right Thrust?
 

I am having trouble visualizing why right thrust is needed. It seems like left thrust would be the key.

could somebody tell me the theory of why right thrust is needed? I assume this meand right as standing from the rear of the craft.

The motor rotates counter clockwise, which seems to me like it would want to drift to the right(as standing from the rear of the plane) and that right thrust would make it worse. Please tell me where the error is in my logic.

thanks in advance

right Thrust?
 

Right thrust is needed because the prop generates a spiral slipstream which pushes on the left side of the rudder/fuse, pushing the nose left. Right thrust counteracts this. Here's a good writeup: http://www.horizonhobby.com/articles/1101.asp

right Thrust?
 

Standing behind it the motor turns clockwise.

right Thrust?
 

Torque reaction causes the plane to roll into a left bank. The right thrust is to compensate for the torque roll. Torque compensation in full scale is often done with vertical tail offset.

right Thrust?
 

Well I'll be damned. I am surprised to see someone other than myself questioning the merits of "right thrust"!

I have built many planes, and assembled several ARFs, and have never built in "right thrust", and am constantly annoyed at having to make the plywood shim to remove this "right thrust" from my ARFs.

Consider the engine thrust line, and you will realize that even with "right thrust", you still have torque... It's simply no longer along the centerline of the aircraft, but acting upon a different axis unparallel to the aircraft's centerline.

I prefer my models to have a centered engine, and compensate for any torque with control surface trims. I have never had any bad experience with neutral thrust in quite a variety of aircraft.

I don't mean to raise any hackles here, but I think "right and down" engine thrust is unnecessary on most aircraft.

Dave

right Thrust?
 

I believe that both prop wash and reaction torque are attempted to be corrected with engine thrust angles. The problem with using surface trim is that those types of trim are proportional to airspeed not engine torque or thrust. If you use thrust angle to correct prop wash and reaction torque, the correction has the same magnitude as the cause. Not so with surface trim.

Having said all that, I have a very difficult time setting thrust angle. My .40 size Extra needs a lot while my 1.20 size Sukhoi has none. I think it depends on your prop, prop speed, airplane design, how straight your airplane is, and a dozen other things. I think the only way to do proper thrust angle trimming is to use the precision trimming techniques that pattern and IMAC guys use. Unfortunately, the successful use of those techniques is not easy.

right Thrust?
 

There are precision angular shims that are commercially available. They come in
1/2, 1, 2 and, 3 degree angles, if I remember correctly. More than one shim can be stacked if larger angles are required. They greatly simplify the process of adjusting the thrust line. A little 3M75 repositionable contact adhesive will hold them in place while the mounting bolts are being screwed down tight. They are easiest to apply with radial mounts.

right Thrust?
 

In spite of what Damantheus says, put in that right thrust unless you want the heading to change every time you change throttle settings. High wing planes and bipes seem to require more right thrust than others. I have had some midwing and low wing planes that require very little right thrust though.

right Thrust?
 

It's interesting that so few people actually want to consider the physics of the matter of engine thrust direction, perhaps due to never having varied from the "norm" which has plagued us all of these years. Those bipes I have built without a hint of right thrust that have acted perfectly normal and only gain altitude with increased throttle are prime examples. But, I'm new here.

If control surface trim is "supposed" to make a plane lose track when throttle is applied, what do we "think" will happen when more throttle is applied with an off center engine that is pulling the nose to the right?

dntmn asks a great question, which means he's probably wondering about the reasoning behind the situation. I'm sure there are aircraft that benefit in some manner or another from engines that are not on the centerline, but I have yet to come across one.

It's a toss up: I choose to trim my planes differently (when it's even necessary) and keep the engine on the centerline. I am an advocate of straight and true regarding an airplane's construction and wing alignment, etc. etc.

Try building a plane with thrust along the centerline before advocating sticking to the norm. If it doesn't work, it's easy enough to add some right thrust. Just my opinion based on my experiences. That's what this board is for, right? Sharing experiences? Don't discount my opinion and experience just because I'm new here, Rodney.

right Thrust?
 

Lest we forget "P" factor. Generally our planes are flying at a positive angle of attack, take-off, figting gravity, pulling toward the canopy in turns. Under these conditions we often find the downward moving blade (right side) at a higher angle of attack than the upward moving blade (left side). If you are unfamiliar) draw a little vector diagram with aircraft velocity you'll see it. This results in a torque force around the "Z" axis (yaw axis) requiring right thrust or right rudder to offset

right Thrust?
 

I'm sorry if I offended you Damnathius, but I also am speaking from about 50 years of experience in flying models, all the way from rubber powered back in the '30's to 1/4 scale and sport since 1958. My mentors convinced me by study and practical experience how important thrust line adjustments are for RC aircraft. True, if you only want to go straight ahead at a constant speed and no aerobatics, you can dial in rudder trim and get by quite nicely and it is not needed. On all my new planes, I often spend quite a bit of time on the initial flights to tune in the thrust settings so that I get no deviation in attitude with throttle changes as it makes the plane so much more pleasant to fly. I have no objection to building one without thrust settings but, for my own pleasure, I dial in whatever it takes to make that particular plane fly correctly under all throttle settings without my having to fight the trim tabs.

right Thrust?
 

No Rodney, you didn't offend me, but it just felt as if you could have summed up your statement by saying "disregard Damnathius", which irked me more than anything. Thanks for the apology... Accepted. :)

You definitely have more modeling experience than I, but I don't believe it nullifies my own... If something works, it works, regardless of the method. I guess I'm just a perfectionist (thanks Dad) and like things straight and true. A few months ago I was shopping for a plane for my Saito .91, and looked at the Modeltech P-51. Beautiful ARF with a semi built up nose. Then I looked down the nose, and was not impressed with the obvious "not-straightness" of it. I put it back in the box and left it in the store, leaving with a .40 size GP Extra 300s.

Of course, this limits my choices of ARFs to those with cowlings, as cowlings can be adjusted to match a centered engine. But I digress.

I guess the way I look at it is if the real plane doesn't have right thrust, I don't want my model to have it. Funny thing is it really doesn't matter to me on planes that have no fullsize prototype, such as my Rascal Fortys. I let the right thrust remain in both of them, but with that nose it's hard to tell it's even there at a glance.

Now you mention you tune your planes to have no deviation in "attitude" when you change throttle settings... Does this mean that they still climb with increased throttle, but do so in a relatively level manner? Or do you mean they don't change "altitude" with throttle changes?

I've pretty much always been able to trim a plane to fly straight, but never got one to not climb with throttle application... More throttle, more speed, more lift?

right Thrust?
 

I'll try to answer your question with this; yes I trim the plane out at low speed, level flight. Now add throttle; if it climbs I increase the downthrust until it stays level, just gains speed. The same with side thrust, if it changes heading with increased throttle, I adjust thrust line right or left until there is no heading change with increased or decreased throttle.

Ahhhhhhhhh, the P-factor
 

djlyon,
I have never been able to get an explanation why it is, that on one page of the private pilots manual etc., "gyroscopic precession" is mentioned as a factor, but then when it comes to the P-factor, that is all forgotten. If "GP" is taken into account, the force created by the higher angle of attack on the downward moving prop blade should not take effect until 90 degrees later, causing a pitch up effect, not a yaw effect. I was just recommended not to engage in a this discussion with the examiner he he.
I do hoowever agree that right thrust is necessary, on 3D planes more than any others. It is obvious on an upline and when hovering. For a speed plane, no side thrust may be beneficial.
DKjens

right Thrust?
 

DKjens
I'm sure your pilots manual is correct. I'm an old retired aeronautical engineer but I'm never looked at the various comparative contributions of the rotating prop to torsion around the 3 (x,y,z axis) either on full scale or models other than to recognize it's there and must be taken into account. I have designed from scratch one full scale prop driven single engine plane and done all the performance analysis but it never got beyond paper and I didn't consider the torsion effects of the prop other than to make sure my Vmc numbers were probably correct.
Having said that I really don't know the comparative torsion effects of prop wash on the fus, on the wings. "Pitch" factor or GP. My gut (often wrong) tells me that on full size, especially aerobatic or heavy metal (prop war bird) GP would be big because of big heavy props. GP I'm sure is used by full scale aerobats to the advantage for many manuevers i.e. spins. Having GP pitch the nose up when applying left rudder would seem to sure help spin entery and transition to a flat spin. And I would guess that GP is a much greater force to deal with than "Pitch" factor . But I really don't know. On a model I would guess that pitch factor is the greater contribution because of the small mass of the prop. But I really don't know. Many of us are using heavier APC props now and that could be making GP a bigger factor.
I suspect that Ollie may be an old aero guy too and probably much more practiced than I. Maybe he can tell us something here as it applies to models.
Now I'm going to have to take one of my aerobats out and try to see what effect switching between wood and composit props has on trim while maneuvering because before I haven't even given it any thought.

