Side- and downthrust

[Connexxion]

Hi guys,

I was wondering what you guys do to get a proper side- and downthrust of a ic-engine.

What tools do you use and how do you use them?

And how can one determine the proper amount of both deflections?

Thanks in advance!

[Ollie]

Mount the engine so that the trust line is easily adjusted between test flights. It depends on the configuration; high wing of low wing, flat wing or dihedral, high thrust line or low thrust line. With test flights, you can learn on your own flying style. Don't be chicken. Don't be an appliance minding attitude with your models. Test fly for CG, decalage thrust line and adjust, adjust, adjust over again till your model's flight is almost perfect.

[yoyoML]

I think you can tie the plane down to a freely rotating (yawing) platform, center rudder and give cruise throttle. Adjust side thrust until the plane doesn't yaw anymore. Might be a good starting point! But if you have a throttle->rudder mixing capable Tx, you'd probably want to set zero side thrust and mix in rudder trim with increasing throttle.

I think the down thrust should give the prop shaft zero AoA when in cruise. So maybe find the cruise attitude and set the down thrust accordingly?

Actually, I just do test flights. :D

[JetPlaneFlyer]

I think you can tie the plane down to a freely rotating (yawing) platform, center rudder and give cruise throttle. Adjust side thrust until the plane doesn't yaw anymore. Might be a good starting point! But if you have a throttle->rudder mixing capable Tx, you'd probably want to set zero side thrust and mix in rudder trim with increasing throttle.

I think the down thrust should give the prop shaft zero AoA when in cruise. So maybe find the cruise attitude and set the down thrust accordingly?

Actually, I just do test flights. :D

Putting the model on a 'turntable' would not work because much of the turning tendency that you are trying to trim out with side thrust comes from torque induced roll. This rolling effect would not be simulated on a turntable. Sitting on a turntable also does not fully simulate the airflow of the control surfaces.
Also mixing rudder is not the answer because the effect of rudder or any other control surface deflection is variable according to flying speed, whereas the turn induced by motor torque and prop wash is dependent on throttle setting.
Flight testing is the only practical way to get it spot on but a good starting place would be to find out what thrust offsets are used in a successful model of similar design to yours and copy it ;)

[yoyoML]

Putting the model on a 'turntable' would not work because much of the turning tendency that you are trying to trim out with side thrust comes from torque induced roll. This rolling effect would not be simulated on a turntable. Sitting on a turntable also does not fully simulate the airflow of the control surfaces.
Also mixing rudder is not the answer because the effect of rudder or any other control surface deflection is variable according to flying speed, whereas the turn induced by motor torque and prop wash is dependent on throttle setting.
Flight testing is the only practical way to get it spot on but a good starting place would be to find out what thrust offsets are used in a successful model of similar design to yours and copy it ;)

Nope, not from the torque induced roll. It's from the helical propwash hitting an up-down assymetric vertical stab. If you had as much vertical stab below the thrust line as above, you wouldn't need any side thrust, though you still need to correct for the roll.

That is how the turntable works, since side thrust fights the yaw.

Since (again) "It's from the helical propwash hitting an up-down assymetric vertical stab." the rudder is naturally the thing to trim, and its effect varies according to throttle in addition to airspeed. Even full size planes need to change rudder trim with changes in throttle.

[JetPlaneFlyer]

Nope, not from the torque induced roll. It's from the helical propwash hitting an up-down assymetric vertical stab.

Side thrust actually can compensate for both ;) or at least it can in models with dihedral. ;)

[Ollie]

I think that jojoML, JetPlaneFlyer and Ollie agree that the complex question of thrust line direction is best adjusted by test flying.

[Rodney]

I agree with Ollie, you need to make the final adjustments by test flying. A good starting point for high wing planes and bipes is 5 degrees downthrust and 3 degrees right thrust. The lower the wing is relative to the CG, the less downthrust required, in fact if the thrustline is high and the wing low, you might even need a bit of upthrust.

[rofujiyama]

Hi
I have found that if I can adjust the motor thrust angle, so at least the rudder and elevator remain centered I wont have to worry about flight trim, on those surfaces,later on. Seems, no matter how much I lateral balance or measure the joining of wing panels I will still carry a little aileron trim :(
Once I get a new plane to fly level, I will go through inside loops and trim aileron and rudder to make it track straight, then I will try outside loops if the plane is capable, to see if the lateral balance is ok and , weight the light wing tip with coins, till both, inside and outside loops track true. Once that is done. I will bring it in to see how the trims are set on the control surfaces. Most times i will be carrying some rudder offset, so i will add or delete side thrust angle to get the rudder centered. Then do more looping tests, till its straight. With the rudder centered I then move on to getting the elevator centered.
If you have a removable wing you can shim the wing to get the elevator centered for level flight and if not a removable wing, I will raise or droop both ailerons and or flaps,if you have them, to get level flight at full throttle. With that done , I then move to adjusting the downthrust angle.
If your plane is Knife Edge capable, you can use the thrust angle to get it to track true on knife edge with full rudder travel, by varying the down thrust angle, positive or negative,BUT, keep in mind that if you need to add a fair amount of downthrust , to get it to track true on Knife edge, the added angle may effect the side thrust angle for looping manuevers, so, you may find you can decrease some of the right thrust again , for upright looping manuevers. :eek:
For high wing, highlift planes, I am not to concerned with downthrust angles, I think , with those types of planes , you get to learn throttle management, rather than flight trim setting, If, it is RC, haven't done freeflight or old timer RC. :D
My settings are for full throttle settings and the centered rudder and elevator help when you decrease throttle. It should track straight and have a controllable decent rate, that you can control with elevator input, when its time to land.

