Aileron Effectiveness

[Pips]

How quickly an aircraft rolls is due to several features, not least of which is the effectiveness of it's ailerons in initiating the roll rate.

The Fokker D.VII was renowned for it's rapid rolling. Yet it only had ailerons on the top wing. Surprising.

The Fokker DR.1 was a poor roller, and it too only had ailerons on the top wing.

The Nieuport N.17 was also a slow roller, and only had ailerons fitted to the top wing.

The Albatross D.II rolled more quickly than the D.III (although it was generally heavy on the controls). That makes sense as the D.II had ailerons on both upper and lowere wings, whereas the D.III had them only fitted on the top.

Both the Se.5 and the Camel had reasonable roll rates, yet they had ailerons fitted to both upper and lower wings.

And the Sopwith Tripeplane was also a slow roller, yet had ailerons fitted on all three wings.


So I guess my question is what is it that affects an aircrafts roll rate the most?

[Dan_San_Abbott]

Cobby:
I had a long conversation with Javier Arango, who has Fok.DR.I. D.VII and E.V, The Sopwith Pup, Tripe and Camel, S.E.5a, Nieuport 24 C1, and 28. The best is the Fok.D.VII with its high aspect ratio ailerons. It rolls better than all the other aircraft including the Sopwith Tripe with its six ailerons. Long and narrow is the word from Javier. BTW the Alb.D.II only has ailerons on the upper wing.
blue skies,
Dan-San

[jumpinjan]

I have a summary of a NACA study that says it depends a parameters of the wing: The wing's planform, the airfoil's thickness & taper and wing tip shape. They mention Dan-San's comment that narrow ailerons gave an increase in effectiveness when used with thick airfoils. "Narrow ailerons (only 7% of the wing area & 15%-20% of the wing's cord) gave excellent lateral control on all of the Fokker airplanes that used airfoils tapered either in planform or thickness". I also remember somewhere saying that on the biplane configuration, ailerons only on the bottom wing seems to be more effective too, instead on the top wing only.

[Barrett]

Nieup. 28 depends on whether the gap between the aileron and wing was "faired" over with fabric. The modification improved roll performance enough to be a common retrofit in many/most squadrons.

Then there's the Fokker Eindecker!

[David_Layton]

I would like to echo Dan's comments. I remember an article by Javier on flying the two DVII's and his comments on the roll and his conclusion that the DVII flew like a 1930's aircraft.

I would go further (and perhaps out on a limb) and state that in my humble the DVII is really a transitional aircraft. You could say it is the first 'modern' (ie post WWI) aircraft. The aircraft of the 1920's and 30's were, in fact, descendents of the DVII and not a continuation of any WWI SPAD, Sopwith or SE.

[Jim]

I agree with all of the above if one assumes a standard aircraft design, but I would place some other factors higher on my list if a rapid roll rate were to be the primary factor for which an aircraft is designed and built.

Aircraft design is an exercise in tradeoffs and one can optimize performance in any one or two traits at the expense of reducing performance in others. For example, inherent stability is normally a very desireable design parameter so that a pilot will be able to control the aircraft and not have to fight every minute to keep it headed where he wants it to go. Adding dihedral to the wings is one of a number of good ways to do this. The down-side to designing such a nice tame aircraft that any pilot can fly is that it will not have the same responsiveness to the controls that you will desire if you are trying to outmaneuver another guy who is trying to outmaneuver you. A strong dihedral will reduce your roll rate and start to become an undesireable trait if your goal is a nimble fighter aircraft. However, as you decrease inherent stability and increase nimbleness such as a fast roll rate, you begin to increase the number of crashes and fatalities of your own pilots who may not have the degree of advanced skill to control the beast. Modern fighter aircraft are designed to be so unstable that even with the finest pilot in the world they would instantly go out of control and crash were it not for the computerized "fly by wire" controls that are built into them to be between the pilot's input on the stick and the resulting movements of the control surfaces. The computers are making hundreds of small adjustments per minute to the flying surfaces to keep it going where the pilot aims it.

Wing length is another design parameter that affects roll rate. Putting mass further out from the axis of your roll means you must move that mass through a very wide arc which is slower than moving that mass through a shorter arc if the wings were shorter. For example, how many bombers can match a good fighter in doing snap rolls? Or, think of a figure skater when she does a spin, she greatly accelerates her spin by pulling her arms and legs in tightly to her body which the center of her axis of rotation. The more closely and compactly she can pull all of her mass into that thin verticle line of her rotational axis, the faster she goes.

[FlyXwire]

Additionally, the differences in aileron deflection possible between the varying WWI-era designs is to be considered:



(from NACA Report No.120 "Pratical Stability And Controllability Of Airplanes")

Also, the human factor enters into the equation, as applying input force to deflect the ailerons on many WWI aircraft required a two-handed effort. From the above chart it can be seen that the horn-balanced Fokker D.VII ailerons are moving and creating more lift (and drag) on the control's downward deflection, increasing desired roll rate, but also probably contributing to adverse yaw. The post-war Frise aileron designs not only helped counteract the effects of adverse yaw, but also helped in harmonizing pilot control inputs too: