Can anyone explain thrust angles to me and how they are used and affect a plane and how they can be changed when you have an engine mounted to an engine mount and firewall. Thanks in advance for everyone's help!.

Thrust angles are used to offset things such as engine torque, and the tendency of a plane to climb excessively under power (especially trainers). So, trainers usually have a bit of down thrust built in, and right thrust built in (down to couteract the high lift of the wing, and right to counteract the torque of the engine, which tries to pull the nose of the plane to the left). Thrust angles can be either built into the firewall when built, or can be added by adding washers between the mount and firewall, either on the top two bolts (for down thrust) or the left to bolts (for right thrust).

Thrust angles are used to counter a plane's tendency to climb, dive, or pull to the left due to torque reaction. What you add in terms of up, down, or side thrust will vary from plane to plane. Most kits and ARF's will have the firewall canted for what that plane needs. If you need more due to modifications, you can place washers (different #'s of, or thicknesses) behind the engine mount at the appropriate corner(s) to change the thrust angle. Sometimes the different mfg's take different approaches to correcting flight tendencies.

As an example, I have a Hangar 9 Cub (80 inch wing)ARF and a 1/5 scale Sig Cub (84 inch wing). Both planes are very similar in size and basic flight characteristics. The Sig has downthrust and no side thrust built into the firewall; the H-9 has neither, but the incidence of the tailplane is much more positive than the Sig, which will tend to keep the nose "down" by keeping the tail "up" in flight. The Sig does the same thing, essentially, by having the thrust of the engine pull the nose "down", opposing the tendency of the flat bottom wing to climb under power.

Most planes with flat bottom wings will benefit from a little down thrust. and most all will benefit from a little right thrust.

What 2slow said, I'll add to that.

Down thrust…The lift of any wing, increases with velocity. This should make sense without needing too much explanation. The faster you go, the more lift you make, hence eventually you take off and fly. Get too slow, and the lift decreases, the plane descends. Typically tailed planes have a nose down pitching moment, i.e. they want to tuck and drill a hole in the earth. To prevent this, the stab/elevator apply a down force which lifts the nose. Ideally, one would trim their plane such that the down force on the stab exactly counteracts the downward pitching moment, hence we have level flight. However, this perfect "trimmed" setup only works at one particular speed. Change speed and wing lift, pitching moment and stab down force change, and they don't change in equal proportions. Note: this nonlinear relation between these forces varies greatly across planes, esp wings. Without getting too technical with moments, etc., symmetrical wings are more behaved in this respect than flat bottomed/under-cambered wings. This is one reason why trainers with flat bottomed wings tend to “balloon� with velocity. Say you trim for level flight at 80MPH. Increase to 90MPH and you will climb, decrease to 70MPH and you descend. There is nothing really wrong with this and might actually be preferred in some models. However, it can be annoying, especially in aerobatic models. The idea then is to set engine thrust slightly down to aid the down force on the tail, based on engine thrust. This makes the assumption that more engine thrust equals more speed. There is a loose correlation there, but it should be obvious that it’s not a perfect relation. I.E., thrust on a 45 up line will be higher for speed X than speed X in level flight, etc. Nothing's perfect, but the general idea then is to set down thrust such that the effects of changing speed on elevator trim are minimized across the typical flight envelope. From this, it should be apparent that changing props will effect down thrust, i.e. 10x6 vs 11x5 for example, the 11x5 will produce more thrust than the 10x6 at X velocity.

Right Thrust…Beyond torque, prop wash on a single engine, single prop plane is asymmetrical. On a standard rotating engine, the wash swirls down the fuse, pushing harder on the left side of the rudder and fuse than the right. This causes a left yaw. There is a correlation between thrust and left yaw. By adding right thrust, the engine will counteract the left yaw. Again, nothing is perfect and right thrust can really only be set perfectly for one given speed, engine RPM and prop. The idea is to minimize the left law effect across the flight envelope. Like down thrust, changing prop will change right thrust.

As for changing thrust, placing washers on the mounting bolts between the motor mount and firewall as mentioned in prior posts is a common practice and works well. Some models will have the firewall offset to begin with. Others may use standoffs cut to varied lengths to create the offset.

Another note, you can simulate down thrust and right thrust, just as good if not better, with a computer radio. Simply mix throttle as master to elevator/rudder as slaves such that as you add throttle, the radio will mix in some down elevator and right rudder. It's not as strange as it sounds and I use this method on many of my planes, esp pattern planes, but I also often do this on sport models too.

