is reduced by 3in MAP until cylinder head temps stabilise, then reduced a further 3in again waiting for CHT to stabilise, and engine speed is reduced to maintain an oversquare condition. This makes a descent quite an exercise in planning and knowing where your top of descent is in relation to your landing destination.

This makes flying your Connie or B-377 quite a different exercise to flying your Spitfire or Comanche. Hope I haven't confused the issue for you. But if you want to know how to fly a radial engine propeller driven airliner correctly, head on over to Tom Gibson's site Calclassics.com where there is a huge repository of information available.


Cheers,
Mike
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Zacke
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Re: avoiding undertorque

Post by Zacke »

Not confusing at all but very interesting and helpful - thanks a lot!

Zacke

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