Roland

[Connexxion]

Thanks alot for your explanation Roland! :)

[JetPlaneFlyer]

Nope, not from the torque induced roll. It's from the helical propwash hitting an up-down assymetric vertical stab.

Just picking up on this point again... I've just built an unusual tailless pusher rubber power model which i think supports my argument on this issue.

http://i3.photobucket.com/albums/y65/Rapier_HP/TT_05_640.jpg

You will see that the fins are mounted on the wing tips, well out of the prop wash, and they are vertically symetrical. Clearly helical prop wash on an assymetric fin is not an issue on this model... However the model still shows a marked left turn under power...this can only be down to torque. Without side thrust the model actually screws into the ground. Adding right thrust (starboard prop tip forward) reduces this turn, more right thrust actually produces straight flight or even a right turn, again this cant be down to the prop wash on the fins, it can only be due to yaw/roll couple 'dihedral effect'.

Steve

[yoyoML]

Just picking up on this point again... I've just built an unusual tailless pusher rubber power model which i think supports my argument on this issue.


You will see that the fins are mounted on the wing tips, well out of the prop wash, and they are vertically symetrical. Clearly helical prop wash on an assymetric fin is not an issue on this model... However the model still shows a marked left turn under power...this can only be down to torque. Without side thrust the model actually screws into the ground. Adding right thrust (starboard prop tip forward) reduces this turn, more right thrust actually produces straight flight or even a right turn, again this cant be down to the prop wash on the fins, it can only be due to yaw/roll couple 'dihedral effect'.

Steve

First of all, you did not add right thrust. You added left thrust instead.

You're right there's a dihedral effect, but it is different from the conventional helical prop wash that asks for right thrust. The torque rolls the plane left, so the plane turns left and with the trim pitching moment it spirals. Instead of adding aileron trim to roll, countering the torque roll, you add left thrust to yaw right. THEN from the yaw/roll coupling the plane gets right roll to counter it.

In short, where you should roll to counter roll, you instead yaw to create roll to counter roll... not good.

A while ago a 747 (IIRC, that is) had a #4 engine failure while in autopilot cruise. With the assymetric thrust, the plane began to yaw to one side, then roll due to the yaw/roll coupling. The (stupid?) autopilot tried to correct with ailerons instead of rudder. After a while the yaw (which didn't get corrected at all) grew too large and the autopilot ran out of aileron to correct the resultant roll. As soon as the pilot assumed control, not knowing the ailerons were at full deflection, the ailerons snapped back to neutral and the 747 did a spectacular yaw/roll/spiral... No it didn't crash, but it took a lot of altitude before the pilot knew what happened and levelled off.

Anyway, the conventional puller has both problems: helical prop wash that requires right thrust/rudder trim, and torque roll that requires aileron trim. Aileron to counter roll, rudder to counter yaw alright?

[JetPlaneFlyer]

I think we are both thinking the same thing...

Obviously thrust offset produces a yaw which relies on yaw/roll couple to counter prop torque. This is not ideal, however unless you go for twin engines or counter rotating props then I cant think of a better way to trim out torque induced roll, especially in free flight or rudder only model where moving the ailerons to compensate during flight is impossible :rolleyes:

As for if I added 'right' or 'left' thrust, I think you are playing with words... I adjusted the thrust line to produce a right hand yaw... I'd call that 'Right Thrust' ... You can call it 'Left Thrust' if you like :) .. (I did add a note saying 'starboard tip forward' to try to avoid any such hair splitting)

Bottom line is that thrust offset, can be, and is regularly, used to counter torque reaction, albeit indirectly, on this we both now appear to agree... yes?

Steve

[yoyoML]

Bottom line is that thrust offset, can be, and is regularly, used to counter torque reaction, albeit indirectly, on this we both now appear to agree... yes?

Steve

I think regularly, the side thrust deals with yaw, not (indirectly) roll. Simply trimming ailerons is much more efficient, which also doesn't care about main wing AoA, unlike using yaw/roll coupling which does. This is what full size planes do anyway, and some even have differential incidences on main wings for this.