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May 17, 2002, 09:50 PM
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Motor thrust angle question

What's the technical reason that some plane's motors must have up/down and left/right thrust angles?

And how do you know if you need it and how much?

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May 18, 2002, 04:00 PM
Will work for planes
omega blood's Avatar
I think it to counter the toque of the spinning prop. It helps for a straiter rog. When you need it I don't Know I've asked the same question my self. Gneraly for most planes it's down 2-3 degrees down and 2-3 right( tractor). hope that helps.
May 20, 2002, 03:24 AM
Registered User
May 20, 2002, 02:36 PM
pushing the extremes
Zoom's Avatar
I will assume that you have the most common type of motor/prop configuration (at the front with the prop spinning counter clockwise when looking at the prop from in front.

Ok let's start with side thrust. The spinning prop creates a helical prop wash. This is swirling air mass which looks like a little tornado extending back towards the tail of your plane. The center of the tail is roughly in the eye of this tornado, but you'll notice the vertical stabilizer shoots upward through this tornado and the propwash ends up hitting the fin on the left side (looking forward). So when you hit the gas, the fin feels this air hitting it in the side and yaws the plane to the left. To correct this we use right thrust. This effectively yaws the plane to the right whenever the you throttle up and offsets the yaw produced by the helical prop wash. Note that the horizontal stab also feels this helical propwash, but since it feels a little force up on the left and down on the right, the net pitching moment is zero. There is a small roll torque produced which wants to make the plane roll right, but this is a good thing as we'll see in a moment.

The other reason we have a little right thrust built in is to offset the motor torque. If you have ever held a plane up under full power (or even more noticeable is just the motor spinning the prop), you will have surely felt the torque. Since the motor is torquing the prop clockwise when looking from the back towards the front, there is an equal and opposite torque which the air puts on the prop and motor. This makes the plane want to roll left under power. We don't have any real way of compensating for this roll torque that is proportional to throttle setting. Mixing some aileron in with a computer radio seems like a good idea, but turns out to be a less than perfect solution because the amount of roll torque the ailerons produce is dramatically effected by airspeed. We use the right thrust to compensate a little for this effect because even though it doesn't really fix the roll torque, we recognize that the left roll tendency will want to make the plane turn more left under power than not. Yawing to the right as a function of power is a less than optimal, but a "better than nothing" solution for general purpose flying. Note that the 3D guys have a special maneuver that exploits this motor torque. It's called a torque roll.

Ok - down thrust.
Sometimes the motor thrust line is placed either above or below the center of gravity of the plane. This is sometimes done for scale reasons or practical considerations like ground clearance. Whatever the reason, if the thrust line were placed below the CG (easy to do on a high wing plane), then a nose up pitching moment will be created which will get larger as the power in increased. To offset this one could angle the motor down to compensate for this effect. Note that positioning the motor higher will also correct this problem, but may not be possible. This is only part of the story though. Lets look at a low wing plane. Here the vertical CG is pretty low and it's easy to put the thrustline higher than the CG. Why is downthrust sometimes still required? This is because as the wind over the wing increases (both from the direct prop blast as well as the resulting increase in airspeed) the wing generates more lift and the plane begins moving upwards (even though it's horizontal attitude is still the same). This upward motion forces air down on the horizontal stab and creates a nose up pitching moment. This moment is loosely related to power setting, but also has a part that is dependent more on airspeed. We compensate for the part, which is power dependent with downthrust, but the other is left uncompensated. This is why even a well trimmed plane will generally still climb when going fast and descend when slowed down, it just won't pitch dramatically when power setting are abruptly changed.

As to how much down or side thrust to use? That's a tough question. Try changing them and see the effect. I usually start with 2-3 deg right thrust and I try not to use any downthrust if possible, I try to position the thrustline up or down instead. Do you know why?

- Zoom -
May 20, 2002, 04:32 PM
Registered User
Zoom & others:

This was exactly what I was looking for. Many thanks to all.

I'm an engineer too and like to know why something has to be done, not just that it has to be done.
May 22, 2002, 08:44 PM
Registered User
actually, up-thrust is a lot more
May 25, 2002, 02:04 PM
Balsa Builder
Paul Susbauer's Avatar
That was an excellent explanation of Righ/down thrust. I understand it now more than I did before.

May 25, 2002, 04:13 PM
dusty bible = dirty life
Majortomski's Avatar
And now for the lone voice in the wilderness answer. I was hoping someone would answer this like Zoom did. He did a great job, unfortunately Zoom has quoted the wrong answers that have been preached as gospel for decades.

First letís get torque out of the discussion. By the laws of physics down thrust or side thrust cannot have any effect on the torque caused by out engines and motors. Unless weíre talking a 2000 Hp engine turning a half-ton propeller then our torque is extremely minor.

Next the mythical slip stream swirl (helical propwash). It's true that on a high humidity morning you can see this spiral around the airplane from the vortex off of the propeller tips. But if you look at it very closely the swirl goes the opposite direction that is given as the explanation for left yaw. So if it did have an effect it should be the opposite of what we are correcting for. Secondly, If the swirl does exist as Zoom reiterated then the same spiral is causing an ENORMOUS ROLLING force to the right by causing more lift on the left wing and horizontal stabilizer and less lift on the right wing and horizontal stabilizer. Ironically in every diagram of the slipstream swirl these forces are always completely ignored, because the vertical fin in the slipstream is the only point the author is usually trying to make. And by looking at the other forces I mention would really mess up this little old erroneous theory. Next watch any aircraft with a smoke system, and at very low airspeeds, and very high power such as a vertical climb or at the verge of the stall, so long as the airplane is still flying the smoke blows straight back under the airplane it never is seen swirling around the fuselage as the theory predicts. And finally we have been analyzing the physics and mathematics of aviation longer than we have been actually flying. In my 25 years as an aero engineer I have never found (and Iíve searched some of the biggest libraries in the country) an aero design book that quantifies the helical propwash. Surely by now some desperate for a topic doctoral candidate has developed a set of equations that for a given horsepower, propeller pitch, flying speed, fuselage length, and fin size will tell you exactly how much to offset the fin for this swirl. These equations do not exist.

So what is the answer? P-factor. P-factor is a difference in thrust on the left and right side of an airplane caused by differences in relative airspeed and angle of attack of the propeller as it moves up on the left side of the airplane and down on the right side of the airplane. In most conventional aircraft this thrust is greater on the right side of the airplane, pulling the airplane to the left. To stop this left turning tendency you can either add left rudder, or aim this lopsided thrust through the CG to minimize this turning force. Thatís what right thrust does it compensates for P factor not torque.

Down thrust is a way of reducing the need to retrim an airplane with power changes. As you increase airspeed by adding power the stabilizer will pull downward with a stronger force, and an airplane will climb. You can either add more down elevator to reduce the climb or you can let the extra power pull the nose down. There is an oft-overlooked benefit of downthrust. When added to an airplane it also lowers the angle of attack that causes P-factor, thus reducing the P-factor, and often eliminating the need for right thrust because the P-factor has been neutralized in the cruise condition.
May 25, 2002, 08:34 PM
Registered User

Thanks for the great explanation of the need for vectored engine thrust. It makes sense except for one question that I have.

Why is there a difference in relative air speed and angle of attack of the propeller blade going up on one side and down on the other side? The bades are moving at the same speed. Further, the main air movement is perpendicular to the rotation plane of the blades. The "bite" of each blade should, it seems to me, be the same, regardless of the position of the blade.

May 25, 2002, 09:56 PM
dusty bible = dirty life
Majortomski's Avatar
Consider the worst case a tail dragger just starting its take off roll. With all three wheels on the ground the airflow is parallel to the surface of the runway. The propeller is tilted back from this airflow around 10 degrees. So on the downward stroke the blade sees an increase of 10 degrees and on the upward stroke it sees a decrees of 10 degrees. Actually the forces due to AOA isn't all that drastic. The real culprit is the speed variations. I know how can the speed be different on a disk turning at a fixed rpm? The answer is vector analysis. For the above airplane rolling down the runway at 30 MPH the downward swinging blade is seeing an increase of 5 MPH and the upward blade sees a decrease of 5 mph due to the resultant airflow vectors at the face of the prop. If you have a 6-foot prop turning at 2000 rpm then the tip velocity is around 214 MPH. So add in the above data and the downward blade sees airspeed of 219 MPH and the upward Blade sees 209 MPH. It is this difference in relative speeds that causes the P factors differential thrust.
Jun 14, 2003, 03:44 AM
Registered User
This all makes reasonable sense, but I'm not convinved on a few issues. A couple of questions:-

