hello everyone at rcuniverse.

maybe someone can answer my question?

i want to build multi taper glider wing and use 4 different airfoils in wing.

airfoils are hq series and all airfoils are exactly same with respect to camber etc. differing only in thickness. thickness starts at 9% at root and then 8.5% then 8% and at tip 7.5% it is 3 taper wing

my question is, do i need to use any washout in wing?

my thinking is since airfoils are exactly same except for thickness then if i design wing with proper lift distribution wing should not need washout.

but i have feeling that maybe since airfoils thin as they go to tip maybe cause aerodynamic washin?

but i not sure so any one can comment i appreciate it.


Hai Dang


i ask this question elsewhere but i still confuse and please excuse my english

This is very commonly done in full-scale, for structural reasons - the airfoil gets thicker at the inboard end, where bending moments are highest, to accomodate deeper, stronger spars, and thinner at the tip, to reduce drag. It is always used in combination with washout. The thinner airfoil at the tip will stall at a considerably lower angle of attack, so it must have a few degrees of washout to avoid tip stall and the adverse yaw that results.

Welcome Hai Dang....your English is very good sir!

Your question is a very good one. I'm interested in hearing an answer from some of the aero professionals that frequent this forum. I've had the opinion that a wing with a thicker tip airfoil percentage would reduce tip stalling tendencies and have aerodynamic washout but I've seen various internet responses to this question from gentlemen with much aero engineering expertise say this isn't necessarily true. I think it has more to do with the zero-lift angle of attack of the chosen root & tip airfoils.

Along with you, I am awaiting a learned response to your question.

Strength wise, what is said above is true, gives more strength for inboard sections to be thicker. However, airodynamically, you want the opposite, thicker at the tips. This helps prevent tip stall but the difference is almost negligable at the model size and speed we use in RC.

For a large...> 3M plane, this effort might pay off, but in the 2M and smaller sizes the effects will be minimal.
Generally, such a multi-section wing uses profiles with similar characteristics, but with the tip section selected to have its greatest effect with an angle of attack where the root is stalling, so the tip remains flying.
Look at the Cl-alpha curves for each profile, and select the tip to have a later stall break than the root.

thank to all for response.

wing is 3.2 metre.

i look at polar for airfoils, root is hq2.5/9 then next section hq2.5/8.5 then next section is hq2.5/8 then tip is hq2.5/7.5

and i think hq2.5/8 and hq2.5/7.5 need a bit of washout so to stall them later than root.

but what if i desing lift distribution eliptical and c/l stronger at tip?

i mean local c/l much higher at tip maybe no washout needed then?

so even though thinner tip airfoils produce less lift washout not needed because I desing wing with much higher local c/l at tip to compsate?


thanks.



Hai Dang

Welcome to RCU......

Your thinking of the Cl's is reversed. To obtain an elliptical lift distribution you need to have a progressivly lower Cl towards the tips. Not higher.

As mentioned washout can be used to delay stalling but the optimum washout twist only works at one angle of attack. Higher angles will produce the desired delay in the stall but lower angles of attack, as when trying to penetrate to get away from sink, can reduce the angles to where the tips are doing nothing or are even below the zero lift angle and are adding drag by being pulled effictively "upside down" through the air by the rest of the glider.

If you look at the polars for that family of airfoils I think you'll find that the thinner versions have a lower best Cd/Cl but that the sideways U shape is narrower than the thicker versions. This means you have an airfoil that has a much lower tolerable angle of attack range and will stall sooner than the thicker options.

If you wish to stick with the HQ series and still taper the wing towards the tips I would suggest that a more suitable setup would be the 2.5-9 out to the first break, then taper to a 2.5-8 at the next break with perhaps 1 degree of washout through that panel and finally the tip panel would taper to a 3.0-7.5 or 3.0-7 at the tip with no further washout or only slight washout. The added camber would provide the aerodynamic equivalent of washout.

My options will need some fine tuning as it's more concept than calculation as I write this off the top of my head but the idea is to have a wing with a minimum of physical washout so that the overall efficiency is not sensitive to the angle of attack like a more twisted wing would be.

I think that the idea of an elliptical lift distribution is a bit flawed. It may be perfect for aircraft that fly at one speed or as an aid to structural design but I think it's equally as important to match the wing segments to the work being done.

For example in your glider I would consider the high cruise speed typically used for exploring as much ground per foot lost as possible. Usually considered to be the best L/D but in truth I suspect most glider fliers tune for a slightly faster speed because they are working with visual cues and probably overestimate the speed needed. Anyway, at that speed I would aim for the inner 50-69% of the wing to do the major portion of the work, the next panel would taper and twist such that at the end of it the Cl was either 0 or very slightly above that value and finally the tip panel would provide no lift under these conditions, being set at the zero lift angle. My thinking being that with no or little lift there is a greatly reduced tip votex formation and thus lower drag.

Then as the angle of attack approaches the stall you need to look at how the airfoils react to the stall onset. If the tip wants to stall first then you can add camber or thickness or both to act like the aerodynamic equivalent of washout to delay the tip stalling.

Obviously this idea would require a bit of playing with something like Xfoil but hopefully you get the idea that I think it's important to study the wing's operation at the extremes of the desired speed range and juggle the factors to best satisfy that range.

thank bmathews,

i think i maybe understand a bit now.

still a bit confused though.

i use a program call liftroll to dising wing and i dising wing with c/l lower toward tip (my mistake in writing to say c/l higher) so lower c/l curve toward tip shows increased lift than at root.

root should then stall first correct? then tip keep flying and tip stall reduced even though tip airfoil is thinner and no washout is applied.

or what if use thinner airfoil at root and accross span of wing and then use thicker airfoil at tip with washout to compsate for zero lift angle difference of airfoils so even though tip is thicker and washout used then wing 'sees' same zero lift angle accros span?

so tip stall prevented and wing works at same effecency accross span and drag reduced.

i read some book by martin simons and i not sure if i understand what he is saying but i think he tell that above is good method.

rcuniverse is a great help and many thanks to all. i learning much now.



Hai Dang

Arranging for the zero lift angle all along the span, which is aerodynamic twist, needs to be in concert with aerodynamic twist, according to Simons, when choosing more camber at the tip..
The difference between the root zero lift angle and the tip zero lift angle determines the geometric washout you will need.
The problem in model sized chords is sufficiently accurate data for the Reynolds numbers the wing will fly at.
Flow breaks down rapidly with lower Rns, as seen in the various wind tunnel data polars, so it's important to keep the Rns where the data looks good.

"The difference between the root zero lift angle and the tip zero lift angle determines the geometric washout you will need."

now i think i understand sure.

if root zero lift angle is say 3.5 degrees and tip zero lift angle is 3.75 degrees caused by increased camber then i need to washout tip by .25 degrees of geometric washout to prevent washin which cause tipstall.

sound correct?


Hai Dang

Yes, your conclusion is correct.