Cheers

AAARRRRRGGGGHHHHH!!!!!

PEOPLE! PLEASE STOP!

Right thrust cannot, will not, and does not in any way shape or form have the ability to cancel out TORQUE. PERIOD. To do so would be a major violation of the laws of physics. If it did, helicopters wouldn't need tail rotors, we'd just bend the main shaft off in a specific direction.

Right thrust ONLY compensates for P factor. Which is differential thrust across the face of a propeller primarily due to different rotational velocities (the downward moving prop combined with the forward airspeed has a higher airspeed than the upward moving prop)(yes, do the math, that is the main source of the forces involved) this is increased slightly by differing angles of attack on opposite sides of the prop face. This, as stated above causes more thrust down one side of the fuselage. Right thrust moves this differential thrust more closer to the CG thus reducing its effect.

Slipstream swirl around the fuselage is a myth started in !QUOT!Stick and Rudder!QUOT! There are no formulas to resolve this !QUOT!correlation!QUOT! between thrust and offset.

Ironically adding only downthrust reduces the need for right thrust, at least for a sport plane in a climb.


PLEASE stop this torque/ offset thrust myth!

This is taken verbatim from the U.S. Department of Transportation, FAA Flight Training Handbook.

-------------------------------------
Torque and P Factor
To the pilot, "torque" (the left turning tendency of the airplane) is made up of four elements which cause or produce a twisting or rotating motion around at least one of the airplane's three axes. These four elements are:

1. Torque Reaction from Engine and Propeller
2. Corkscrewing Effect of the Slipstream
3. Gyroscopic Action of the Propeller
4. Asymmetric Loading of the Propeller (P Factor
-------------------------------------

Yes, the word torque is not being used in the strict physical description, and I have a large physics background so I know what torque is, and I try to keep an open mind, but until someone actually offers evidence that only #4 on that list is correct and the other 3 don't apply in any way shape or form to yaw, I'm sticking with my myth.

Cheers.

[ John please read my answer in the context of the thread. Steven asked what do thrust angles do for an airplane The first three answers all said they haves something to do with torque.
Within the context of his question I repeat my simple answer and stand by the simple fact that an offset thrust line cannot have any effect on reducing the effects of torque.

Right thrust cannot have an effect on left rolling of an airplane due to items 1 and 3 on your list.
Right thrust cannot have an effect on the slipstream spiral, if you choose to believe it exists, it would simply point it further to the left of the fin, making the slipstream effect more pronounced. so there goes #2. By the way you will only convince me that this myth exists when you show me the set of formulas, in any aviation performance book that mathematically resolve the magnitude of this supposed slipstream for a given horsepower/torque and number of propeller blades. (I've looked at all 38 performance books in the FAA library at Oklahoma City and those equations don't exist. Most of the books quote Stick and Rudder verbatim. Books that predate Stick and Rudder don't even discuss spiraling slipstream.

That leave #4 what causes the majority of left yaw on a single engine plane and what defines the critical engine on a multi engine plane, off set thrustline from P factor.

As for quoting the FAA, I believe the current version of that book has eliminated all reference to spiraling slipstream. But my copy's at work.

And poor Copernicus was excommunicated and is spending an eternity in Hell because he dared to challenge the official position.

Teaching something wrong will never make it right. It will always be a bad explanation for what is really happening. Aero engineering and the associated performance issues are precise sciences. I cannot stand this constant dilution of that science by pseudo science incorrect explanations.

T. Solinski
Aero Engineer
FAA Aviation Safety Inspector
Private Pilot, SEL, MEL.

Geeeeee, you sure are smart[:-].... I wish I were that smart[]... everyone,,, look how smart he is[X(].... and a real live pilot too!!! WOW!!!

Now Now,lay off the engineer.
You know engineers can never be or will be wrong.

Sounds like it's getting pretty heavy to me. As someone mentioned, getting back to the original question, Steven, a plane is a bunch of compromises. You have drag from the wings, fusleage, landing gear, etc. and this doesn't line up with your thrust so the plane in flight tends to not fly straight. This means you may need some changes in thrust and controls to make the plane fly the way you want it.

