Espritmodel.com Telemetry Radio
Reply
Thread Tools
Old Jan 04, 2013, 09:53 PM
PaulG
Newcastle, NSW Australia
Joined Dec 2004
302 Posts
Discussion
Aileron Differential, Camber,Speed

Normally we use aileron differential to counteract adverse yaw. Dr. Drela has published an opinion that no differential and compensatory increased aileron/rudder mix is the lower drag solution to entering and exiting a balanced turn. Despite Dr Drela's highly regarded reputation, I think most of us still use aileron differential together with aileron/rudder mix to get a low drag entry into a balanced turn. Certainly the full sized gliders I use to fly had aileron differential built in and we were taught how to balance the turn with judicial use of the rudder pedals. It may be of interest that different gliders needed more or less rudder to get into the turn and it had to be applied either before the application of ailerons or simultaneously with the ailerons. once in the turn, there was always a slight in-turn pressure on the rudder pedals and correction of bank angle also needed rudder movement. Of course the problem with aerodynamics is scale, and you cannot necessarily apply a full scale solution to a model problem.
Generally, when we fly a model faster, less differential is needed, when we fly slower, generally more differential is called for. When we fly at different speeds we often use different camber, more for slow, less or negative for higher speeds.

Is the amount of aileron differential needed for smooth entry into a balanced turn due to speed, camber (altered aerofoil) or both?
pigly is offline Find More Posts by pigly
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 05, 2013, 12:28 PM
Registered User
wakumann's Avatar
Canada
Joined Jul 2003
2,451 Posts
Quote:
Originally Posted by pigly View Post
Normally we use aileron differential to counteract adverse yaw. Dr. Drela has published an opinion that no differential and compensatory increased aileron/rudder mix is the lower drag solution to entering and exiting a balanced turn. Despite Dr Drela's highly regarded reputation, I think most of us still use aileron differential together with aileron/rudder mix to get a low drag entry into a balanced turn. Certainly the full sized gliders I use to fly had aileron differential built in and we were taught how to balance the turn with judicial use of the rudder pedals. It may be of interest that different gliders needed more or less rudder to get into the turn and it had to be applied either before the application of ailerons or simultaneously with the ailerons. once in the turn, there was always a slight in-turn pressure on the rudder pedals and correction of bank angle also needed rudder movement. Of course the problem with aerodynamics is scale, and you cannot necessarily apply a full scale solution to a model problem.
Generally, when we fly a model faster, less differential is needed, when we fly slower, generally more differential is called for. When we fly at different speeds we often use different camber, more for slow, less or negative for higher speeds.

Is the amount of aileron differential needed for smooth entry into a balanced turn due to speed, camber (altered aerofoil) or both?
Depends on the model and numerous other factors, best to try it out practical.


.....what was the question again
m2c
thomas
wakumann is offline Find More Posts by wakumann
RCG Plus Member
Old Jan 05, 2013, 12:37 PM
-----
Woodstock 1's Avatar
Ireland, County Kerry, Kerry
Joined Dec 2005
6,897 Posts
It's of academic interest really, seeing as most of us have no way of altering differential on a continuous basis as we fly..... And I don't get involved in academic "discussions" about aerodynamics on forums. Ever !

Chris
Woodstock 1 is offline Find More Posts by Woodstock 1
Reply With Quote
Old Jan 05, 2013, 01:35 PM
PaulG
Newcastle, NSW Australia
Joined Dec 2004
302 Posts
Fly the plane or the air?

The question is about whether there is value in continuously fiddling with the settings of the plane to get it to fly in the most efficient manner or, as I do, get the plane pretty right and then concentrate on the air that you are in to make the best of it.
A famous full sized gliding world champion, Paul McCready (the inventor of the McCready ring that gives you best speed to fly between thermals) once said that he did not care if he was slipping or skidding so long as he was going up faster than anyone else - that's probably a paraphrase.
We once put a yaw vane on the nose of one of our aircraft with a camera looking at it. The glider was flown low, so it was really visible and flown to give as balanced a turn as could be achieved. The video, however, showed a certain amount of slip (vane pushed to the outside of the turn). The explanation was that the fuselage being fairly short the air could be coming over the nose at an angle but still be going over the wings parallel to the chord. Probably more important than this was how much the vane moved about as compared to a full sized glider.
pigly is offline Find More Posts by pigly
Reply With Quote
Old Jan 05, 2013, 02:37 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
I don't recall such a statement from Drela but my understanding of differential is that it is far less efficient and less consistent than an equivalent rudder mix and therefore should be avoided at all costs -- with a notable exception:
- Differential reduces the tendency to tip stall by limiting the downgoing aileron so it has great benefit for, well, just about every plane.

