Why down and right thrust?

[KenP]

I am converting a 1/8th scale gas plane to electric and I noticed the ply gas motor mount has no down or right angles. I always put down an right on all my electrics, I do it because I have always been told to do it. So what I am looking for is the why do you put down and right thrust angles?

[gnofliwr]

Down and right thrust help compensate for trim changes due to throttle changes and it is more common on trainers and sport models - most pattern planes use no offsets.

Right thrust compensates for P-effect which wants to yaw the plane to the left. When the plane flys an angle of attack, the downward moving propellor blade strikes the air at a higher angle of attack than the upward moving blade. For a clockwise (from the plane) rotating propellor, this cause slightly more thrust from the right side than from the left. More throttle, more off-center thrust. Pointing the motor a couple of degrees to the right compensates.

Down thrust is compensates for the tendency of a plane to nose up when throttle is applied. The wing is flying at a greater angle of attack than the stab so when you throttle-up, the wing will generate relatively more lift due to the speed increase than the stab, so the nose will pitch up.

If you try to fly the plane inverted, the right/down thrust becomes left/up thrust which is why you have to hold down elevator and left rudder to fly straight. Pattern planes are often built with 0,0,0 offsets. That is, the motor, wing and stab are all at 0 degrees to the datum line and there is no right thrust. This way the plane flies the same right side up or upside down. (Now if the computer radio would only figure out that you meant to push instead of pull after that low inverted pass, we really have something ;) )

-Roger

p.s. I just realized, I gave the wrong explaination for needing down elevator when flying inverted. The down thrust actually helps here, but the upright positive angle of attack on the wing becomes a negative angle when inverted. Also, flat bottomed and semi symetrical wings have to fly at a higher angle of attack when inverted to generate the same lift.

- Roger

[KenP]

Ok that all makes sense.
Thanks

[schembfr]

How would I go about finding the correct R and downward angle to mount an outrunner on a converted cub?

[KenP]

I seem to always use 3 deg down and 3 deg right, that seems to always work for me. But you did bring up the other thing I would like to understand why do some gas planes not seem to have right and down angles.

[Aio_1]

schembfr,

It comes down to testing. There is no accurate analytical way to choose these angles.

I don't agree totally with Roger's explanation:
The pitch change when you alter throttle setting is primarily due to the pitching moment of the airfoil which isn't quite the same as the lift. For most aerofoils the Cm is pretty much constant for a wide range of angle of attack. So when you speed up the effect of Cm increases and the force applied by the tailplane must be altered to maintain trimmed flight. In addition to Rogers explanation the asymetric lateral/directional effects of the motor are impacted by the shape of the fuselage. The prop causes some swirl in the airflow which can alter the pressures on either side of the fuselage causing a yawing and/or rolling moment.

If a plane doesn't have any thrust line offset built in it is probably because the plane is not well designed or trimmed. I don't think most of us don't properly trim our planes. I know I don't. I put in some effort but I rarely keep going once I get reasonably close. My flying isn't good enough to really notice small trim errors. Competition precision aerobatic pilots on the other hand put a lot of time into trimming and would probably be horrified at the state of most of our planes!

In general there's no real difference between electric and IC planes aerodynamically. The only big difference I can see that would apply to some types of plane is that electrics often use bigger props which would increase the distances and areas over which these asymetric forces act and thereby increase their moments.

Aidan

[lrsudog]

Even a prefectly semetrical airframe can need right & down thrust to eliminate the unwanted effects of power changes. The only practical way for most of us to determine thrust angle changes is by trial and error. Generally, for high wing planes I start with 2 degrees right & 3 degrees down. Low wing or mid wing planes I start with 1 down & 2 right.

[Aio_1]

I left out the thrustline position in my explanation above. If the thrustline does not go directly through the center of mass the thrust will cause a pitch change regardless of aerofoil shape.
A perfectly symetrical airframe will not still be symetrical when trimmed for stable flight. Unless the CG is on the neutral point (which would make for a pretty uncontrollable airplane) the elevator has to be deflected to maintain a positive angle of attack for level flight. This makes the plane non-symetrical.