Most non-giant RC planes run a fairly high revving 2-stroke engine with a short prop. The high rpm and short prop means you won't get much reaction, be it torque, P-factor or spiral prop blast, compared to a giant with a lower turning engine with a long prop. Full scale planes use multi-cylinder engines and much longer props in relation to the plane so they have a great reaction.

You will also find that if you fly at high alpha (angle of attack), really nose up at low speed, a harrier maneuver or a hover, for example, you do get a fair left pull. Most 3D planes use some right thrust to compensate for whatever it is.

To change the thrust line, most people will put a washer or two behind the engine mount to cock it over. With a cowled airplane, you normally offset the engine mount to the opposite side so the crankshaft comes out the center of the cowl.

As for whatever it is, I went to pilot training in 1959 right after graduating from engineering school. My only prop time, except for a few hours here and there, was in a T-34 with a tri gear. I seem to recall we had a torque (that's what my instructor called it) pull to the left on take off and I know we weren't getting much P-factor sitting level on the gear. The guys who flew T-28s had even more torque and they also sat level, so again, it wasn't P-factor. I know the engine rotates one way so the plane wants to rotate the other. I think, and I could be wrong, that if the plane can't rotate, the torque reaction is translated like on a gyroscope into a yaw instead of a roll.

Since I started flying powered model planes in 1950, my model experience is greater than my USAF flying. I recall free flight guys using a fin on the bottom to use the spiral prop blast to counter the torque turn to the left. Maybe they didn't know what they were doing, but it worked. I also flew control line planes with the fin on the bottom for the same reason. The usual CL flying direction is counter clockwise which makes torque (whatever) pull you to the inside ot the circle. The underside fin countered this negating the need for tip weight. Looked terrible, I'll admit.

Sounds to me like I need to do a little experiment on spiral prop blast and torque for my column just to see if it exists.

Thanks everyone for your help, I am sorry to see the little conficts between you guys but hey, if there wasn't anyone to challege "theories" and "facts" we might still think the world is flat... Anyway, I thank you guys for your wisdom and I think I know at least the basics of thrust angles, some of it was kinda going over my head but as I said, I think the basics were presented. Thanks again...

That is an interesting little diatribe Major -- feeling better now?

I'm sure that you don't really care, but I do agree with you regarding the refrences to right-thrust and torque --- but --- I have visually seen the spiral airflow off propeller tips -- it can be quite visible in humid conditions, and I'm not close to being alone in this observation. It has even been photographed.

I have also seen it curling around the fuselage of a Harvard & most assuredly striking the fin & rudder --- on the left side. It was a notable topic of conversation among several of us one morning about 45 yrs ago, as we watched some of our fellow pilots starting up & taking off.

I'm rather a newbee at this, but I'll join the fray just for the fun of it.

It seems like JohnW and MajorTomski can both be right. John notes that "... torque ... is made up of four elements which cause or produce a twisting or rotating motion around at least one of the airplane's three axes". It seems to me that the key here is that the torque can be felt on any of the three axes.

1. Torque Reaction from Engine and Propeller
This would be torque (twisting moment) on the roll axis as the airframe's reaction to the drag of the prop in the air and the force to accelerate the prop, if the RPM is changing. Down thrust or side thrust would not affect this.

2. Corkscrewing Effect of the Slipstream
It seems to me that a turning prop must impart some type of turning (corkscrewing) motion into the propwash to preserve angular momentum. Assuming that the propwash continues back along the thrustline, it will impart torque on the roll axis through its effect on the flying surfaces. However, there may also be torque on the yaw axis because there is a lack of symmetry from the fin/rudder being above the trustline and nothing being below it. It seems to me that the tendency would be to push to the tail to the right, and this would require engine right thrust or right rudder to offset it.

3. Gyroscopic Action of the Propeller
If I remember my physics correctly, this is called Gyroscopic Precession, and would only occur when the plane was yawing or pitching, and it would produce a torque in the other axis (not the prop rotation, roll axis). I would expect this effect to small except, perhaps, in extreme spins (when the pilot would be occupied with other, larger, forces).

4. Asymmetric Loading of the Propeller (P Factor)
Asymmetric loading of the prop occurs when the angle of attack of the prop blades varies through a full rotation of the prop. For this to occur, the prop must be moving foward into the air at an angle that is offset from its axis of rotation. Thus, P-Factor will be most pronounced during takeoff when the tail is on the ground (ground effect issues aside) and during high alpha attitudes. This will produce a tendency for the plane to pull to the left. (This also occurs in boats where the prop shaft is not in line with the boat's motion.)

