cameron

What factors affect the rate that the plane will rotate in a torque roll?

(Assuming the pilot is able to keep the plane in the best attitude for rotation)

Weight?, Wingspan?, motor?, prop?, C.G?

Thankyou,
Cameron McDonald.

EXCAP232

My Feedback: (12)

The vertical attitude seems to change it most. If you drop the nose a bit it will go into a hover. Some planes tork faster at slightly different attitudes. Sometimes a little on it's "back" will cause a faster rate.

EXCAP232

cameron

The plane I fly is completly symmetrical, so the best attitude is dead verticle, it does torque roll, but very slowly, even if held dead vertical.

I watch TOC videos and alike, and see those big 40% ers spinning like tops.

What factors, other than the pilots ability affect the roll rate?

Thankyou,
Cameron McDonald.

3D Joy

Try leaning back. When aircrafts are leaned back, you must keep a little down elevator and normally some right rudder to stay there. These inputs are the same as a negative left snap apart from the ailerons which could be added in torque rolls.
With this method, I have more difficulty slowing down the torque rolls than getting them to turn. Only my opinion, though.

cameron

Doesn't the plane rotate fastest when the tail and thrust line are in line vertically?

Isnt this why most ships need to be tipped back to the canopy?

Because my plane is completely symmetrical (ie everthing set at 0-0), and has no up or down thrust, leaning the plane back is theroetically the same as leaning it forward.

Therefor the best position for my plane to torque roll should be dead vertical?

Or am I completely off track with my thinking?

Jleyland

My Feedback: (1)

try a heavier prop...

3D Joy

Why would a heavier prop help? The only thing that could help is the way the flier puts his plane and the diameter of the prop. The larger the diameter, the better the aircraft will turn and remain controllable because of more air over surfaces.

In fact, a heavier prop should be harder to fly with because the engine will spool up slower and small, fast corrections become harder to perform. An instant blast is what you should want.

Jleyland

My Feedback: (1)

From what I gathered he wants to t-roll faster. A heavier prop will produce more torque. This will make the plane t-roll faster.

3D Joy

Why do a prop will "produce" more torque than another. I taught that it was the engine that was producing torque... With the same engine and RPM, you should have the same torque even if the prop weights a ton.

Gary Seeloff

A smaller diameter prop might help. It seems to me that a smaller prop would produce less thrust, and therefore need a higher throttle setting to have enough thrust to hover the plane. More throttle = more torque = faster torque roll. Just guessing.

I would also think that less wing area at the tips would help. Shorter span, or a tapered chord. Also just guessing.....

Gary

Jleyland

My Feedback: (1)

Hi 3-d Joy - Its that physics stuff - I think "torque" falls under "for every action there is an equal and opposite reaction" Assuming the same rpm, and taking the same shape of the props for comparison, the value of the equal and opposite reaction is reliant on the wieght of the prop. One of the equal and opposite reactions to a propeller spinning is torque. These are just my findings from trial and error and may not be correct - I know theres much more knowledgable people who can explain the physics behind it.

SloFlight-RCU

My Feedback: (2)

Guys...the whole heavier prop is all about flywheel effect. Think about the tools they use in space. Since there is no air ( there for no friction), & no gravity, the drills they use must have a weighted motor turning opposite that of the "tooled" end...otherwise...they could never get any torque on the bolt/screw.

If you were to (hypothetically) hover an aircraft in a vaccume (yes, I know its not possible, but stick with me), after some period of time, the airframe would be spinning as fast as the prop, and therefore, the prop wouldn't be spinning at all in relation to the aircraft.

Now, with that said...we are not in a vaccume and we must think about air resistance of the wings, tail & airframe. All of these factors will limit how fast the aircraft will spin while in a torqe roll.

The heavier the prop, the more influence it will have on the airfram & wanting it to "match speed" with the prop. Also, a larger diameter prop will have the same effect since the tips will be farther out & therefore more of a flywheel effect.

This is not necessarily scientificaly correct & I will not try to explain all of the physics behind it...since frankly, I dont know them all!!

Just how it works out in my head (which could be a scary place for anyone else).

Jleyland

My Feedback: (1)

Hi Sloflight - Interesting stuff - I have just one question about this statement -

"If you were to (hypothetically) hover an aircraft in a vaccume (yes, I know its not possible, but stick with me), after some period of time, the airframe would be spinning as fast as the prop, and therefore, the prop wouldn't be spinning at all in relation to the aircraft. "


The thing i'm wondering about is in relation to the pilot in the airplane the propeller rotates clockwise - a torque roll rotates the plane counterclockwise. - Would that mean the the plane will rotate the same rpm but opposite the direction in a vacumm? Or am I missing something with the use of tools in space relation? This is all interesting stuff thanks

Yellow Jacket

My Feedback: (6)

Torque is the cross product of r and F where F is the force and r is the vector from the rotational axis to the point of action. The magnitude of torque can be found by simply multiply F by d, where d is the perpendicular distance from the axis of roation to to the line of action. In the case of props, d is simply the length from the hub to the center of mass of each blade (find the torque of each blade individually then, add them together)

So, to increase torque, you can increase either F, d, or both. F is a function of mass, so a heavier prop will increase torque. d is function of prop diameter, so a large diameter, low pitch prop will also increase torque.

Also, inertia is defined as mr^2. Where r is the radius (in this case equivalent to d--that is, the distance from the center of mass of one blade to the center of the hub). This shows that increasing m and r will increase inertia, and due to Newton's 3rd law, the rotation will be faster.

Additionally, rotational kinetic energy is 1/2Iw^2. Where I is inertia (above), and w (omega) is angular velocity. Increasing the rotational kinetic energy is done by increasing I or angular velocity (~RPM). For the prop/plane system to be in equalibrium, the plane's rotational kinetic energy must be equal and opposite of the prop. Increasing the kinetic energy of the prop will increase the TR rate.

So, to increase the rate:
Increase mass, angular velocity (RPM), and the diameter.

As I mentioned above, I is a funtion of r squared, and kinetic energy is a funtion of w squared and I. Because m was not squared in any of the equations above, angular velocity and diameter will play a larger roll in TRing than mass of the prop.
As somebody mentioned, increasing mass will increase spool-up/down time and make corrections harder. Due to this, I would try to increase diameter and speed before increasing mass.

The above discussion assumes all other things equal and is just to get the most torque out of the little spinny thing.

Factors hindering the speed of rotation are wind, surface area, and weight.

Weight increases the inertia of the plane and means that more force is required to get it to spin. (Newton's first law--an object will remain with constant velocity until acted upon by another force). Reducing the weight will allow the plane to spin faster (and accelerate faster to that final spin rate)

Surface area increases drag and air resistance, creating more forces to be overcome. Wind is just another source of air resistance. Reducing surface area (wing, fuse, tail, etc.) will make it easier for the plane to spin, and it will be less affected by wind.

I don't think CG will affect spin rate. AoA should be such that the engine thrust line is vertical.

Hope that helps.

-Eli Liechty
[email protected]

mc71

Another mechanical engineer I see. I agree entirely with yellow jacket but don't share his views on wind, once in a torque roll you are at the mercy of the wind moving at the same velocity disappearing of into the distance, so once you are moving at the same velocity as the wind, the relative velocity is zero. Its heaps harder to control, but no slower once your plane is moving.

Yellow Jacket

My Feedback: (6)

MC71,

You're right that if the plane is moving at the same velocity as the wind, it will not be affected by the wind.

However, if the plane's vertical axis remains stationary as the wind blows (ie. the pilot keeps it there), then won't the wind slow the spin rate?

-Eli

3D Joy

Maybe I should keep silent but strongly feel that the "F" in your formula has nothing to do with prop selection. If you prop an engine to turn 8000 RPM and then change that prop to a larger diameter smaller pitch prop that also loads the engine to 8000 RPM, then you have EXACTLY the same torque. Torque comes from the burning gasses of the engine, not the propeller. Horsepower is obtained by multiplying RPM and torque with a constant. Given the same RPM, you get THE SAME TORQUE. That was the part I am sure of .

Then the one I am not sure of ...
As for the weight of the prop, I don't think it changes that much things. If an aircraft needs its engine and prop setup to turn 6000 RPM to torque roll, then the kinetic energy of the prop has nothing to do there since kinetic energy has remain the same when speeds remain the same. Let me explain with an example : a fully loaded truck and an empty truck rolling both at 60 Mph. They both have the same engine and both engines work the same to stay at that 60 Mph. Kinetic energy enters the scene only when the trucks want to turn, accelerate or decelerate. If you try to remove or add kinetic energy, then you have a reaction but if you keep that constant 6000 RPM then kinetic energy does not change.

