2 Degrees Down , 2 Degrees Right

[Screamn1]

It seems that this is kind of the rule-of-thumb for most planes and I've always kind of took it as fact and never questioned it. But getting back to the basics, I understand that 2 degrees right is to counter the torque of the motor but what about the 2 degrees down. What happens if you have more / less than this? I can see where when you throttle up, you gain speed which increases lift and the 2 degrees down maybe doesn't allow the plane to climb as much. Is this correct? I'd just like a bit more explination on this.
Thanks.

[BMatthews]

Part of the preffered amount of downthrust is related to the CG placement as well. The further forward the CG placement the more pitch stable the plane is but also the more it'll respond to extra airspeed by nosing up to kill the airspeed. In effect models with forward CG's and the wing to stabilizer decalage angle to match are one speed models and will try to nose up to dampen down any extra speed. Adding downthrust helps limit this reaction. Conversely the further back the CG the less you need downthrust to combat the highly reduced nose up pitching from extra speed.

Some rudder only models use as much as 6 and 7 degrees of downthrust. Pattern planes that are intended to be set up very close to pitch neutral often don't have any. 2 to 3 degrees is common for sport models and 3 to5 is common for trainers.

[Rodney]

The thrust line is adjusted to cancel out the effect of drag. Most drag is from the wing so a high winged model usually requires more downthrust than a mid or low wing. Since the drag force times it's moment arm (the distance above or below the CG) is greater the farther the wing is from the CG, the more thrust offset is required. It also depend on whether or not the thrust line is above or below the center of drag.It is really just a matter of having all the thrust and drag forces cancel out relative to their moments about the CG.

[gouch]

I understand that 2 degrees right is to counter the torque of the motor

Could someone explain that? That dosen't make sense to me?

The torque reaction of the motor would be along the motor shaft line opposite to the prop rotation, so how does turning it to the right counter this? Going off this idea, it has only moved it to a skewed line relative to the aircraft. If this where the case, then why not left thrust? I am under the impression it has more to do with the whole "air over the fin thing" than torque reaction from the motor.

Cheers
Paul

P.S not critising anyones idea here, I just think there's more to it

[Cuda]

Gouch, Part of 2 degree's of right thrust is to counter the P-factor or the prop factor. This is when the prop is turning the air is moving with a spinning force around the fuse which pushes on the left side of the verticle stab. This will cause the plane to waw to the left when under power and the plane is at slow speed. This is why when flying full scale aircraft and most models you have to apply right rudder during takeoff to keep the plane flying straight down the runway. When I am flying the Cessna 172 (Full scale) during slow flight, I have to keep a lot of right rudder in to it or the plane will fly in a big circle to the left.
Another factor that is taking place is when you pitch the nose of the plane up the right side of the prop is at a higher angle of attact in relation to the wind which gives it the affect of more pitch on that side than the left side of it.
And last, the engine torgue will try to roll the plane to the left which will cause the right wing to create more lift than the left one which add's to the reason why the plane will yaw (turn) to the left under slow speed and high power. Hope this helps

[vintage1]

Theres a combination of things - torque, spiralling airflow, the angle of attack of a prop disc to airstream, that in general tend to roll and yaw a model left.

Use of right thrust and dihedral together tends to counter it. its teh quick fix for a model that wanst to turn left under power.

I have used up to 15 degrees downthrust on a R/O model - no throttle, no elevator - in my youth, in order to tame dramatic zooming under power.

[gouch]

Cuda, thanks for the info. I was skimming on the right track then :D

Another factor that is taking place is when you pitch the nose of the plane up the right side of the prop is at a higher angle of attact in relation to the wind which gives it the affect of more pitch on that side than the left side of it.

This bit has me stumped, I can't picture it yet. I can understand how a helicopter has this in forward flight as one side is moving toward the front of the helicopter and the other is going back, causing a difference in lift on either side, but I don't understand what you mention.

If only you could see me at the moment, I have one of my RC planes in
front of me trying to nut it out. At least I'm not making brooom sounds!


And last, the engine torgue will try to roll the plane to the left which will cause the right wing to create more lift than the left one which add's to the reason why the plane will yaw (turn) to the left under slow speed and high power

I think I get this one, just confirm/correct if you could.

The yaw effect is a secondary effect due to the right wing lifting DUE to the the torque reaction of the motor/prop. Just the same as if you simply banked to the left with aileron only. an uncoordinated turn if you will. Yaw is not a DIRECT reaction of the torque. This is more a question than a statement, especially if it's totally wrong! :D

One more thing, what if you had a single engine aircraft that had twin fins, that weren't in the wash if the props? would you still need the right rudder at low speeds/taxiing? I'm guessing not, but that dosen't mean much when I really don't know !!

Cheers
Paul

P.S do full size use right thrust? . I have noticed on a few jets (bizjets and the like) that they have some downthrust. Does your cessna have any downthrust?

[gouch]

I took so long to write the above , Vintage has come in before I had it finished. :D

Any thoughts on what I have mentioned in the above vintage?

Cheers
Paul

[vintage1]

Until and unless you can totally remove the swirl from the slipstream, engine torque will try and roll the plane left.

Removing the swirl at the top of the plane with a high fin will partly cancel the roll, but induce a left yaw.

Prop angle to airflow will if positive induce a left ywa as well. With downthrust you get a bit of right yaw.

Roll yaw coupling via dihedral makes it all into turning moment anyway.

Its so bloomin complicated, I just give up and go by rule of thumb. On a high wing rudder model a couple of degrees of right, and as much downthrust as is needed to make it fly relatively straight, or if its scale, use the mixers to avoid ruining the looks of the model.