So, I don't see that there is any torque from the engine rotation that needs to be countered, and therefore MajorTomski is correct that offset will not counter torque on the roll axis. (Aileron trim might be used to counter engine torque on the roll axis.) However, P-factor and asymmetric loading of the tail surfaces (fin and rudder) can induce torque on the yaw axis, and engine thrust offset can used to correct this. So I agree here with JohnW.

But, if you will look very carefully the vortex rings off the prop tips go in the opposite direction that the slipstream theory predicts.

Another strike against the slipstream wrap theory that the classic hits the tail from the left causing more lift on the right theory contradicts all other foms of airfoil/ airflow theories. It spirals the wrong way.

And no I don't feel better.

Well, of course the torque can't be "cancelled out." If it were, then there would be no rotation of the propeller, because that is what causes the torque--rotational forces. However, this isn't a physics class. The guy isn't climbing into a jetliner and hauling hundreds of people across the coutry. Get real, and get a life. The torque of the engine definitely has something to do with the leftward yawing tendency, and placing right thrust into the firewall will help to compensate for this. This is all the explanation that a beginning RC PILOT needs.

"The torque of the engine definitely has something to do with the leftward yawing tendency, and placing right thrust into the firewall will help to compensate for this. This is all the explanation that a beginning RC PILOT needs."

Then let's also tell him that lift is genrated by majic pixie dust that in put in the special balsa wood, styrofoam and coroplast we make our models out of.

If you're going to learn something about aerodynamics, then go through the effort to learn it right.

Back to the first post. Right thrust ONLY compensates for P factor, one of the several yaw/torque related issues that have been discussed on this thread. Right thrust has absolutly no effect on torque. Adding right thrust cannot diminish a roll, and subsequent turn caused by tourque, only the lift off of a wing can do that.

My tirade as some of you view it was kicked off out of a desire for all of us to use the correct terms, that's all. Sorry to cause you all so much grief.

Your Tirade is nothing more than the tirades my physics teachers in college would throw--just to make sure everyone around knows they are smarter than you. I hope you feel better. I bet his plane will fly all the same...

I'm looking back nearly 50 yrs, but I'm pretty sure that I recall seeing the torque rings spiraling from left to right over the top of the left side of the airframe & under the right side of the airframe. That would fit with the slipstream theory.

WRT P-factor -- while your explanation of differing angular velocities is correct, somehow you have failed to note the more important effect from the differing right-left angles of attack, resulting from a positive AOA. This amplifies the effect of assymetric angular velocity.

Getting back to torque effects -- you are looking at this issue solely from the point of aerodynamics & full scale aircraft. In that sense I agree with you, however, full scale situations frequently do not relate directly to model operations. In the case of FS aircraft, the effects of torque are minimal, -- largely because the huge majority of FS planes have limited power output (very limited compared to models) and they usually operate from hard surface runways. Those that operate from grass etc have wheels that are large in relation to surface roughness. Models are strongly affected by surface roughness & they have very powerfull engines. Even a low powered 40-size trainer has a power (or thrust) to weight ratio exceeding that of WWII fighters. They also have considerably lower polar moments of inertia. Engine torque assymetrically loads the landing gear of models to the point where the differential wheel drag alone will cause a model to swing left. In other words torque reaction has a pronounced effect on model tracking during take-off.

I have to leave at present, but I will return later to continue this discussion.

I don't know if my physics teachers were smarter than me. They certainly knew more about physics than I did, and that's why I went to class.

Exactly.................Great point.

Brit brings up a good point. If the effect of the different angular velocities of the upward and downward portion of the prop arc are what create the leftward yaw, and the yaw has nothing to do with torque, then why is the yaw most noticeable at takeoff, when airspeed through the prop is minimal?

Because, that same minimal airspeed will not allow the rudder to be effective enough to counteract the differential thrust. Even though the value is small, there is nothing to counteract it.

Have a look here;

http://www.zap16.com/images/kb04%20B...take%20off.jpg

C-130's are great for this. Note the spiral goes up and over the top of the fuselage (wing in this case) from right to left. If you visualize this spiral wrapping around the fuselage, you can see that it would strike the fin on the right side, causing higher AOA on the left side, resulting in a yaw to the right. The classic slipstream explanation has the spiraling air going the other way and thus left yaw.


If you do the math, and Peter Lert did a great job of this in a Pilot magazine many years ago, the forces due to the angular velocity differences were an order of magnitude higher than those caused by the differing AOA's.

I agree on the ground tracking point, torque loads on the landing gear could cause veering to the left. But this torque is countered by the landing gear pressing against the ground. Once the aircraft is airborne, a right engine off set still can't stop or effect rolling due to torque.

Thank you for making this a discussion!

dmanson's and majortomski's explanations about engine torque having VERY LITTLE to do with the need for down/sidethrust are correct. The reason for side thrust is mainly due to the effect of the spiralling slipstream against the fin. During high-alpha flight (take off and slow speed flight) the "P-effect" may be noticable depending on the propeller pitch and rpm. On full scale aircraft this is usually compensated for by using rudder trim.
Gyroscopic effects are only noticable during yaw and pitch movements. The effect of gyroscopic precession is to "shift" the force 90 degrees in the direction of rotation, e.g. assuming a propeller rotating in the normal clock-wise direction when viewed from behind, a yaw to the left will produce a nose up pitching moment and vice versa.

2slow2matter wrote:
In a way you are right but the effect is the opposite of what you think. The engine torque (which for our model aircraft engine is very small) has to be compensated for by a a very small rolling moment to the right which has to be provided by the wing. Assuming a normal 5 lbs, .40 size aircraft with a 1 bhp engine rotating a prop at 10000 rpm the rolling force needed to counteract the torque is approximately 0.7 N (approx. 0.16 lbf) which is approximately 3% of the total lift generated by the wing in order to keep the aircraft flying straight and level. The differential lift will cause the induced drag on the right part of the wing to be slightly higher than the induced drag on the left part of the wing. This will give the aircraft a slight tendency to yaw to the right which should be compensated for by introducing LEFT thrust or LEFT rudder.

As you are aware, what is usually needed is not left thrust but rather RIGHT thrust and that is because the effect of the spiralling slipstream on the rear part of the fuselage and part of the wing is a much stronger effect than the effect of the induced drag described above.

/Red B.

Now we're getting somewhere -- obviously 50 yr-old memories are unreliable, additionally, I accept correction re. the relative magnitude of P-factor causitive effects.

I'm gratified at your acceptance of on-ground torque effects on models & I also agree with your post-liftoff commentary re. torque reaction. However, I'm not sure exactly what the point of the discussion was wrt torque -- have we been discussing take-off roll perturbations, or post lift-off perturbations?

WRT P-factor -- it is not an in flight issue with model AC, except at high AOA's, which don't often occur with models. Additionally, P-factor is not a perturbing force with trike-gear models (or FS for that matter) during take-off, until rotation. For models, it is a brief transient force that dissapears quickly following take-off, due to their very low wing loadings and high acceleration rates. This essentially means that we are only discussing P-factor wrt tail draggers during initial take-off roll & at the moment of rotation. Is that the sense of the discussion, or am I getting off base?

Since torque-reaction & surface roughness validate the use of right-thrust during take-off roll, this leaves us with right-thrust during in-flight situations. Here is where I have some problems with right-thrust. I don't use it on my own models -- I use rudder to hold track, but it is commonly applied, even to sport aerobatic & 3D types. It is also applied to at-least some modern FS types -- if my memory serves me correctly (there it is again) the Beechcraft T34C uses right thrust. What perturbing force requires in-flight right thrust? Is the modelling industry out-to-lunch? Somehow I don't think so --- which brings us back to spiral slipstream. Since my own memories of it are not quite right, I'm now confused about its effects.

I think some are being a little to hard on MajorTom, the dude has credentials, which I didn't know until his second post. All I wanted was some evidence my beliefs were wrong, and if so, I would correct them, but I needed evidence first that was more credible than what I'm working with. If MajorTom is indeed right, I can understand the frustration in his first post. Where I'm having the problem is personal model flight experience, I am finding it hard to believe that p-factor alone is the only reason for right thrust. I have no proof other than gut feel that something else is going on, and I've alwasy been told until now that there was something else. Is there any way to easily compute the P-factor for a model such that the typically right thrust on a model (say 1.5 to 2.5 degrees) makes sense that p-factor alone is causing the yaw? Cheers.