Why don't prop planes need to counter the prop like a helicopter?

For some reason I feel a strange compulsion to point out that not all single-engine prop planes have left turning tendencies - some designs with props rotating the opposite direction have right turning tendencies for all the same reasons, just in mirror-image.

Another aspect of aircraft design that helps correct for the left turning tendencies of a single engine prop plane: the engine is often offset to the right. Take the cowling off of a Piper Warrior and look at the engine and you can see it.

I also recall reading something about an Italian WWII fighter plane that had one wing longer than the other to offset some of the engine torque.

It’s not uncommon for twins to have counter rotating props.

Can you translate this? Why is having counter-rotating props a bad thing?

Yeah, some do, some don’t. Preferably they all would have counter rotating props, but that means the engine manufacturers have to produce two versions of their engines which adds cost.

Sure, if you think about it there has to be some method of roll control at hover independent of any engine effects. Just wind gusts and lateral weight unbalance would demand it.

Ummm, right. But the same system that is used for compensating for wind gusts and weight imbalances is ALSO used for the (very minor) problem of torque.

As Berkut points out, the Harrier has a pretty slick engine design. And thanks for letting me know that compressor air is used for the control nozzles. I suspected as such, but wasn’t sure.

It’s a good thing. Most twins that use counter rotating props are designed so that as the engine turns, the descending blade is closest to the fuselage. This is good for controllability if one engine fails.

However, most of the P-38s had counter rotating propellers that turned outwards due to problems with buffeting. Still corrects for P-factor, but not as good if you happen to lose an engine.

Could you explain why this is good for controllability?

And as a side note, the P-38 had in-line, liquid cooled engines which turned at relatively high rpm and required a gear reduction between engine and propellor. This made counter-rotating props a cinch; all that was required was an idler somewhere in the gear train to reverse the prop direction. With radial engines an entirely different engine would have to be built and stocked resulting in added logistic complexity. It’s just simpler to let the pilot worry about the torque since all of them are already trained to do so. And of course on the radial engine planes where torque is really a problem, single engine fighters, counter-rotating props are not an option.

And the reason having the props turn outwards is not so good?

The thrust from a prop is offset towards the side of the prop disc where the blades are going down. If you have a clockwise rotating prop (as viewed from behind or the cockpit), the thrust line is shifted to the right rather than coming straight through the centre.

On a twin engine aircraft with props rotating away from the fuselage, an engine failure on either side will result in an operating engine that produces thrust further out from the aircraft centre-line than if it was rotating the other way. This means that more rudder is required to counter the turning moment produced by the offset thrust.

As control effectiveness reduces with airspeed, more rudder is needed to keep the aircraft flying the slower you go. On the above aircraft, you will run out of rudder control at a higher airspeed than if the prop was rotating the other way. You have less useful airspeed range available to you.

Reference my above post. The closer the thrust line is to the aircraft centre line, the easier it is to control the aircraft. A single engine aircraft has only a slightly offset thrust line (due to the down going blades producing more thrust) and is very controllable. A twin operating on one engine with props rotating inwards has a significanlt offset thrustline and is controllable but care must be taken. A twin on one engine with the prop going outwards has a thrustline further offset and is still controllable but more care must be taken and there is less margin for error.

Sorry, meant to include this:

It can be seen that the third scenario is the worst. On a twin with counter-rotating props turning outwards (P38), an engine failure will always produce scenario 3. An engine failure on a twin with counter rotating props turning inwards will always produce scenario 2 (a good thing). On a twin with props rotating in the same direction as each other, there is a 50/50 chance of scenario 2 or 3.

Certainly.

A single engine propeller aircraft with a “standard” clockwise rotating propeller has a left turning tendency at high angles of attack. One of the reasons for this is that the descending blade of the propeller has a higher angle of attack than the ascending blade, and is producing more thrust. This tries to yaw the airplane to the left, which you can correct for with right rudder.

Now, imagine the same thing on a multi-engine airplane, with a clockwise engine on each wing. You will have the same left turning tendency, which you can correct for with right rudder. But what if one of the engines decides to quit? With the engines mounted on the wing, away from the aircraft’s centerline, the airplane will try to yaw into the failed engine, because all your thrust is on the other side.

Now on your twin with two CW engines, it’s much worse to lose the left engine than the right one. The reason for this is that your right engine’s descending blade is towards the outside of the wing, away from the fuselage. The farther away your thrustline is from the centerline of the aircraft, the worse off you are. It makes your asymmetric thrust situation worse. However, a loss of your left engine won’t be as bad, since the descending blade (with its greater thrust) is closer to the centerline of the fuselage, reducing the tendency to turn into the dead engine.

The obvious solution to this is to make it so both engines have their descending blades closest to the fuselage centerline (counter rotating). That way, no matter which one you lose, you are keeping the thrustline closer to the fuselage. But, on the P-38, the descending blades are farther away from the fuselage centerline. Single engine performance suffers.

Yeah, too bad they put them on the wrong wings. :slight_smile: But, they had their reasons.

Or, what he said. :slight_smile:

Really, every little bit in a light twin helps. When you lose an engine in a twin, you don’t lose 50% of your performance, it’s more like 80%. Counter rotating props (when turning the right way) help out a lot.

Yes. The first airplane I flew for the AF after pilot training was the C-27A. (Bonus: I took that picture!) The props on that thing were NOT counter-rotating: they both rotated left (looking forward). This made a right engine failure more critical than a left engine failure (even more so during a single-engine go-around when the operating engine is at max torque). Guess which engine was always “lost” during checkrides? :cool:

Yeah, while doing my Instrument Rating, their favourite was to fail the left (critical) engine in the Beech Baron midway through the (left) base turn of an NDB approach.

I suppose in theory, and maybe with today’s high performance planes with their greater power and much steeper climbouts than with piston engined aircraft, there is a significant difference in up and down propellor blades.

However, in my copy of the Pilot’s Operating Flight Instructions for the Martin B-26 (which I have open before me right now) I find no mention of any difference in controllability depending upon which engine fails on take-off.

In training for this aircraft and for the equivalent Douglas A-26 there was no mention of this effect either. My guess is that for aircraft of that era, like the P-38, the effect was so small compared to all the others in the case of engine failure on take-off that it faded into insignificance and was not mentioned.

You don’t mention whether the B-26 has contra-rotating engines or not. If it does then it is a moot point as there will be no critical engine.

It has nothing to do with extra performance or anything like that. Any aircraft may have an angle of attack at low speeds as high as 15-20 degrees. That is, the angle between the wing and the relative airflow, not to be confused with the angle between the wing and the ground, climb angle is irrelevant. In cruise flight the angle of attack may be around 4 degrees. this all translates to the engine itself sitting at an angle to the airflow and so the down going blade will have a high angle of attack than the up going blade and will produce more thrust, hence the thrust is offset towards the down going blade.

This is a factor for all prop driven aircraft from 60hp Piper Cubs to a C130 Hercules.

From here:

It sounds like the B-26 was a handful for new pilots to learn how to fly.

What’s the date on that flight manual? The saying in aviation is that every rule is written in blood; my father’s T-38 manual (Dash -1) is a heck of a lot different than the one I got 14 years ago, and I’m sure that mine is different than what the students are getting now. As lessons are learned, warnings are added.

The manual I quoted was for the B-26 F and G models which were the last off the line. However, traning was in B-26A, B, C models.

The A version was the original, 65 ft. wingspan version which was increased to 71 ft. for B and subsequent models.

The early problems in training were the result of several factors. Lack of experienced instructors. Most of them were not long out of cadet school themselves, all the experienced pilots being overseas. Difficulties with the Curtis Electric Propellors which tended to run away. Lack of engine power. The A model had 1850 HP Pratt and Whitney engines which were later changed to 2000 HP engines. In addition the airplane was more critical of weight and balance than the Army was accustomed to; maintenance personel were insufficiently trained. All of this comes from the book The Martin B-26 Marauder by J.K. Havener who was a pilot in the 497th Sqdn., 344th Bomb Group, a B-26 outfit.

Jimmy Doolittle was given the task of straightening out the problems and he selected Capt. Vincent Burnett, who Doolittle knew from barnstorming days, to actually do the job. After Burnett got done the Army knew how to fly, instruct for and maintain the plane and it had the lowest loss record of any WWII bomber.

Nevertheless, there is no mention in the handbook, as of 15 February 1944, of a difference in single engine performance depending upon which engine is out and there was no mention of it training in the airplane.