What turns an airplane - discussing rudder

[RonP]

I did not want to hijack the thread on the “What makes an airplane turn” topic so I started a new thread about rudder. In the previous thread someone mentioned an R/C (radio controlled) model that just had rudder control. Having grown up in the infancy of R/C rudder only was very popular. With just rudder control you can turn, climb, dive, loop, roll, split S and Immelmann. Everything except sustained flight upside down. Bear in mind we had no elevator, aileron or throttle control. The magic wasn’t just the rudder but a combination of wing incident, dihedral, speed, stabilizer angle, airfoil and engine thrust. The point being although there was no pitch, elevator or engine speed control the fixed state of those were not static but had dynamic affects at different phases of flight. Although these were models the physics of flight doesn’t change. This demonstrates to me it takes all forces to turn an airplane, not just 1 control.

To those not familiar with how an airplane flies it seems impossible to perform a sequence of maneuvers with just the ability to turn the rudder left and right. We did it every Sunday all day. If anyone needs a better understanding how this is possible let me know.

 

[Salty]

I’m not sure how you can “climb, dive, loop, split S and Immelmann” without a throttle or elevator. I’ll give you climb until the fuel runs out and dive after the fuel runs out.

 

[Hang 4]

Dihedral is a big part of the answer.

 

[RonP]

I am glad you asked……

When the airplane is trimmed correctly with engine thrust, wing incidence, stabilizer angle and dihedral the airplane will climb slightly with power and a shallow descent with power off very stable with no control inputs. All the above is key.

A banked turn is accomplished with rudder inducing a yaw. The outer wing is moving faster than the inner wing and the outer wing generates more lift causing a bank.

To climb fly straight and level. To lose altitude turn sharper causing more bank and less lift in the vertical direction. A shallow turn will maintain altitude. Using this technique you can put the airplane at any altitude.

All maneuvers start with full rudder causing the airplane to spiral and losing altitude. Trading altitude for speed is the key. At the bottom of the spiral level the wings with rudder and the excess speed increases lift and the airplane will do a loop.

To do a roll do a spiral. At the bottom of the spiral level the wings and give full rudder. The excess speed makes the rudder have more authority and the airplane will roll, sloppy but a 360 degree roll.

A spilt S same start with a spiral. Level off at the bottom of the spiral and give full rudder to start a roll. 180 degrees into the roll when the airplane is upside down level the wings with rudder and it will continue the 1/2 loop back to level.

An Immelmann starts with the spiral. Level the wings at the bottom of the spiral and begin a loop. At the top of the loop give hard rudder. At the top of the loop the airplane is almost at stall and the prop wash hitting the rudder will roll the airplane level.

This is how we could do maneuvers with just rudder control.

 

[Dana]

A banked turn is accomplished with rudder inducing a yaw. The outer wing is moving faster than the inner wing and the outer wing generates more lift causing a bank.

Everything else in your post is a good explanation of doing aerobatics with a rudder only R/C (those were the days!), but rudder induces bank not because the outside wing is moving faster, the speed difference isn't enough to matter, but because of dihedral. When the plane yaws the underside of the outer wing is presented to the air increasing its effective AOA, and vice versa on the inside wing. That's why rudder only planes (not only R/C, the earlier Quicksilver ultralights had no ailerons so rudder was on the control stick, but they had lots of dihedral).

An airplane with no dihedral, like the Kolb I used to own, won't roll at all with rudder.

 

[RonP]

Dana,

The reason I go with the outer wing lift theory is in order to have the dihedral cause the bank would mean the airplane must be in a constant skid to keep the bank. However I never scored 100% on any test so there is always doubt with my answers.

Going back in time we entered R/C as escapement was fading and pulse rudder only was in vogue for those who could not afford a multi.

Even harder to explain than rudder only flight is Galloping Ghost R/C systems providing rudder, elevator and throttle with one actuator and the rudder and elevator constantly moving!

 

[Dan Thomas]

An airplane with no dihedral, like the Kolb I used to own, won't roll at all with rudder.

Marcel Jurca said that his Sirocco, which had no dihedral, acted like it did. It was likely due to blanking of the inside wing when a skid was introduced. Another factor is the damping of the tip vortex on the outside wing in the skid, which limits the spillover of the air from the underside to the top, and increasing lift that way.

But I wouldn't want to have to fly such an airplane without aileron control. Any turbulence would make it very difficult.

 

[midlifeflyer]

A banked turn is accomplished with rudder inducing a yaw. The outer wing is moving faster than the inner wing and the outer wing generates more lift causing a bank

Of course. I have demonstrated that with students as part of the coordination discussion. You can turn the airplane with rudder alone or aileron alone. Turn with rudder alone, you yaw and the bank will lag and the catch up. Turn with aileron alone, the airplane will briefly yaw in the opposite direction and then catch up.

is that what that other long thread was about?

 

[Dana]

The reason I go with the outer wing lift theory is in order to have the dihedral cause the bank would mean the airplane must be in a constant skid to keep the bank.

As long as you hold the rudder deflected the airplane will be in a constant skid.

Marcel Jurca said that his Sirocco, which had no dihedral, acted like it did. It was likely due to blanking of the inside wing when a skid was introduced. Another factor is the damping of the tip vortex on the outside wing in the skid, which limits the spillover of the air from the underside to the top, and increasing lift that way.

That's interesting. You get dihedral effect on a high wing aircraft due to the increased pressure under the wing from the fuselage side being presented to the relative wind in a skid. I never considered the top vortex effect but that makes sense.

Swept wings also behave like they have some dihedral. I see the Sirocco has a straight wing with a swept leading edge inboard to accommodate the retracts, that may also contribute.

As I said, my Kolb with no dihedral had absolutely now yaw/roll coupling, but it didn't have a fuselage, either. That also meant slips were useless... it would slip fine, but without any fuselage side area to add drag it had no effect on glide angle. But ultralights are light and draggy enough that you can just dive at the runway threshold without worrying about gaining speed and floating down the runway.