# Rolling a space plane in the absence of gravity

• dbaezner
In summary, rolling an aircraft allows for more thrust to be applied in a direction change. Yaw just rotates the aircraft around its own axis, while roll changes the direction of the lift force (which is always perpendicular to the wings).

#### dbaezner

Why roll an aircraft?

If an aircraft can pitch and yaw, when does it need to roll?

Thanks,
Dirk

Rolling can allow a lot of thrust to be applied for changes in direction. You might want to search some basic posts on aeronautics.

Yaw just rotates the aircraft around its own axis. It doesn't change the direction you are flying - it just makes the plane "fly sideways". (The direction of flight will change slowly, but that's not much practical use).

Roll changes the direction of the lift force (which is always perpendicular to the wings) so there is a component acting sideways relative to the direction of flight. That provides the centripetal force you need to fly around a circle. More roll gives a circle with smaller radius.

It's quite possible to "steer" a plane by rolling it without any yaw, though that isn't a very comfortable way to fly. You really need to do both at once: fly around a circular path, and keep the plane pointing in the same direction that it is flying.

It's easer to see the difference between roll and yaw in flying a helicopter than flying a fixed wing plane, because a helicopter really can fly sideways or backwards. A fixed wing plane can't do that, because the thrust from the engines is always in a fixed direction relative to the plane.

Thanks for the Wiki article. I now get why a rudder-based turn alone would cause a skid to the outside of the turn.

However, it also says, "If the pilot were to use only the ailerons to initiate a turn in the air, the airplane would tend to "slip" toward the lower wing." From reading through the references, it seems like a slip is simply the opposite of a skid (both purely sideways movements of the aircraft).

Is that right? If so, what is it about the airflow over the two wings that pushes it sideways in the direction of the lower wing?

Thanks,
Dirk

Sorry, I didn't see the previous post to mine. Got it. Thanks, everyone.

AlephZero said:
It's easer to see the difference between roll and yaw in flying a helicopter than flying a fixed wing plane, because a helicopter really can fly sideways or backwards. A fixed wing plane can't do that, because the thrust from the engines is always in a fixed direction relative to the plane.
A fixed wing glider can't "fly" sideways either. In order to generate lift, a glider or any fixed wing needs to be pointed in the general direction of travel so that the wings can generate lift, which limits the amount of yaw that a fixed wing aircraft can still generate sufficient lift for normal flight. Some fixed wing aircraft (models) could fly backwards, assuming an elevator or more likely a stabilator that wouldn't break apart due to the stresses.

A helicopter get's it's lift from a rotating wing (the rotor), and can fly in any orientation, including upside down if the collective allows for sufficient negative pitch, which is common for aerobatic radio control helicopter models.

coordinated turn
At the cruising speeds of most powered aircraft, very little rudder is required for a coordinated turn, mostly just elevator input once the aircraft is banked.

For a glider in a tight thermal turn, the inner wing experiences less relative air speed than the outer wing and to compensate, opposite (outwards) aileron is required along with a lot of (inwards) rudder input to produce a coordinated turn.

rudder only turn
Most aircraft have dihedral (wings angled upwards) which wil result in some roll response to rudder inputs. Once the aircraft is yawed, air pushes up on the upwind wing and down on the downwind wing, resulting in a roll response. Some radio controlled aircraft only have movable rudders and elevators, and rely on dihedral to produce a roll response. The turns aren't fully coordinated, but it's good enough for a model.

Free flight models have no movable surfaces and rely on dihedral and preset rudder and elevator positions to fly in large circles and land randomly somewhere downwind. The next step up from this is rudder only models, which provides roll and turn control.

When (only) rolling an aircraft to make a turn, is it the flow of air above/below the wings that causes the turn, gravity, or both?

dbaezner said:
When (only) rolling an aircraft to make a turn, is it the flow of air above/below the wings that causes the turn, gravity, or both?
It's a combination of the lift produced by the wings and gravity, assuming an aircraft makes the turn at constant altitude, which requires up elevator, depending on the amount of bank angle. For a constant altitude turn, the radius of the turn is affected by the bank angle and airspeed^2. Rudder is used to keep the tail from "slipping" downwards, although at "cruise" speed for most powered aircraft, weathervane effect will limit the amount of "slip" even if no rudder input is used.

Here's the tricky part (probably belongs in the sci-fi forum at this point): if I remove the effect of gravity but keep the lift produced by the wings, could an aircraft still turn by rolling? I have spacecraft flying through a medium similar to air (more like mud, actually) and have the ability to roll, pitch, and yaw. Can I use those maneuvers to turn the spacecraft around, or does it still require gravity?

Thanks,
Dirk

dbaezner said:
Here's the tricky part (probably belongs in the sci-fi forum at this point): if I remove the effect of gravity but keep the lift produced by the wings, could an aircraft still turn by rolling?
Assuming level flight with no gravity, then no lift is being generated. Rolling will just rotate the aircraft about it's central axis. Pulling back on the elevator will cause the aircraft to loop, and if it's rolled sideways first, then cause the aircraft to loop horizontally, which would be a "turn". Rudder could also cause a plane to turn, using the fuselage similar to a wing.

Here's one for the masters. This is for a sci-fi story. I cross-posted to the flight physics folks as well.

I have a space plane with the ability to yaw, pitch, and roll (with rudder, elevator, and ailerons) in the air, but am trying to understand what would happen if the plane no longer experiences gravity (while still flying through the air). Would it still be able to roll (turn) in the air or would I need something more (maneuvering jets?) in order to still make the turn?

Thanks,
Dirk

What air? Planets have an atmosphere precisely because the planet gravitates. There is no air in outer space.

If you really want airplane-like maneuvers would certainly have a different wing design to avoid lift. I would expect that it starts to roll, followed by a pitch towards the new direction. No jaw, as the pitch goes exactly in the right direction (the whole "wing" force changes the direction).

D H: A large artificial habitat?

dbaezner said:
Here's one for the masters. This is for a sci-fi story. I cross-posted to the flight physics folks as well.

I have a space plane with the ability to yaw, pitch, and roll (with rudder, elevator, and ailerons) in the air, but am trying to understand what would happen if the plane no longer experiences gravity (while still flying through the air). Would it still be able to roll (turn) in the air or would I need something more (maneuvering jets?) in order to still make the turn?

Thanks,
Dirk

Multiple posting is not allowed at the PF. I have merged your two threads. Please do not cross-post again here.

The plane is still in the atmosphere. I'm exploring what happens to the flight physics if the plane and its contents cease to be affected by the force of gravity (think of it as an on/off switch). Can I still turn by rolling? Or do I need gravity to accomplish that? I need a design that works with gravity and without, hence the attempt to keep with a near standard aircraft design.

Thanks for the responses. mfb, it seems like roll and pitch would work. Thanks.

Apologies for the cross-post.
Dirk

dbaezner said:
The plane is still in the atmosphere. I'm exploring what happens to the flight physics if the plane and its contents cease to be affected by the force of gravity (think of it as an on/off switch). Can I still turn by rolling? Or do I need gravity to accomplish that? I need a design that works with gravity and without, hence the attempt to keep with a near standard aircraft design.

If gravity suddenly gets switched off, you are going to have a serious problem controlling the airplane, IMO. The only way to counter the typical 1g of lift would be to point the nose down a fair amount, which will hinder the rest of the control surfaces from doing their job. To do a right turn, you would roll right 90 degrees and do a 1-g turn or more...

If you switch gravity off suddenly, your airplane will accelerate upwards at ~1g. Without roll, I would expect it to stall quickly. It won't fall down (as gravity is switched off), but that is certainly not good for the airplane. With roll to account for that, you make loops.

berkeman said:
If gravity suddenly gets switched off, you are going to have a serious problem controlling the airplane.
The elevator would just need to be trimmed to produce no lift flight. For a common aircraft, the fuselage would be nosed down a bit, since the wings are typically angled up slightly.

For an aerobatic aircraft, the wings and tail section are symmetrical, and aligned with the the fuselage, so zero lift flight corresponds to zero elevator deflection. In addition, aerobatic aircraft usually have neutral stability, the center of lift is located at the center of mass, so that the aircraft can travel in a straight line in a dive or climb without having to adjust elevator inputs regardless of the change in speed (unless the speed becomes too slow in a climb).

LOL...posted about normal planes...didn't see the "space plane" in the topic.

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