There are three major causes of adverse yaw:
1. Increasing lift (camber) on one wing will increase the induced drag and likewise reducing lift (reflex) on the other reduces drag.
2. If the wing is at a positive angle of attack, deflecting an aileron upward will decrease frontal area (profile drag) and vice versa.
3. While the plane is rolling, the lift vectors of each wing tilt as shown by Drela here.

While the first two effects dominate with small course corrections and as the roll is initiated, it is the lift vector effect that dominates throughout the roll and thus is the most significant for large roll angles such as turn entry/exit. And since these effects occur at different times, there's the need to apply rudder at a different time than the ailerons: When the ailerons are first deflected the wing is not yet rolling so the "cambered" and "reflexed" wings each see the same AOA and the difference in lift between them is great (points #1 and 2 above) and that large lift difference is used to accelerate the massive wings to a new roll rate. Once a significant roll *rate* is established, Drela's diagram above shows that the corkscrew path taken by the wings reduces the AOA of the rising wing and increases that of the falling wing, and thus the net difference in lift between wings is very small, despite the large difference in camber/reflex. In this case point #3 dominates as #1&2 fade.

The points above explain why adverse yaw effects are different at the beginning/end of a roll than they are while steadily rolling. In models, the beginning/end are often brief enough to ignore but the effect is quite significant in certain aircraft such as F3B/F sailplane pylon racers as well as large precision aerobats.

Lastly, note that all the major causes of adverse yaw increase with CL. Therefore the plane will behave differently with ballast, camber, speed, and elevator input. And the CL relation means that there is zero adverse yaw at CL=0 and any differential or rudder mix will have the opposite effect when flying inverted.
vespa is offline Find More Posts by vespa
Last edited by vespa; Jan 09, 2013 at 02:27 PM.
Reply With Quote
Old Jan 05, 2013, 03:10 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
It's a common misconception to relate adverse yaw and coordinated turns. Adverse yaw is the undesirable yaw coupling that prevents the pilot from applying a pure roll input. Coordinated turns require the pilot to counteract the deliberately built-in roll and spiral stability of the aircraft. There is really very little connection between the topics. If the aircraft has a very large amount of dihedral it may require inside aileron to be held during a turn, in which case differential and/or rudder mix will of course help, but this is not really an issue of adverse yaw.

That said, any reasonably stable airplane will require inside rudder to maintain a coordinated turn. A side effect of turning is that the outside wing goes faster and creates more lift, rolling the plane steeper into the turn. While this is not generally the case with highly stable Cessnas and such, it is a significant effect on neutrally stable high-performance aircraft and thus requires *opposite* aileron to be held in order to maintain perfect coordination.

Since aileron deflections spoil the wing efficiency it is sometimes preferable to circle with a slight slip -- using insufficient inside rudder and taking the fuselage drag penalty in order to allow the dihedral effect to provide the outside rolling moment instead of ailerons. Increasing the dihedral will increase this effect, lessening the need to slip but there is no perfect balance that holds true for all bank angles so most designs have just one "sweet spot".
vespa is offline Find More Posts by vespa
Reply With Quote
Old Jan 05, 2013, 04:23 PM
PaulG
Newcastle, NSW Australia
Joined Dec 2004
302 Posts
Thanks

Thanks, that explains it reasonably clearly, the question now is do you use aileron differential and if so in what circumstances?
pigly is offline Find More Posts by pigly
Reply With Quote
Old Jan 05, 2013, 08:28 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
I think ideally you should have zero differential for any low CL flight such as pylon racer straightaways, counteracting the small bit of adverse yaw entirely with ail-rud mix. For high CL flight such as pylon turns or thermal circling you want a lot more differential to avoid tip stalls. And for very high CL flight such as winch launching sailplanes with a lot of extra camber you want a tremendous amount of differential, typically such that the ailerons only move up, never down.

