slowflyer airfoil for AP

[Bill Harris]

As you recall, MX was building a dedicated plane for AP, discussed in this thread:

http://www.rcgroups.com/forums/showthread.php?s=&threadid=170493

The results were that "the thinner flat bottomed airfoil resulted in a much faster flying plane - too fast for my AP needs."

He used a Selig 3021 profile.

I need an airfoil recommendation for a dedicated AP plane. It needs to fly a slowly like a GWS Slow Stick but has to penetrate for windy conditions. What I envision is a flat-bottomed airfoil with a hinged TE (much like full-span ailerons) that could be lowered to simulate an undercambered airfoil for low-speed, high-lift situations and put level with the bottom of the wing for penetration and speed. Flaps and spoilers could be a side use of this. Much like reflexing the flaps on a full-house sailplane.

Thanks,

--Bill

[Sparky Paul]

This will do that.. without flaps.
For penetration, add ballast..
Gentle Lady..

[Bill Harris]

Ah, the venerable GL airfoil...

That is a good beginning.

--Bill

[Sparky Paul]

I'm curious as to the speed that's too fast.
When I put an AP up about 600 feet, it takes several seconds for it to fly far enough to get an image of a different part of the subject with a reasonable of overlap to splice two or more inages together.
The cycle speed of the camera also requires some time between images.
Could this the "speed" problem?

[Bill Harris]

I assumed that it was a camera cycle speed or a "shutter speed" issue since he uses a Pencam.

It might be a good idea to find out why it was "too fast" before tossing out that profile.

I'll ask him.

--Bill

[Bill Harris]

I asked. It is a landing speed issue.

Understandably.

--Bill

[builder]

Here's one you might consider, a 12 percent thick section that has a lot of drag at very high angles of attack, making for a slow and short landing. It was tested with an open structure wing and the results agree with the polars shown here.

[Ollie]

Builder,
The polars of the Ew1 are very impressive and appear to be superior to some well established high performers like the S3021.

Would you tell us where to find the coordinates? I would also like to know how the open bay version was tested and where to find the raw data from those tests. Where was the test section open bay? Where was the test section sheeted? What was the chord and what was the rib spacing? What was the zero lift angle of attack? What were the stall characteristics? What was the pitching moment coefficient versus angle of attack polar?

I couldn't find the Ew1 in the NASG or UIUC data bases.

[Bill Harris]

Builder--

That looks good. 12 percent with a Phillips Entry.
--Bill

[Sparky Paul]

My copy of Profili 2.13d doesn't have the EW-1, but I see he used 2.14.
I guees it's time for an upgrade. :)

[builder]

The EW1 is a home brew airfoil using the common section S 7055 as a starting point. The wing tested was an open structure with no sheeting. The ribs were 2 inches apart and the chords were 8.25 and 5.75. There was almost no sweepback of the leading edge, and the span was 41 inches. I used a 2 degrees positive angle on the wing. Compufoil says 1 degree is ideal, but maybe for a faster flying or bigger model. Here is a picture of the test model. The first flight was in conditions windy enough to roll the model backwards down the runway (they said 20 KMH) but once airborne it handled it no problem. I could not have flown my Pico cub in the same conditions. Pointing the nose down a bit gave me very good penatration, and trimming back the airplane would slow it to a crawl. The landing was made at almost zero speed in the 20 K wind. Those of you with Profili might want to test it against the 7055. A friend who has Profili tested the EW1 airfoil for me. I have ordered it and will be doing more work on airfoils.

[djcougarshuttle]

i have been playing with airborne video and aim to design a plane for that purpose too. i like the ew1 airfoil idea and put together a prototype build structure for it with some of the techniques i have been using with reasonable success. all comments appreciated.

i'm not sure what to do about washout or tapering. the purpose of the plane is to lift a relatively heavy video transmission system to as high as possible and land it safely - sometimes in smallish spaces - with my rookie pilot skills. throttle/rudder/elevator.

[Ollie]

Taper and twist affect the lift distribution of the wing at various angles of attack and were the wing will stall first (how much if any tip stall margin). There is an on line applet available that will do the calculations to help you make these design decisions. See:
http://aero.stanford.edu/WingCalc.html

[djcougarshuttle]

i went and tried some numbers in WingCalc and i see the curves. my instinct was to try to get the curves (co. of lift and fractional co. of lift?) close together and niceley rolling off all the way out (like an ellipse...)

but really, i don't know what the implications of these curves (from taper and twist) are for my design. i assume a lot of wings get built without taper because of ease of building? i wouldn't mind the build for greater lift : drag. but wasn't high drag at high alpha one of the good characteristics of the ew1? i'm confused :rolleyes:

as for washout, i will spend a lot of time climbing so avoiding a stall with my nose in the air all the time will be important. i'm thinking positive washout would cause the tip to stall first and automatically correct the aoa in many situations? is that a good idea?

thanks.

[Mike Taylor]

thinking positive washout would cause the tip to stall first
This is true, and is NOT what you want. If one tip stalls, the other (still flying) wing tip pulls you into a spin. Tip stall is bad - especially if both wings are moving at different speeds - like when you are turning.

You want that stall to start in the center of the wing...

[djcougarshuttle]

thanks - sorry this is a little elementary for this forum.
now i understand the tipstall.

so negative washout then would help avoid stalls in a stable way.

any thoughts on taper for this wing?

[Mike Taylor]

Lowering the AOA of the tips in relation to the root (washout) will make the wing less prone to tip stalling.

Having the tips at a higher AOA than the root (washin) is a bad thing.

The stall should start in the center of the wing, where there is no long level arm like there is in a wing tip, and where there is a higher AOA. This drops the nose (by reducing the lift) gently in the middle of the wing, and both tips are still flying.

Even with wing tip washout, tip stalls are still possible, but harder to acheive.

Other ways to reduce tip stalling is to use a thicker section or one that is more symetrical as you near the wing tip, to add turbulators to the outer portion of the wing, etc., but washout is easiest for the builder to build in.