What factors influence roll rate?

[DeaninMilwaukee]

More specifically, why are some planes very fast to roll and others are'nt?

Factors that should make for fast roll:

1) short span
2) big ailerons
3) big aileron throw
4) tapered chord wings

I had all these items neatly pigion holed in my mental flight characteristics file, but my new jk aerotech P-51 has kinda screwed this list up.

The only thing on that list it has going for it is the tapered wing, yet this plane rolls as fast ( a blur! ) as my old MAT Tumblwewatt did, and that plane had all four items going for it plus light weight.

The only things I can see that might have an influence here is that the wingloading is high on this plane, and its a low wing with some dihedral.

At this point, I realize I don't really know how to predict how fast a new design will roll, and I was hoping one of the sharper guys here could shed some light on this subject.

Thanks in advance.
Dean in Milwaukee

[joao]

Hi Dean,

As you say the P51 has a high wing loading, that is usually associated with a fast flyer. A fast plane has "more airflow" over the wing and so also over the ailerons- > more force on the ailerons-> higher roll rate.

Beeing a low wing sure usually helps increase the roll rate.

I think all the 3 first factors you mention are correct. For factor 4 you mean something like the Spitfire version with the "cut" wing tips for low altitude dogfight ?

Just my 0.1 cents;)

João

[joao]

REPLY UPDATE;)

"more airflow" is way too incorrect. Please think of "more airspeed".

João

[markdrela]

Originally posted by DeaninMilwaukee
More specifically, why are some planes very fast to roll and others are'nt?

Factors that should make for fast roll:

1) short span
2) big ailerons
3) big aileron throw
4) tapered chord wings


A pretty good approximate formula for the roll rate is:

roll_rate = (airspeed/span)*net_tip_twist_angle

"net_tip_twist_angle" is the apparent tip AoA change due to aileron deflection. The ideal wing for fast rolling with minimal drag would be a "wing-warping" design like the Wrights used, with a linear twist twist distribution across the span.
It is then equivalent to an extremely high-pitch freewheeling propeller with constant pitch across the blades, er... wings. Every spanwise airfoil is then lined up with the local flow direction for minimum drag. Chord taper does not improve the roll rate of such an ideally-twisted wing.


Since wing warping is structurally impractical, it must be approximated with hinged ailerons. To mimic the linear twist, the ailerons want to have an inverse taper with zero chord in the center, and want to extend all the way to the tip. The zero center chord is of course impractical, so some compromise is necessary. Having the aileron all the way to the tip is quite practical and effective, however.

Here's a numerical example...
A wide-chord aileron extending all the way to the tip, deflected say 30-35 degrees, might be able to produce

net_tip_twist_angle = 20 degrees

With a 5-foot span flying at 60 mph = 88 ft/s, the roll rate is then

roll_rate = ( 88ft/s / 5ft )*20 deg = 350 deg/s

or almost 1 roll/second. Either doubling the speed OR halving the span will double this roll rate.

[DeaninMilwaukee]

Hmm. I think I get this, ( well sorta!).

The p-51 has a fairly small chord at the tip, so the constant chord full span ailerons become a progressivly larger percentage of the total chord as they get nearer the tip, THEREFORE, the net tip twist angle becomes progressivly greater as it gets nearer the tips, giving greater authority than if it was a straight chord wing with the exact same chord and throws.

This combines with the fairly high flight speed of this plane to create the extremely high roll rate it demonstrates.


Do I have this right? :)


Dean in Milwaukee

[Sparky Paul]

I have a DynaFlite P-51.. built with strip ailerons at first. Roll rate way too fast for a scale airplane at high rate.
Bandsawed off the trailing edge along the line the of scale ailerons and flap hinges, and added scale flaps and ailerons.
Cut the aileron area down at least 50% (which moved the active area out a bit). Roll rate STILL too fast at high rate.

[GWRIGHT]

as many have said, airspeed is something you didn't list in the first post. In addition to overall roll rate, roll inertia plays a big part in how nice a plane flies. If the roll inertia is high, it will accelerate to full roll rate slower when you deflect the ailerons, and stop rolling slower when you release ailerons. I like the lowest roll inertia possible for a very "crisp" feel on the ailerons. The key to this is distributing as much mass towards the center. For example, imagine a large model with plug-in wing panels. The wing panels are 1 lb ea. If you balance an individual wing panel spanwise and it stays level when balanced an inch from the tip (mass distributed WAY out towards the tip) it will have the same overall roll rate as an identical set of wings that is balanced 1 inch from the root, however, it's going to be slow to accelerate and decelerate which will make it very hard to do nice point rolls and "index" at the proper positions, and the aileron control will simply feel "loose" (best word i could come up with :) ). Equip the plane with an identical set of wings (still one lb ea. just like the others), only with the spanwise balance point 1 inch from the root and the plane will be very "crisp" and have a light feel on the ailerons, basically flying MUCH MUCH better. Throw in large ailerons, high deflection, and a bit of speed, and you get really fast rolls, that start and stop instantly. This is a bit of a digression from the original topic, but roll inertia is a very important thing to how well a plane flies and I thought it would be appropriate to mention. A good analogy would be to take two identical sports cars, and accelerate from a dead stop to a particular speed, say 30mph. In one car, do it in 1st gear, in the other car do it in 3rd gear (if will start moving :) ) , then let them coast to a set speed, say 10mph, without touching the brakes . The 1st gear example is sort of how a low roll inertia plane will feel on ailerons, the second example is how a high roll inertial plane will fee. first one will feel much more "peppy". I know, bad analogy, but the best I could come up with.

[MikeSt]

Mark,

Originally posted by markdrela
A pretty good approximate formula for the roll rate is:

roll_rate = (airspeed/span)*net_tip_twist_angle

Here's a numerical example...

A wide-chord aileron extending all the way to the tip, deflected say 30-35 degrees, might be able to produce

net_tip_twist_angle = 20 degrees




How did you get 20 degrees? - how does 20 deg relate to 35deg deflection ?

How does the chord affect the formula?

Mike

[markdrela]

Originally posted by Mike Stramba

How did you get 20 degrees? - how does 20 deg relate to 35deg deflection ?


The 20 degrees is the change in the airfoil's zero-lift angle. This is typically 50-60% of the flap or aileron deflection, at least before major separation sets in.

[MikeSt]

Originally posted by markdrela
The 20 degrees is the change in the airfoil's zero-lift angle. This is typically 50-60% of the flap or aileron deflection, at least before major separation sets in.

Ok, thx.