Those were my feelings since I am not a mechanical engineer but am very interested in this kind of stuff. I hope I do not cause any bad reactions.

Yellow Jacket

My Feedback: (6)

3D Joy,

I disagree with both of your explanations, although I'm not sure I can backup my point any further than I did initially. Maybe somebody else can chime in one way or the other?

The "burning gasses of the enine" is work potential that is converted to kinetic energy. My understanding of torque is that it is the moment acting upon a given point, or in this case the line of thrust, and it is a result of the prop (for the reasons I mentioned earlier). The force is produced by the engine, but the torque is due to the prop.

F=ma Force is a function of mass. Assuming acceleration remains constant (which may be a big assumption here), increasing m increases F.

Your truck example is not true because KE=1/2mv^2. The loaded truck is heavier, so it's kinetic energy is proportionally greater. The loaded truck has more kinetic energy when at the same speed as the unloaded truck, regardless of acceleration or turning.

I'm not an expert by any means. I'm just a student at Georgia Tech (aerospace engineering)

Can anybody else shed some light on this?

-Eli

cameron

Thanks guys this is good, Starting to look like physics at school, arghhhhhh, but I guess there is alot of physics in the forces acting on an airplane.

So now I understand why blipping the throttle works, accellerating the prop creates little bursts of torque to the airframe? Right/Wrong?

And the Diameter, RPM thing makes sense.

But I have also heard that increasing the pitch of the prop will speed up the torque roll, how does this work?

Reallity (that I have seen anyway), seems to go against the RPM theory, as the smaller the engine/faster the rpm, the less torque produced, ie little engines high rpm vs big gassers at low rpm?

Or is this an indication that the prop diameter is more powerfull than RPM in creating torque?

Or is this just a scaling effect at work here?

Thankyou,
Cameron McDonald.

Yellow Jacket

My Feedback: (6)

Yes, blipping the throttle increases acceleration of the prop, and increases torque. This is because torque can also be defined as moment of inertia times angular acceleration. The moment of inertia stays the same (prop), and blipping the throttle increases acceleration, thus increasing torque for a brief instant.

3D Joy

Yes, theorically, the acceleration should not change ; it remains at zero. The engine has theorically constant speed and so the aircraft. If the acceleration is at zero, then the "F" in you formula will be zero (the weight of the prop do not change anything).

In real world, you must gently rock the throttle to stay there and you help torque rolling when the prop accelerates (now you formula enters the scene) but as soon as you accelerate the engine, you must slow it down so you don't gain altitude, so the prop decelerates and in turn, slowing the torque roll until you get equilibrum.

The only way to gain torque on a given engine is to prop it so it turns in its peak torque range. A better way is to get a 4-stroke which for the same thrust, get higher torque since it is turning slower.

mc71

your right about the wind yellow jacket, I just think it looks cooler if the plane just dances across the field under the power of the wind.

The whole reason behind torque rolls is there is an unbalanced torque from the engine and prop that is being balanced by the turning aircraft, blipping the throttle does two things, it creates more torque for an instant, it also allows the plane to fall back on its tail for an instant. Meaning the airframe is getting even less help from the propwash to resist the turning moment.
I think that made sense.

seanychen

My Feedback: (4)

Some of the below points have been mentioned already. Below is a systematic method to categorize factors that will affect torque roll rate:

Input to torque roll: prop mass, prop drag

Plane's resistance to torque roll:
1) inertia resistance: plane's mass, how close is CG to the thrustline, gravity's effect on non-vertical torque roll
2) drag resistance: wing location, wing span, wing area, wing shape

*Make the thrustline as close to center as possible.
*Make the wing as close to center as possible.
*Concentrate the plane's mass close to the centerline.
*Light airframe, as the prop's counter torque has less aircraft inertia to rotate
*Small wing, preferably short wingspan: the wider the wingspan, the more it will resist torque roll. For this reason, biplane torque rolls faster than similarly loaded & powered monoplane.
*Highly tapered wing, like the Edge. For the same reason as above, only in this case the wingtip's wing area is reduced to help roll (flying roll as well as torque roll)
*Pure vertical attitude. Deviation from verticality will cause gravity to work against the torque roll.

I hope this captures every contributing factor to this system concerning torque roll. I did everything short of drawing out the free body diagram.