Scale effect?
 

I always build planes with no right thrust in the actual airframe, (firewall offset) and depend on the trimming process to show me what's required. Flying is the only real proof. Also, since different engine sizes and prop combinations might affect the amount of offset thrust required, a straight airframe offers more choices.

I don't think comparing these needs to full-size aircraft is necessarily valid. After all, (especially with aerobatic types) we fly at radically different Reynolds numbers, with aircraft that often are turning props at 5 to 10 times the speed of the full-size aircraft, and moving the aircraft at scale speeds often equal to several hundred miles per hour.

I've found that for true "neutral" response, as in a Pattern plane, right thrust is nearly always required, when the airframe is straight, symmetrical, and balanced. Trying to compensate for this with control trims would not only lead to other problems with aerobatic precision, but it would increase drag as well.

One example:
The Raytheon King Air, which I've documented for a scale model I'm building ( http://www.nextcraft.com/b200_construction02.html ) has a one degree twist built into the vertical stabilizer, producing a "right rudder" effect.

right Thrust?
 

There seems to be some good discussion regarding trimming of the airplane and thrust effects. I suggest that you go to your local hobby shop and purchase the November 2002 issue of FLYING MODELS and read Dean Pappas article starting on page 40. Most of your questions will be answered.
Hope this info will help.
George Frazier


You can reach Dean Pappas @ Flying Models
PO Box 700,
Newton, New Jersy
07860

right Thrust?
 

Thanks for all the replies. Lots of good info here

right Thrust?
 

This is the reason that I have always wanted to build a twin engine plane with counter-rotating props :D

right Thrust?
 

Counter rotating props would work get. I am a professional pilot and have been flying R/C for one year. I would suggest adding right thrust. It will take care of your yaw at all speeds. You can cheat with the rudder but now your plane will be crabbing all the time. Try moving your rudder trim three clicks left. You will have to add aileron trim to offset the rudder trim. Your plane is now out of rig and that is not good. Remember you need up or downthrust adjusments for different spped right? I would have to think it would be the same with right thrust. For you experts can you mount your engine just left of the vertical centerline to add right thrust? I think this would work, but I am not sure. I saw a picture of a U-CAN-DO-3D and his engine most of been 5 degrees to the right. It did not look very good with a cowl.

right Thrust?
 

That's how I do it on cowled planes. To keep the shaft in the center of the cowl at the front or prop plain I offset the engine to the left. So it's the offset to the left and the right thrust that provides the torque around the Z axis (when I'm looking for it)

right Thrust?
 

Where can one purchase angular shims for adjusting large motors with down thrust and right thrust?

Stewart Tittle
Klamath Falls, Oregon

right Thrust?
 

Good question. After my post on the subject, I went looking for shims in the Big Tower catalog and all I could find were in the R/C car section. I assumed they would be of questionable utility. However, I do clearly remember engine mount shims being available some years ago.

Lacking shims, stacks of fender washers can be used with a radial mount.

right Thrust?
 

Here is a more permanent fix better than shims.

1. Use washers to determine the best down/right thrust.
2. measure with an insidence meter etc..
3.remove the motor and mount from the firewall
4. keep the motor bolted to the mount.
5. use a belt sander or grinder to grind down the back of the motor mount to the proper angle.
6. temporarly install and recheck to ensure the thrust is as before, resand/grind as necessary.
7. reinstall the motor and mount.

Hope this helps

Right thrust
 

DNTMN said his prop rotataes counter clockwise. I think he means ccw as if seated in the cockpit. Would this not change the right\left thrust angle In a full size F4U you needed right rudder to hold straight on take off and in the British Seafury, with it`s opposite rotating engine it needed left rudder. It seems, right or left thrust, depends on the engine`s direction of rotation