Airfoil reflex help

[merrelland]

I need some help to understand airfoil analysis.

I'm trying to make the questions clear. If it makes any difference, I'm thinking of a glider in a conventional planform. Something that is similar to most slope racers. Can assume a V-tail if that helps.

Below are polars at Re1,2M from 3 airfoils. 1 is RG14 thinned. 2 is same airfoil with 2 degree flap (reflex) at 20% chord with NO adjustment to centerline. Airfoil 3 is same as airfoil 2 WITH centerline adjustment.

Airfoil 2 and 3 are the same I know. I have included both in the polars just so my questions are clearer. It may be of interest to some to note that polar pg1 shows no difference between airfoil 2 and 3 (because they are the same) but polars page 2 and 3 appear different (because the graphs are scaled on AOA).

I'm not asking what happens if you reflex the airfoil while flying (which may be the source of some confusion in any case). My questions relate to analysis of the difference between airfoil 1 and 3 if treated as different airfoils. My polars are at Re 1,2M only because it shows maybe a plausible scenario where you might prefer reflex. Actually it has nothing to do with my question.

Ok, finally to the questions.

Question 1. If I am choosing wing incidence for the 2 different airfoils 1 and 2, I think I would want to pick a cruising speed and then mount the wing with the AOA that gave me the best L/D at that speed.

So, if I have decided that I want a cruise at Cl of .1 at Re 1,2M, then,
(from polar page 2).
for airfoil 1 I would choose A= -1.0
for airfoil 3, I would choose A= 0.2

This is just to set up the incidence for optimum AOA for cruise I think. I'm not looking at any other flying conditions for the moment.

Based on polars, I would expect airfoil 3 to perform better at this cruise speed (Re1,2M) in straight and level flight. Are there any other implications to mounting the two airfoils differently? Is my thinking process correct?

Question(s) 2

(From polar page 3) At those respective AOA determined in Question 1 for optimum cruise, airfoil 1 returns Cm = -0.22 and airfoil 2 returns Cm = -0.09. Both are negative Cm.

A. What are the implications of this if we compare the airfoils for use on the same planform?
How does this affect stability and/or handling, or....?

B. What would be the normal tweak to decalage or planform or tail moment or.....? if you were working with airfoil 3 as compared to airfoil 1.

I understand that there are many factors working together but I'm trying to focus my understanding on a narrow condition.

Thanks,
Merrell

[Ollie]

Read about Dr.Drela and Phil Barnes:
http://www.rcgroups.com/forums/show...+of+incidence++
http://www.rcgroups.com/forums/showthread.php?t=438965

[merrelland]

Thanks for the links Ollie!

So for question 1 which is refering to the wing incidence angle (wing mount to fuselage centerline), your saying it doesn't make any (much) difference.

Per Phil's post"
The first thing to understand is that the only thing that matters is the decalage, or the angle of the wing relative to the fixed portion of the horizontal tail. We talk about setting the tail at an angle relative to the boom and the wing relative to the fuse etc. but that is all just a way of indirectly getting a certain relationship between the tail and the wing. If you are flying a slope racing plane or maybe even an F3B plane then you might be interested in having the fuselage meet the air at just the right angle for minimum drag at high speed but for the purpose of a thermal duration plane I think we can neglect that for now and just worry about setting the tail relative to the wing."

Since this is higher speed, I assume that my thinking then is correct to line up the fuselage with my desired highspeed cruise AOA.

Now to question 2 regarding Cm implications...

I didn't find anything in the links regarding Cm implications between the 2 airfoils. I know the design dynamics are a grand symphony with blind men feeling elephants and all...... but,

For the two airfoils flying at Re1.2M with wing incidence as above, what would be the implications of the different Cm of the two.

No predicted difference?
Probably would require different CG and decalage? Not
Probably would be more pitch stable? or less?
That's what I'm trying to get to without building the two wings and trying it.

Maybe I'm thinking too narrowly, but I'm thinking ....
take a trimmed out plane with airfoil 1 and replace it with same size/shape/weight airfoil3 at new wing incidence. What would I expect at the cruise speed of Re1,2M?

And on the other hand, what would I expect to change in design from airfoil 1 to 2 if I'm starting from scratch with the planform.

Thanks for your help,
Merrell

[Ollie]

The airfoil is not the wing. The airfoil polar plots apply for the two dimensional part of the flow around the wing. The third dimensional part of the flow accounts for induced angle of attack and induced drag. The wing design accounts for aspect ratio, sweep back, taper, twist and AoA. See:
http://aero.stanford.edu/WingCalc.html
it is a simple wing with one taper.
For a complex planform wing see:
http://www.amadistrictii.org/cjrcc/wing2/wing.html

You are not ready to compare airfoils in design. First consider the space in the airfoil for the spar and the spar strength and stiffness. Begin with the design of the spar. Apply the wing lift distribution and the maximum airplane G's for the spar load.

You are not ready to apply the airfoil polar graphs. First study aerodynamics.
See:
http://www.carstens-publications.com/
Book Hanger
Model Aircraft Aerodynamics (A11007) Simons.
The fourth edition has been almost completely revised and redesigned, as well as new material added. 344 pgs.; Sftbd.
$19.95

[BMatthews]

You're concentrating so hard on one aspect that you're forgetting the rest of the flight.

Reflexing an airfoil like this will reduce the pitching moment and reduce the need for stabilizer area. But remember that you're only flying fast with a reflex for part of the flight. When it's time to slow for that light lift or crank down the flap for the landing then you increase the camber of the airfoil and the pitching moment radically. So you need to set the stabilizer to be large enough that it can handle the worst case condition.

The camber changes of the airfoil as the flap is moved around does require changes in decalage or elevator trim. This is why many of our sailplanes use full flying stabilators in order to avoid the drag of a cambered airfoil shape in the stabilizer when radical trim is needed. The decalage required also is speed sensitive so as speed changes so does the decalage in order to trim to the new speed. Another good reason for a full flying stabilizer.

CG is set primarily by the areas and chords of the wing and stab and the tail moment length. While my own thoughts are that the airfoil camber (and thus pitching moment) plays a part it's apparently a fairly minor part with more or less "normal" tail moment arm lengths and tail areas. Certainly I have not seen a tail volume calculation or associated CG selection equation that includes the airfoil pitching moment in the process. However tail volumes are not something that follows a set value. There is an acceptable range of values and I suspect that highly cambered airfoils are happier with tail volumes on the higher side of the range.

[Ollie]

Oops.

[merrelland]

You're concentrating so hard on one aspect that you're forgetting the rest of the flight.



You are right, that's what I'm doing. I promise I'll remember to consider the rest of the flight before I launch. :)

Your comments are right on point, thanks.