• Advertisement
Sign in to follow this  

Physics of flight: turning

This topic is 3225 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

I've got my lift, gravity, thrust and drag equations sorted out. Now I'd like to be able to turn my plane. Could someone please explain to me how a plane changes its heading? I'm under the impression that the rudder is only used to make minor adjustments, but turning is handled by other control surfaces. I can see how raising and lowering opposite ailerons changes the roll of the plane, and I can see how if the plane had rolled to the left by, say, 45 degrees, the lift now has a sideways component. But what I'm missing is how the plane actually changes its heading. The only things I can think of are you either have to use the rudder, or "pull back on the stick" to "climb" at your 45 degree angle. But I don't think either of those are correct. Any help appreciated.

Share this post


Link to post
Share on other sites
Advertisement
I think that's correct. The lift generated by the wings is perpendicular to the surface. If you bank the plane, the lift force wont be vertical, but also pulling towards the side.

If you want to turn really sharp, you roll the plane 90 degrees, and pull hard on the stick. If you want to increase the rate of turn while not banking so much, use the rudder to make corrections in the horizontal plane.

Also, rolling 90 degrees means, you no longer have vertical lift, so you'll loose altitude. Rudder can compensate for that in a way.

Share this post


Link to post
Share on other sites
Ok, thinking of it a different way...

In the plane, pedals control the rudder, pulling back on the stick changes the pitch of the plane, and moving the stick side-to-side rolls the plane.

If I disabled the rudder, and locked the stick so it could only go left or right (no adjustment in pitch from the cockpit allowed) and while facing due North I rolled 45 degrees to the left, would the plane start to turn West, or would it continue to face North while drifting North-West?

Share this post


Link to post
Share on other sites
If you want to make it really nasty look for the gyroscopic effect (pitching an airplane with a spinning propellor creates a roll moment). That's just kind of annoying/stupid for most practical flight simulators though (but something I wasn't even aware of until recently).

Share this post


Link to post
Share on other sites
>> If you want to make it really nasty...

Nope. Not trying to make it nasty. Just trying to make my flight sim work.

I can make the plane roll but that doesn't change its heading (in my flight sim). I was wondering if that was correct behaviour or if I've missed some vital force or effect that would make the plane change its heading. Hence my question involving disabling rudders etc.

I just want to know if my flight sim is behaving correctly, or if it's possible to change heading by banking the plane - does physics / gravity / lift / something else change the heading if I'm flying along with my wings in 45 degrees away from horizontal because I rolled to get there, or do I continue facing due North (while drifting North West due to the horizontal component of lift)?

Share this post


Link to post
Share on other sites
When the plane is rolled to the side, the pitch axis also is no longer horizontal. So, move the elevator to cause the airplane to pitch about the pitch axis, and the pitch changes causes the nose to move to the left or right, effectively changing the heading. The more you pitch, the faster the turn. The more you roll, the more effective the pitch. Gradual turns require only a small amount of elevator deflection to generate a small amount of pitching motion. If the a/c pitches rapidly to a new direction, it still takes a while before the a/c will be moving in the new direction. The new thrust axis has to accelerate the a/c to change the velocity to be aligned with the new orientation of the fuselage.

Turning can be accomplished with the rudder also. It just might take longer....the rudder is not as efficient at turning as using ailerons to bank and elevator to pitch are. When the rudder is deflected, the fuselage is yawed to one side, and if you hold the rudder in place, the thrust (which is aligned with the fuselage) will be pulling in the direction the fuselage was yawed to. Eventually, the aircraft will become moving in that new direction. Turning with the rudder alone can be annoying. The aircraft is directionally stable. When the rudder is deflected, the a/c will, to begin with, stabilize roughly in the starting direction of travel...it'll find equilibrium in the direction it is already moving in...and will slide sideways for a while. This is called "sideslip." But, eventually, it's direction of motion will align with the thrust axis.

Share this post


Link to post
Share on other sites
Quote:
Original post by timgradwell
If I disabled the rudder, and locked the stick so it could only go left or right (no adjustment in pitch from the cockpit allowed) and while facing due North I rolled 45 degrees to the left, would the plane start to turn West, or would it continue to face North while drifting North-West?


You would drift to the west (and loose altitude), but I'd say you would not incur a change of heading, unless there is some weird effect taking place that would generate a yaw effect. Maybe the interaction with gravity (not being aligned with the plane down axis anymore generate some instability) changes the plane's heading, but dont quote me on that.

If you pull on the stick, your heading will definitely change. I suppose it would be interesting to find out for real, or with someone who has practical knowledge.

Share this post


Link to post
Share on other sites
I achieved what I think you're looking for in my game by ensuring the wing forces are applied just in front of the aircraft centre-of-mass. This gives me a torque which when the aircraft is rolling leads to a change in heading. May need to 'trim' it in normal flight to remain level though.

