Please help me to learn to interpret Profili data

[The Other Dave]

I am getting ready to cut some wings for a flying
wing that will be used as a sloper for both
combat and wing racing.

I have multiple flying wings but all e-powered.
This will also be my first dedicated sloper. I have
sloped some gliders and e-powered ships but no
pure slopers.

I was playing around w/ profili trying to decide on
an airfoil and making my own just to mess around.

So here we get to the meat of the question.

This is what I understand (or think I do)

cl(cd) is a measure of the minimum drag of the profile.
cl(alpha) is the lift vs angle of attack
cd(alpha) is the drag vs angle of attack
cl/cd(alpha) is the lift/drag vs angle of attack

Here is where it gets murky for me
cm(alpha) is has something to do with pitch moment vs angle of attack.

But how do you know what is good or bad for cm(alpha).
Please remember this is for a flying wing so it needs to be
fairly pitch stable.

I have included some examples from my playing around.
The David-1 airfoil was an experiment.

If I am interpreting this correctly.
From cl(cd) the HS522 has the lowest possible drag
with the David-1 and the Clark YH fighting for 2nd.

From cl(alpha) the Clark YH has the highest lift
David-1 second until > 5 deg pitch.
HS522 3rd

From cd(alpha) the HS522 has the lowest drag for most of
the positive AOA's but itsn't that good for neg.

From cl/cd(alpha) The David-1 has the highest lift but only at an AOA of 3-5 deg. And it has a narrow peak. Where
the other 2 climb more gradually and are smother.
I think this (David-1) would be twitchy to fly as it's max lift point is narrow.

Now here is where I am lost.
Please look at the cm(alpha) graph. Does this mean that
the David-1 is more stable in pitch than the other 2 or less.
I had allways heard that the clark Y series of airfoils would pitch forward around the stall point. Does that mean that up around 11-12 deg AOA that the David-1 airfoil is very pitch happy?

I also notice that around 0-1 deg AOA the David-1 airfoil is non-linear. This would seem to be bad.

It would seem like you would want the pitching moment
to be = 0 at the zero lift angle.

So based on all of the information I have at this point.
The David-1 airfoil (which was purely an exercise in makeing the tool work) would be a bad choice for a flying wing.

So basically I am looking for information on how to interpret these graphs.

Thanks

David

[BMatthews]

If this is a plank style flying wing then your airfoil isn't stable enough. But if it's a swept flying wing with washout to provide the pitch stability then it looks pretty decent. If it's swept you may want to check out the Panknin wing twist Excel spreadsheet for calculating the amount of washout required for the swept wing. You can download it from www.b2streamlines.com .

Airfoils for plank style wings need to have positive pitching moments. All 3 of the ones above are negative pitching moments for a wide range of angles of attack.

[Ollie]

http://www.aerodesign.de/english/profile/profile_s.htm
http://my.athenet.net/~atkron95/pcsoar.htm
http://www.b2streamlines.com/Culver.html
or
This single PDF document has all four installments of a series on the interrelationships between the center of gravity, pitching moment, sweep angle, and design coefficient of lift. The Panknin methodology is used extensively to provide relevant examples. Originally published in RCSD* during 1994 and 1995 to answer some questions asked by Bill Kubiak.
*
http://www.mh-aerotools.de/airfoils/index.htm

[The Other Dave]

Thanks guys.

Ollie I can't find the pdf you are talking about
on


http://www.mh-aerotools.de/airfoils/index.htm

Thanks,

David

[Ollie]

Find pdf at:
http://www.b2streamlines.com/OTW.html