Basically I try to setup my planes with just enough differential to tame tip stalls and fill in the rest with rudder mix. Even better is to mix differential with elevator (if your radio is capable) such that you have more diff in the thermal/pylon turns, less in the acrobatic rolls, and reverse differential when inverted.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Jan 06, 2013, 03:02 AM
PaulG
Newcastle, NSW Australia
Joined Dec 2004
302 Posts
tip stall

I fly only thermal duration models, just to get that clear and we are talking about thermal turns. Are you recommending that I reduce aileron differential, increase aileron/rudder mix to compensate until I start getting tip stalling (outer wing?) and then increase the diff a bit to eliminate the tip stalling?
Thanks, Paul
pigly is offline Find More Posts by pigly
Reply With Quote
Old Jan 06, 2013, 12:39 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
Yep, that's basically it. It may be worth doing a couple of flights with pure diff vs.pure rudder just to get a feel for each extreme. Lastly, Dr. Drela's advice is always worth heeding.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Jan 06, 2013, 01:05 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
Ah, here's the Drela post you mentioned regarding zero differential. Though theoretically ideal, the problem I have seen with this approach is that when turning near max CL the inside wing tends to stall when applying down aileron to roll out of the turn. This is resolved either thru better piloting or differential. I choose the latter.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Jan 06, 2013, 05:26 PM
F3B and F3K
RetoF3X's Avatar
United States, TX, Dallas
Joined Mar 2009
1,546 Posts
Agreed with Vespa, in F3B I apply differential as a function of CL:
100% at max Cl on winch launch to 0 differential in speed run.
In distance and speed I want a neutral pitch response upon aileron input and set up the differential accordingly. Any adverse yaw is compensated by rudder input.

Reto
RetoF3X is offline Find More Posts by RetoF3X
RCG Plus Member
Latest blog entry: more Tweagle testing
Reply With Quote
Old Jan 06, 2013, 05:54 PM
Detail Freak
target's Avatar
Harbor City, CA
Joined Oct 2003
21,348 Posts
I must be doing it wrong, I usually increase diff with speed, and decrease rudder mix with increased speed.
This has been (seemingly) working for me so far, but I haven't tried the opposite yet.

R,
Target
target is offline Find More Posts by target
Reply With Quote
Old Jan 06, 2013, 07:26 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,505 Posts
Yeah the pitch influence is another factor. Aileron differential is essentially the addition of reflex across the entire wing and as such it has a pitch effect.

Target, your reduction of rudder mix with speed makes sense but I can't think of any advantage in increasing differential. What is the behavior that inspires that?
vespa is offline Find More Posts by vespa
Reply With Quote
Old Jan 06, 2013, 07:49 PM
RV7guy
Chandler, AZ
Joined May 2004
320 Posts
All different

Every airplane is different. Only through lots of work can you get things right for you. That is the important part. Get it right for you.

Remember, trimming is a verb. An ongoing, iterative process. Regarding the differential, I followed Mike Smith's numbers for my Aspires. Seems pretty good!!!

One tip that I got from someone a long time ago was to use reverse expo on the rudder in the aileron rudder mix. That is, I have more rudder coming in early when the the ailerons are moved. I've found this gives a very smooth coordinated turn.

I also have less rudder mix than in previous set ups where I didn't use reverse expo.

Works for me!!

Darwin N. Barrie
Chandler, AZ
Team Futaba
dnbarrie11 is offline Find More Posts by dnbarrie11
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Aileron Differential RC Man Electric Plane Talk 10 Aug 14, 2012 02:08 PM
Discussion Aileron differential on a warbird? 10AE Electric Warbirds 4 Jul 10, 2012 06:39 PM
Question Aileron Differential Mr. Wiz Hand Launch 27 May 04, 2012 03:03 PM
Discussion 12FG setting dual ailerons with differential Roblister Radios 2 Apr 22, 2012 12:45 PM
Discussion Aileron Differential - Rudder FSD Slope 29 Mar 05, 2012 08:21 PM