Do real prop planes also use downthrust and rightthrust?

[addict]

I have always wondered if this is a crude diy fix that only modellers use, or if full size single engine prop planes also set the angle of their prop with a couple of degrees of downthrust and righthrust? I also have always wondered why the prop pulls to the left instead of the right? Anyone know the answer to these two questions?

[vintage1]

Definitely. Have a look at a 3 view of the Supermarine Walrus, for one of the most skewed engine mounts ever seen..

[KenSt]

I believe the Trojan T-28 has noticable downthrust.

[N74463]

There are a couple of reasons that prop airplanes tend to turn left. P-factor and torque are often blamed and they do contribute. But the big reason in most planes is the spiral slipstream. The prop accelerates air backward to provide thrust (that action/reaction thing). However, the air doesn't flow straight aft. It blows back in a spiral. On most airplanes the vertical stabilizer is on top of the fuselage. So the spiraling slipstream is only blowing on the left side of the stab. And that turns the plane left.
Tilting the engine to the right helps to compensate and, as vintage1 said, full size airplanes do indeed have right thrust. In fact, many full size planes also have the vertical stab angled slightly to the left for the same reason.
The adjustments in full size planes are aimed at making the plane go straight in cruise, which is to say less than full power. All the light planes I ever flew needed right rudder at full power.

Joe

[addict]

You are all my professors. Thank you to you all

[Brandano]

The torque can be neutralized by aileron trim, and the P effect will only be noticeable when the plane has a high AOA, or in any case the prop disk isn't perpendicular to the airflow. Since the engine is mounted to be aligned to the airflow while cruising the only effect left is the spiral flow from the prop, exactly like Joe said. Adjusting the rudder angle probably also reduces the need to add aileron trim to fight the torque induced roll.

[macboffin]

In practise, the engine is not always aligned with (cruise) airflow ;due to airplane configuration the thrust line mught be high, or in rare cases low, so the thrust line is angled to take care of that. Most models have a high thrust to drag ratio, ( a little like full size warbirds) and so a tilted thrust line could be important The "Walrus" amphibian mentioned a good example, to lesser degree almost any flying boat type, where the engine/s high mounted.

[vintage1]

If you do the analysis, the torque and the spiral airflow are halves of the same thing..its the acceleration of the air into the spiral that essentially is what 'creates' the torque and 'undoing' it when it hits a part of the plane creates an opposite torque.

Remember that sidethrust doesn't counteract torque directly..essentially that, and any rudder adjustment will yaw the plane slightly and then you need dihedral to translate the yaw into a rolling moment. Planes with no dihedral like pattern planes never have side thrust, and have to rely on aileron input.

[N74463]

I guess that's true. It seems reasonable that most of torque is the reaction to accelerating air into a spiral. I'd guess that the remainder is the reaction to turning the mass of the prop and engine components. And yes, torque tries to roll the airplane, so sidethrust doesn't counteract that directly. To counteract the roll you need ailerons (as Brandano said) or dihedral and a little sideslip, as vintage1 says.

But side thrust does directly counteract spiral slipstream effects. Because the vertical stab sticks up into only half of the spiral, it yaws the plane. Side thrust yaws the plane in the opposite direction.*

So torque rolls the airplane left, spiral slipstream impinging on the vertical stab yaws the airplane left, and P-factor (asymmetrical prop disk loading) also yaws the airplane left. It's clearly a liberal conspiracy. ;^)

Joe

* Yeah, I know, there is a tiny component of roll when the slipstream hits the vertical stab, but it's roll to the right and it's pretty small.

[Brandano]

So torque rolls the airplane left, spiral slipstream impinging on the vertical stab yaws the airplane left, and P-factor (asymmetrical prop disk loading) also yaws the airplane left. It's clearly a liberal conspiracy. ;^)
I wonder if this puts the weird engine arrangement on the SU26 in a completely new light...

[RCAV8R13]

If you do the analysis, the torque and the spiral airflow are halves of the same thing..its the acceleration of the air into the spiral that essentially is what 'creates' the torque and 'undoing' it when it hits a part of the plane creates an opposite torque.

Remember that sidethrust doesn't counteract torque directly..essentially that, and any rudder adjustment will yaw the plane slightly and then you need dihedral to translate the yaw into a rolling moment. Planes with no dihedral like pattern planes never have side thrust, and have to rely on aileron input.
All pattern planes have right thrust because the vertical stab is not symmetrical (for the same reasons you stated before). The more symmetrical and centered on the thrust line the vertical is, the less right thrust is needed.
RCA

[AndyOne]

RCAV,

I second that.

We had a talk from a pattern flyer at our club a while ago and he stressed the need for some quite radical side-thrust.

Andy.

[vintage1]

I thought the whole point of a pattern plane was that it WAS symmetrical, by and large.

I accept that if the slipstream is hitting the stab more above than below its going to yaw left..but the few planes I have seen have a slightly high thrust line relative to the (deep) fuselage, and hence a lot of side area below the thrust line, balanced by the fin above..and no sidethrust.

[AndyOne]

Vintage,

The whole point of a pattern plane is to have the most neutral possible flying airframe hands-off so the pilot only has to concentrate on flying the manoeuvres as accurately as possible. Any tendency of the airframe to fight back has to be reduced to an absolute minimum. So all the trimming tricks in the book are used to achieve this and side thrust is definitely one of them. Pattern flyers have even been known to saw wings in half to adjust the dihedral in order to eliminate adverse yaw.

Andy.