[globemaster3c17]

gouch, take your model and position the prop so that it is horizontal. Look at it from the side while holding the nose up say 20 degrees. Imagine that the plane is flying level at this nose-high attitude. Look at the left side and you will see that the left prop blade is almost vertical. Now turn the plane around so that you are looking at the right side. Notice that the right prop blade is say 30 degrees from vertical. We will pretend that the prop doesn't see a forward component of relative wind in our example, so the left side has an AoA of 0, while the right has an AoA of 30 degrees. Higher AoA means higher lift (thrust in this case) provided it isn't stalled. So the right side is making a lot of thrust, while the left side is making very little thrust. This results in the center of thrust moving to the right side of the plane. Since the drag is centered on the plane's centerline, and the thrust is displaced to the right, the plane will yaw to the left. This is what p-factor is all about.

[hwhall]

Interesting discussion! Have I absorbed some of it right: If the plane is a pusher, sidethrust may not be needed because of airstream swirl (if the vert stab is not in line with the prop) but the p-factor may still require some sidethrust. Did I get that right?
--Wayne

[mrbadger]

P.S do full size use right thrust? . I have noticed on a few jets (bizjets and the like) that they have some downthrust. Does your cessna have any downthrust?

Normal full size aircraft don't use down or right thrust.
They have trim tabs. E.g. on a small Cessna, e.g. 152, 172 etc.
there is a fixed tab on the back of the rudder that at cruise airspeed
pushes the rudder slightly thus counteracting the slipstream effect.
At other airspeeds e.g. climbing or descending the pilot will need to apply
a little rudder pressure to "keep the ball in the centre".
As you go towards more powerful aircraft, e.g. Cessna 206, Beech Bonanza
the trim tab is pilot adjustable so there's less effort on the part of the pilot.

Almost all full size aircraft have a movable trim tab on the elevator or can change the angle of attack of the tailplane. This is pilot adjustable, so you don't need downthrust either.

Jet propulsion works on a completely different principal from props so I don't think the apparent downthrust you might see on a bizjet has the same purpose at all.

[gouch]

globemaster3c17:

That has helped clear up my understanding of it, thanks. It's still not imbedded in the grey matter yet, but getting there! :D

mrbadger: I "thought" I was seeing it (downthrust on rear mount bizjets), but I think I'm just looking at the engines relative to the "on the ground" attitude, not relative to normal flight attitudes.
And I totally didn't even think of the trim tabs. Interesting to note that you mention not all of them were a manual adjustment either. Interesting.

Cheers
Paul

[vintage1]

Interesting discussion! Have I absorbed some of it right: If the plane is a pusher, sidethrust may not be needed because of airstream swirl (if the vert stab is not in line with the prop) but the p-factor may still require some sidethrust. Did I get that right?
--Wayne


Not quite.

As long as the sliptstream has swirl, there will be a torque reaction from the prop imparting the swirl - conservation of momentum etc etc.

P factor holds always.

Th etorque induces rolling momet. Getting rid of te swirl induces olling moment in the opposite direction, but, if not symmetrical abut the CL, also a yaw moment.

So if you use an under slung fin, that would give a right yaw. A symmetrical fin would give no yaw, and to some extent counteract prop torque with a roll moment in the opposited direction.

Siilraly a high mounted engine may give right yaw on a fin if the lower half of the slipstream hits the fin top only.

Like I said,its complicated.
.

[globemaster3c17]

Glad to help gouch.

[Thomas B]

Normal full size aircraft don't use down or right thrust.
They have trim tabs. E.g. on a small Cessna, e.g. 152, 172 etc.
there is a fixed tab on the back of the rudder that at cruise airspeed
pushes the rudder slightly thus counteracting the slipstream effect.
At other airspeeds e.g. climbing or descending the pilot will need to apply
a little rudder pressure to "keep the ball in the centre".
As you go towards more powerful aircraft, e.g. Cessna 206, Beech Bonanza
the trim tab is pilot adjustable so there's less effort on the part of the pilot.

Almost all full size aircraft have a movable trim tab on the elevator or can change the angle of attack of the tailplane. This is pilot adjustable, so you don't need downthrust either.

Jet propulsion works on a completely different principal from props so I don't think the apparent downthrust you might see on a bizjet has the same purpose at all.

Take a look at a Cessna Caravan the next time you see one. This large single engine Cessna turboprop has a load of right thrust AND a load of down thrust. I could not find a pic that showed the right thrust, but I attached a small pic of a Grand Caravan that shows the downthrust clearly. The Caravan is a little like a model: big prop on a gearbox, lots of P-factor and spiral propwash. Similar to our geared electrics, in a way.

A number of other light aircraft have thrust offsets as well.

If more light aircraft had some thrust offset, you would not have to hold so much right rudder in a full power climb..;)

The downthrust you see on bizjet jet engines is simply there to minimize any effects of the engine thrust line being well above the wing of the aircraft. Without the downthrust, it will make the aircraft have a bigger pitch effect for power applied, until the aerodynamic forces are all balanced again.

The Lockheed Martin S-3 Viking had some real issues during carrier suitability due to the thrustline/pitch effects when changing power during approach. (engine pods underwing, even with some down thrust caused the aircraft to pitch up when power was applied and to pitch down when power was removed.) The downthrust did help this, but they also had to add a black box that mixed elevator to throttle position to hold a constant pitch angle during power changes.

Stab and elevator trim does mitigate the need for downthrust in a typical ligh aircraft.