1) From the above, it seems that the dominant yaw is a function the engine/prop thrustline's angle of attack - and thus extemely variable in flight. So, in trimming for stable flight I guess you should only apply side thrust to a model if the engine/prop comination is at some angle of attack in stable, staight and level flight. This could in theory be left OR right thrust depending on whether the thrustline is up or down in this trimmed flight ?

2) Quote from Thomas Solinski, above: "the swirl goes the opposite direction that is given as the explanation for left yaw" - Are you implying here that the swirl goes in the opposite direction to the prop, OR, the same direction but we have our "impinging on fin" logic reversed ? It seems to me that a conventional prop turns clockwise from the rear so any helical propwash as conventionally described would seem to "hit" the fin from the left side and induce left yaw - requiring corrective right thrust ?

3) "The mythical helical propwash" - why do wind tunnels / ducted fans etc. have vanes to "staighten" the flow if this helical propwash doesn't exist ? Perhaps that's not their function then. Although not as knowledgable as Thomas above, I think that basic laws of physics (friction, airfoil drag etc.) infer that some degree of helical propwash must result as the prop "drags" a little air around with it, although perhaps this effect is relitively minor.
Jun 22, 2003, 03:47 PM
Voices through wires? Ha!
Chas's Avatar


Also, the P-factor argument, while definitely a powerful LEFT yaw force in positive-alpha situations like take-off with a taildragger, should produce RIGHT yaw in a pilot-induced dive.

Torque, is a ROLLING reaction and can never be corrected in the yaw plane (by sidethrust, rudder, opening the door, waving to Mom, etc etc).

There are a million myths out there in the Naked Aeromodelling City! That's what makes it interesting.
How about the "pressurised muffler" baloney, or the "downwind turn"?

Jun 22, 2003, 11:05 PM
Registered User
Chas, I know what the "downwind turn" is all about (ground speed vs. air speed) but what is the "pressurised muffler" baloney ? So called back-pressure ?
Jun 23, 2003, 10:11 AM
Voices through wires? Ha!
Chas's Avatar

Oh-oh ... here we go...!

Hi Mike

Here I go with the "exposed flank" again! I've had a lot of trouble with some contributors thinking I've been some sort of know-it-all Father Superior, but at the risk of changing my avatar to Poindexter's Snobbier Uncle, ........

The engine manufacturers for years have responded to modeller's insistence that a "pressure nipple" be fitted to mufflers. The idea being, as I'm sure you know, that a pipe is connected from that nipple to the tank vent and this somehow
"pressurises" the tank and improves fuel feed to the engine.

It's a waste of good neoprene, of course, because there is no muffler pressure. You can't pressurise something with a great big hole in the back of it. Just perhaps, if the exhaust outlet on the muffler were reduced to something ridiculous (like say the diameter of a piece of fuel tubing! ) there might be some pressure build-up, but you'd be experiencing horrendous engine problems by this time as you're effectively plugging the exhaust.

One more item on the "torque" issue. If torque is causing that left yaw, how come it doesn't cause a right yaw when flying inverted?
The "right sidethrust" that was built in will then be aggravating the yaw, not trimming it out.
I think we have to accept that torque, a rolling reaction (which works on the ground ) is the least powerful of the factors at work. P-Factor is powerful but only when the main airflow is divergent, and by a lot, from the propeller disc.
The only thing left is helical propwash, and this diminishes with airspeed just as you would expect it to. It doesn't show its face on pushers, or twin-boom aircraft with the tails out of the wash. For me, that's QED.

Jun 23, 2003, 10:55 AM
Registered User
Chas, Nah, you're not exposing anything to me. I was referring to 1:1 car people who are always talking about exhaust systems and its 'backpressure' which is really a standing wave effect. I've never been a gasser so I know nothing 'bout pressurizing fuel systems. Which does sound like an exercise in futility

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