At the field I saw an ARF Trainer with 6+ degrees of right engine thrust. Would the amount of forward thrust be reduced with this setup?

Looking at the problem strictly from a mathematic point of view, the thrust is reduced by the cosine of 6 degrees, which is about .9945. So you lose about 1/2 of one percent of the thrust. By the way, 6 degrees is a tremendouse amount of engine offset. One or two degrees is usually enough.

Thanks. I told him I thought it was a lot. He said that is the way it's supposed to be. Still don't believe that but...

Did it fly ok? Sounds like an awful lot to me too. Must have been swinging a huge lead prop- or maybe the tail feathers were crooked...

It was an Avistar with a 10-6 wood prop on a OS 40. Just seems like it should have had a little more thrust. I don't think there was a large amount of rudder trim used.

This is not unusual for a high wing trainer, they typically require about 5 degrees right thrust and 3 to 5 degrees down thrust to fly well without having to move the trim tabs every time you change throttle settings.

Thanks Rodney, I didn't know that. Never had that much on mine. I'll take a closer look at the palnes at the field and see what the set-up most often used.

Hi guys,

I've searched for a technical explanation as to why right thrust is necessary. I don't dispute it, I just want to know why (can't help it, I'm an engineer. Obviously not aeronautical though....) Can anyone please shed some light? I don't need formulas , just a simple explanation as to why right thrust is necessary, and consequently, when it may not be necessary. Thanks.

Cary

Right thrust is usually needed in propeller-driven aircraft (even full-size) because the rotating prop not only imposes a rearward impetus to the airstream, but a sideways component as well. This sideways component is in the direction of the propeller rotation. Some of the airstream is thus moving aft and towards the right-hand side of the model and will strike the tail. The rightward force on the vertical surface will push the nose left, so you need to counteract it. That's why you need right steering on takeoff.

Once in the air, the effects are much less, because the forward motion of the model reduces the effect (a vector diagram will show this), but some of the left-turning force is still present. To reduce the amount of changes, especially with throttle-up and throttle-down, a thrust angle is built into the engine mounting system.

An examination of a full-size Aircoupe (Ercoupe, and other variant spellings) will show significant thrustline angles. You can also see this on a DC-9 airliner. The engine pods are angled downward towards the rear, giving the same effect as downthrust on an airplane with a propeller in the nose.

Regardless of the actual direction and magnitude of the thrust angles, they are built into an aircraft to help deliver the handling characteristics desired by the designer.

Here are the basics, I think.

Two reasons.

#1
Looking from behind:
The prop spins clockwise so conservation of momentum would suggest the plane wants to spin counter-clockwise. Thus, the left wing tips down and the plane wants to turn left.

#2
The air coming off the propeller is swirling. At the rudder the air flow comes from the left. This give a lift vector at the rudder that pushes the rudder right and also make the plane turn left.

Both of these effects change with throttle setting. So the rudder needs to be adjusted with throttle to make the plane fly straight.

If you trim the surfaces to fly straight at low throttle and then trim the right thrust to fly straight at full throttle, you will have a plane that doesn't change heading much with throttle setting.

If you do search on trimming or trimming charts you can look-up the best ways to check the right and down trust.

Carl

Guys, thank you very much Bill and Carl.

That makes so much sense that I should have thought of it myself.

With regards to the propellor washing over the the rudder, then for every propellor turning clockwise (looking from rear), matched with a vertical stabilizer mounted on top, you'd always have to add some right thrust to the engine. A vertical stab mounted on the bottom then (odd, but just an example) would require left thrust.

I don't quite understand the torque effect with respect to rolling the airplane left and causing it to turn left. It seems like the axial torque would definitely change with throttle setting and want to roll the plane but it would take a lift vector component in the "left" direction for the plane to want to turn left (ie banked left) and I imagine we always subconscously correct for this.

Thanks for the search terms to try and thanks for the replies. I've always been curious about this....

Cary

EDIT: Changed polarity of "left" and "right" engine thrust needed as Bill corrected me below.

No. When you look forward from the rear of the aircraft, some of the airflow strikes the left-hand side of the vertical fin and rudder, imparting a nose-left force. You want RIGHT thrust to counterract it. If the vertical surface was below the fuselage, it would be struck on the right-hand side, imparting a nose-right force. You'd need LEFT thrust in that case.

The actual torque effect is minimal because of the fact that the wings impart a huge damping force with respect to the actual torque reaction of the engine.

Within the past few months, Peter Garrison of Flying Magazine, did an article on this which analyzed the forces that cause the nose-left tendency of an aircraft, and why some aircraft have an offset built into the verical fin and why you need to add right rudder during takeoff and climb in an aircraft with a clockwise-turning propeller (when viewed from the cockpit.

Got it, thanks Bill. I understand it but wrote it wrong (opposite). Thanks for correcting me and pointing out the magazine article.

I'm going to edit my orginal post in case someone else down the line reads this and I confuse them for no reason...

Cary

In the same vein ot side thrust, I have been experimenting with using out thrust on twin engine airplanes to counter yaw in an engine out condition. I am using 8 degrees out of both engines. Cos 8 = .99 so I am still getting 99% of forward thrust, but I am getting 14% side thrust.

The test plane, a Goldberg Tiger 2 converted to a twin with 2 OS .46AX engines weighs 9.5 lbs and has a 32 oz/sq ft wing loading, pretty heavy. I have taken off on 1 engine and alternately fueled each tank 1/3rd full and flown at full power, doing maneuvers until the engine on the low tank quit. The plane handles well on 1 engine, turns either direction, rolls and loops, although going over the top is slow and it does yaw there.

As a second test, I pulled the OS engines and replaced them with a Thunder Tiger .46Pro with a Tower muffler on the left and a Thunder Tiger .42GP on the right. There's an unsynched pair for you. Still flies well. Can't seem to tell much difference in handling, although with the 2 OS AXs, it had more power. The OSs are in another twin and this one is still flying with the mismatched pair. I am so convinced about out thrust that all my new twins get out thrust.

8 degrees looks huge, but as an engineer, I go with what works.

I used Ernst thrust plates bought at Tower for the side thrust. They come in a package of 3 deg, 2 deg, & 1 deg for $2.95 or so. Takes me 3 sets.

Cary/Bax,

I am in the same boat with Cary, in that I have a good physics/engineering background but the aerodynamics stuff is new to me.

I tend to agree with Bax that when you are accelerating (the take off run or pulling a vertical) the swirling air from the prop is the main effect. And this is what is being corrected for in the right thrust. The trimming charts I have seen adjust the right thrust to pull a straight vertical.


The torque effect (I think -- Bax, please correct me if I'm going astray) is seen more as changes in trim at cruise speed. At cruise the prop should not be swirling the air behind it so much as it corkscrews through the air near pitch-speed (pitch*rpm). If you trim the plane to do a straight level hands-off pass at half throttle, then make a hands-off pass at full throttle the plane will veer left. I've been told this is due to the torque effect. My guess is the roll rate from the change in torque is only a few degrees per second, so in a 3-6 second pass it looks like a gently banked turn to the left. After 600 yards or so the pilot has to turn the plane around.

Computer radios let you mix some rudder into the throttle. Is this usually used for correcting cruise trim or for making take-offs easier? Seems that if you used it for take-off (acceleration effects) it would mess-up the cruise trim when changing throttle settings.

Carl