Share this post


Link to post
Share on other sites
Basically it is just like you say. Rudder is only used for minor corrections or to fight crosswind in landing and take off situations.

By deflecting the ailerons you roll your plane. As soon as the wings are no longer horizontal you experience a lift force that is no longer directly opposing the gravity. Instead it points to the left or right as well as up. That is why you begin to drift sidewards and start falling as Oliii said.

Now put that in combination with what Aph3x said: The layout of the plane decides if this causes the plane to have remarkable changes in heading. The inertia tensor of the plane when viewed as rigid body decides how it reacts to the torque.

Finally, keep in mind that other flight sims you might yours comparing with use simplifications of the flight model to make their sim feel like what most players would think how an aircraft acts [grin]

Share this post


Link to post
Share on other sites
Quote:
Original post by Aph3x
May need to 'trim' it in normal flight to remain level though.

That should be quite ok since all aircrafts need trimming depending on their current weight, velocity, winds, and maybe a hell of lot more influencing forces.

Share this post


Link to post
Share on other sites
The overall center-of-lift is normally behind the center-of-mass, not in front. This ensures stability. Eventually you'll find trouble with your simulation if you keep applying the lift ahead of the center-of-mass. To understand why, think of an a/c in level flight. Remember, lift is proportional to angle of attack under normal conditions, and angle-of-attack increases when you pitch up while maintaining the same relative heading. If you pitch up slightly, lift increases. Lift in front of the center-of-mass will cause an increase in the pitch that will cause the a/c to pitch up even more. Unstable. Consider instead the case I mentioend with center-of-lift being behind center-of-mass. In this case, if you pitch up, lift increases, causing a counter-acting torque...the increase in lift causes the a/c to pitch back towards neutral. Stable. This is the reason for there being two primary lifting surfaces. The main wing either provides lift lift that has minimal effect on pitching torque (for conventional aircraft with the wing in front/stabilizer in back) or guarantees the primary lift is behind the center-of-mass (for canard style a/c with the main wing in back/stabilizer in front). The stabilizer is used to achieve trim as center-of-mass moves due to fuel burning off, different passenger weight distribution, etc., and to control pitching motion for maneuvering, take-off and landing, changing altitude, etc. Properly designed, the stabilizer cannot provide enough lift to cause the center-of-lift to move ahead of the center-of-mass, but does provide enough torque...with it's large lever arm and (in the case of a conventional configuration, small) lift....to affect the pitching motion of the a/c. The only types of fixed-wing a/c that have the center of lift ahead of the center-of-mass are aerobatic aircraft (and these are usually neutrally or mildly stable...center-of-lift still behind center of mass but not as far behind as passenger a/c) and fighter a/c with computer controlled stability on an unstable configuration (think X-29).

Share this post


Link to post
Share on other sites
Quote:
Original post by timgradwell
I've got my lift, gravity, thrust and drag equations sorted out. Now I'd like to be able to turn my plane.

Could someone please explain to me how a plane changes its heading?

I'm under the impression that the rudder is only used to make minor adjustments, but turning is handled by other control surfaces.

I can see how raising and lowering opposite ailerons changes the roll of the plane, and I can see how if the plane had rolled to the left by, say, 45 degrees, the lift now has a sideways component.

But what I'm missing is how the plane actually changes its heading. The only things I can think of are you either have to use the rudder, or "pull back on the stick" to "climb" at your 45 degree angle. But I don't think either of those are correct.

Any help appreciated.


A few general notes that might help you think about this...

- The rate of turn of an aircraft is dependent on the horizontal component of the lift. As the aircraft rolls the lift vector changes, the more it rolls the more of a horizontal component you have. Look up 'banked turn' on Wikipedia.

- Rudders generally aren't used to turn, in fact it is a common misconception that rudders are for that. Really, they are used just to alter where the nose is pointing. An aircraft rolls due to the change in lift on each wing when the ailerons when they are moved. This causes the drag on each wing to change, which causes the aircraft to yaw in the opposite direction of the turn. Rudders are used during turns to counteract this and maintain what is known as a 'co-ordinated turn'.

- However, yes you can turn an aircraft by only using rudders. It isn't an efficient way of turning. Nor is it directly a turn, it really is an indirect effect that causes the turn - basically the yawing of the aircraft changes the lift on the wings...causing a roll. Instrument pilots are often taught to use rudder for fine controlled small corrections to maintain flight on a given heading.

- Pitching will change the rate of your turn as people have explained. Pilots usually adjust pitch slight during turn to make up for the loss of vertical lift - remember some of the lift component is now more horizontal in a roll, so some vertical lift is lost. Lift is increased by pitching up. The point is...even if you're doing a simple sim take note of the vertical component of lift.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement