Anyone have the correct zero lift angle for sa7035/36/38?

[hwangan27]

i look eveywhere and found difrent values for zero lift angle on thes 3 aerfoils and i have no idea which are ones correct and i need correct ones for my wing desing thanks to anyone can point me to correct figurs

hwang27

[JetPlaneFlyer]

Here is an X-Foil generated graph.

[hwangan27]

thank jpf. i see that graph to say sa7035 -1.9 then 7036 -2.3 then 7038 -2.8
i am very beginer at this an i hope i read corect

hwangan27

[JetPlaneFlyer]

hwang27,

Yes have the right idea... I'd say that it was closer to -1.9, -2.2 and -2.6... but now I'm splitting hairs.

To make it easier to make out the exact point where the curves cross the line here is a graph zoomed in on that point:

[Ollie]

The problem is the geometry profile plus the boundry layer. So, any measure for polars includes effects of the thickness of the boundry layer. The thickness of the boundry layer changes with reynolds number including separation bubbles. At low speeds, expect the zero lift angle of attack to shift a bit with change of reynolds number.

[hwangan27]

ok ollie i think i unerstan a littl of what you say in your pos
queston--the numbrs jpf suplied mus be close for therml speeds i wan fly at so what if i jus add a little bit mor washout to "counter zero lift angle of attack" (quot from othur site) to be on safe side? better bit mor than not enough i think....

hwnangan

[hwangan27]

ok ollie i think i unerstan a littl of what you say in your pos
queston--the numbrs jpf suplied mus be close for therml speeds i wan fly at so what if i jus add a little bit mor washout to "counter zero lift angle of attack" (quot from othur site) to be on safe side? better bit mor than not enough i think....

hwnangan

[hwangan27]

sory for doubl post.

[Ollie]

The three airfoils are used in one wing. One airfoil at the root, second airfoil between (root and tip) and the third airfoil at the tip. The reason you want the three airfoils have zero lift angles of attack is because the aerodynamic twist (wash out). The wing effect washout is the sum of geometry AOA and aerodynamic AOA. At aircraft stalling air speed, the wing has taper then the root chord has a reynolds number a smaller reynolds number between and the tip chord has smallest reynolds number.

[hwangan27]

Hello Ollie, I am Hwangans' english teacher and my name is John. Hwangan has asked me to clarify a point as his english is at (beyond actually) it's limit in this discussion. I know nothing of these subjects but am happy to see one of my students striving like this so offered to lend a hand.

Question--Since as you say in your reply "At aircraft stalling air speed, the wing has taper then the root chord has a reynolds number a smaller reynolds number between and the tip chord has smallest reynolds number." does Hwangan choose the washout value based on the stalling speed at each individual chord station reynolds number? relative to three different airfoils?

He thanks you Ollie for your patience with his questions.

John Patterson

[Ollie]

John,
I suggest that Hwangan has two language problems english and math. Understanding the polar graph math is one nub of his problems. Another nub is applied use of the polar graphs in advanced beyond just math. He must understand aerodynamics before using wing design. What is his purpose? I must understand Hwangan's background before giving him good advise.

[Ollie]

There is more:
The popular airfoil SD7037 was later developed for a whole improved family with SA7035, SA7036 and SA7038.

Hwangan's question about wing washout depends which airfoils from root to tip (airfoils first to last), which wing planform (chords), which sweepback, which range of AOA (or Cl), air speed, wing span, etc. It is not simple question because it depends on lots of aerodynamic knowledge.

[biber]

Be aware of the fact that Xfoil and other codes have some trouble with finding the correct zero lift angles of airfoils.
Consider the zero lift angles calculated by these codes as a rough estimation.
Other things, like e.g. drag polars might be predicted far more accurate by them.
OTOH if you want to compare just very similar airfoils, it might work quite well, though.

biber

[BMatthews]

That may be but in model building and flying we seldom achieve absolute accuracy to the point that the errors from the Xfoil program really matter. And besides, I would expect the errors to be consistent within this narrow range of shapes to the point that using the data to design a wing would still result in the airfoils all reaching their zero lift or very close to it at the same time if the wing was designed using these numbers.

As for the Reynolds number situation we need to look at how the model will be flying at or close to the zero lift angle.

Technically the wing would be in a vertical dive (or nose straight up to a stall :D ) when operating at the zero lift angle since zero lift means no support. At small angles of attack approaching the zero lift angle we're talking very low lift coefficients which implies speed runs such as fighting to get back upwind or an F3B model in a timed speed run. At those speeds you need to figure out the Reynols numbers as well but they will all tend to be higher due to the speed of flight that goes along with operating at low lift coefficients and low angles of attack.

As the speed drops and the lift coefficient goes up to compensate and the reynolds numbers lower due to the lower speed the zero lift angles loose their importance and the focus of attention shifts to how the airfoils and chord lengths and resulting Reynolds numbers affect the wing as it approaches the minimum sink speed.

None of these factors are ever constant. Every aspect is changing radically as the wing alters its flying speed.


I'm guessing that the idea is to use these three airfoils in conjunction with some washout twist but so that for high speed all the sections of the wing will reach the zero lift at the same time. That would suggest that the 7035 would be used at the root and the 7038 be used at the tip in conjuction with only about 0.7 degree of washout. Now that's all fine in theory but in reality you'll find that this small an amount of washout will not work well for flying the model and that you'll need a couple more degrees of washout in order for the model to have more friendly flying charactaristics. But that means that your wing won't all reach the zero lift angle at the same time.

As with many things in life compromise is the key.

[Ollie]

Please read this:
http://www.geocities.com/jebbushell/garton2m/garton2m.htm

[hwangan27]

thank to all for reply, the mike garton 2 meter is exacly what i want to buil (thank ollie for tht link as it explan exactly so well what i wan to do) only i want to buil glider with 3 meter span insted of 2 meter and i thinking having the sa7036 aded to wing would help with make wing more efecient and "tip stall resistant".

if adding sa7036 make things to much more comlicated then i will go with sa7035 across span to sa7038 at tip an use known washout valu of .7 degre that mike garton tells in his buil story

but i am stil thinking that sa7036 can be used if washout required to match zero lift angel of all 3 aerfoils is used, say sa7035 root not washout then sa7036 with .3 degre washout an then sa7038 at tip with .7 degre washout.

Hwangan

[Ollie]

Your simple question doesn't go to your true purpose!! Please, study better questions and their answers.

Please read this:
http://www.amadistrictii.org/cjrcc/wing2/wing.html
http://www.geocities.com/jebbushell/COOKBOOK.htm
Download Liftroll
You need this for your wing design!!!!

[hwangan27]

sory for not ask perfect questons or knowing which questons to ask i will try to find a place for beginer such as mysef where anger is not in some replies

H

[Ollie]

H,
I am sorry but, sometimes the truth is painful and hard. If you don't like my version of the truth, please, skip over it. It is your choice.

[BMatthews]

Hwangan27. It seems like you're trying to design this model to be as perfect as it can be according to the numbers. But at some point you will need to fly it as the pilot.

And that is where compromises will need to be made. The wing you seem to want to design is trying to use the very small differences in airfoils to have a built in washout without having to twist the wing. But it's quite well proven that you need more washout than what the airfoil choices will give you if you want a model that is easy to fly in most, but not extreme, situations. With as little geometrical washout as you'll have with just the airfoil shape the wing will still want to tip stall if you try to turn at more than a very shallow bank angle. With a two degree twist along with the airfoil choice it would be a lot more gentle for the pilot to fly.

Besides, while it may seem that using the airfoil shape to provide the washin is a good thing this is not the best way to get the best performance. A lot of the drag in a wing comes from the tip votices. Using wing twist of 2 to 4 degrees may have some small drag penalty at higher speeds but at lower speeds this wing twist results in the airfoils at the tip not working as hard and a reduction in the strength of the tip vortex and thus a reduction in one of the major sources of drag. The more twist the lest drag you can have at one speed but the more narrow the speed range. The less twist the wider the speed range where the angles won't work against you but more drag from the vortices you have at thermalling speeds. Again a compromise needs to be made in order to get the best performance at the focus of where you want the model to fly.

It's the same with the Center of Gravity position. If you put your CG at the neutral point the model will be as efficient as it can be but the pilot will not be able to fly it out to a long range where it's hard to see because he will need to be constantly making small elevator corrections. So again we compromise by putting the CG slightly in front of the Neutral Point at a position that matches our flying ability and comfort for flying in difficult conditions.

I hope that helps.