Even for full size planes I believe prop effects can't be fully predicted. Some of the asymetric properties can't be trimmed out at all. For example I did a test flight in a Jetstream 31 flying lab last week where we set up the plane in bank then released the controls and left it to it's own devices to check the spiral stability. That design is stable in the spiral mode to the starboard but unstable to port.

Aidan

[Virgil Kee]

For what it's worth, the thrust settings on the Graupner Mini Piper are 5º down and 1.5º right. This is consistant with the discussion above, in that it reflects the fact that the (very high-lift) wing is mounted far above the thrustline, thus requiring a lot of down thrust to compensate. To readily grasp the necessity of downthrust in this case, using armchair, non-scientific "logic", imagine an exagerated example of a thrustline that is far BELOW the wing: suppose the motor were slung UNDER the fuselage, maybe a couple of inches! Power it up in your mind and notice what happens to the vertical stability! The plane would try to LOOP!!!!! You'd need a HUGE amount of down thrust, and quite possibly you'd be picking up pieces long before you could trim it out!

Virg.

PS - Anyone want to know how to type the DEGREE symbol as used above? EASY: Hold down the ALT key and key in 0186 on the num pad. Cool, huh? :D

[Salto]

0186

Damm, doesn't work on my Mac!

[kallend]

If a plane doesn't have any thrust line offset built in it is probably because the plane is not well designed or trimmed.

Very strongly disagree with this statement. It all depends on the pitching moment coefficients and other design parameters. A well designed plane can need no offset.

[vintage1]

A totally symmetrical plane does not need offest and will fly equally well upsode down as right way up.

The moment you add a littel built in pitch stability though, yiu need a little downthrust to compensate. That is if you want whacking the throttle wide open to result in just more speed, not increased climb rate..

[Aio_1]

Very strongly disagree with this statement. It all depends on the pitching moment coefficients and other design parameters. A well designed plane can need no offset.

For stable flight the CG is forward of the planes neutral point. The pitching moment caused by this is balanced aerodynamically by all the other pitching moments present. This includes that due to the aerofoils, the thrustline, and the tailplane aerofoil/incidence/trim.
The pitching moment due to the CG position varies depending on the aircrafts attitude. It is maximum for level flight but at any other attitude the CG can contribute to rolling moment and/or yawing moment as well and the pitchig component varies. The requirments for the balancing forces vary depending on aircraft attitude. In level flight these forces will need to stay constant regardless of airspeed. Change in airspeed affects the magnitude of the aerodynamic forces and so adjustments need to be made. Since airspeed in level flight is determined by the thrust it is possible to make powered planes autotrim themselves by using offset or angled thrust-lines. Thus as the speed varies the thrust component is used to keep the sum of the moments in equilibrium. If you want to use throttle to control climb rate rather than or as well as airspeed then you won't necessarily want all the forces in perfect equilibrium.

Aidan

[Virgil Kee]

Salto

0186

Damm, doesn't work on my Mac!

Sorry, Mate! My advice (in Post #9 above) on typing the degree symbol only works with the Windows OS. Mac users should try this site:

http://ml.hss.cmu.edu/mlrc/links/keyboardhelp.html

The degree symbol is shown as the first item under the FRENCH or ITALIAN character listings.

Virg.

[Salto]

Like this Í

Or this Í

Que? I don't think my Mac speaks Spanish after all.

Graham.

[kallend]

For stable flight the CG is forward of the planes neutral point. The pitching moment caused by this is balanced aerodynamically by all the other pitching moments present. This includes that due to the aerofoils, the thrustline, and the tailplane aerofoil/incidence/trim.

Fine, but what if you don't want stability? What if you want to optimize 3d hovering? Or same flight characteristics upright/inverted. Zero-zero thrustline does not imply bad design.

[Aio_1]

When I say stable flight I do not mean a setup for beginners. I mean that the result of control inputs in pitch is convergent. If you move the CG further aft than the horizontal surface neutral point you will get a divergent response in the pitch axis. I'm pretty sure that the reaction speed required to control such a setup is beyond any RC pilot.

Aidan

[vintage1]

Oh. I ALMOST managed it till there was a crunching noise :D

[Aio_1]

I should also have said that even if the pilot could manage it they'll want damn fast servos. :)