Airfoil Ideal Angle of Attack

[Chuck Clemans]

Compufoil lists the Ideal Angle of Attack for the Clark Y airfoil as 0.91 degrees. How do I make use of this information? Should the chord line of the wing section be at + 0.91 degrees relative to the chord line of the stabalizer?

My application is a medium size electric powered rc aerial photography platform.

Thanks to all who respond.

Chuck

[Ralph Walton]

1 degree will get you in the ballpark. If possible, make wing (and horzontal stabiliser)adjustable, then you can optimise your setup, then permanently fasten/glue into position.

[nmasters]

From the charts I'm looking at it looks like that's the AoA for minimum drag. Using the cl at that AoA reduced by a correction factor (say ~10%) you can find the weight that a given wing can support at a given speed, or how big you need to make a new wing to carry the weight

--Norm

[No Step]

That's about what I end up using most of time. I set the wing (Clark-Y is what I use much of the time) at +1 degree common to the datum line of the fuselage, which is parallel to the stab. center line ("0" on a flat airfoiled stab). Even then, I find myself having to trim in some down elevator much of the time due to the high lift of the Clark-Y,... so yeah, an adjustable stab would be a good thing to do, but many times not able to do. Which reminds me of a question I have on Stab. incidence. I'll start another thread so as to not distract from this one.

Btw, Hello from Everett.

Fred https://home.comcast.net/~guilfoyle72/

[BMatthews]

It's a little more complicated than that.

For your case I'll assume that there's an angle you will want the fuselage to fly at in normal cruise such that the camera is held at the optimum angle. Meanwhile for that cruise speed your wing will need to fly at a lift coefficient that is determined by the wing's area and the flying speed. That lift coefficient is based on the angle of attack and the airspeed for that wing. So if you want the fuselage to sit level in flight and the wing needs to show an angle of attack of 3 degrees then you'll want to set the wing's angle of incidence at 3 degrees to the fuselage center line. Keep in mind this will only hold for that one single airspeed.

A quick and dirty but effective way to figure this out is google for foilsim and configure the built in test wing and airfoil to something close to your design. Then "fly" the wing at a realistic cruise speed and alter the angle of attack until the wing makes the same amount of lift as you expect your model to weigh. The angle of attack that produces that much lift is pretty much what your wing will need to fly at for the real model.

[Chuck Clemans]

No Step, when you mount the Clark Y at +1 degree relative to the fuselage datum is this with reference to the Clark Y chord line or the bottom of the airfoil?

I'm looking forward to the first flight of my Orb Aerial Photography plane when the weather improves in the Bellevue/Everett area. This is a Mike Evans design which can handle anything up to a DSLR camera. The Orb has a Telemaster type flat bottom airfoil mounted at about +1 degree relative to the bottom of the airfoil.

Orb Build thread:
http://www.rcgroups.com/forums/showthread.php?t=905733#post10302561

Chuck

[nmasters]

Chuck--

That's got to be the cutest AP rig I've ever seen :D

Fred--

Thanks for the link to your fleet. I love pictures of naked wooden airplanes. I especially liked the KA-8a

Then "fly" the wing at a realistic cruise speed and alter the angle of attack until the wing makes the same amount of lift as you expect your model to weigh. The angle of attack that produces that much lift is pretty much what your wing will need to fly at for the real model.

Coincidentally I posted a spreadsheet in this forum that dose a lot of what you described here just a few hours ago. The twist part is specific to flying wings (it's Walter Panknin's formula) but the lift part in the bottom yellow rectangle is pretty generic. just put your planform and design cl into the input column. leave the airfoil characteristics set to 0 and set the static margin to something low. I just ran profili on the Clark-Y at Re=80,0000 and Re=15,000 and it showed the minimum drag angle to be -1 degree (that's negative one). The cl at that AoA is between 0.1 and 0.3 depending on the Reynolds number. Use my spreadsheet or the program BMatthews suggested to find your Re and I'll send you the polar with some notes.

--Norm

[No Step]

I use the actual chord line, not the bottom surface for measuring. What a fun looking model! That is so funny! Talk about drawing attention at the flying field! I need to read through that build thread. Shoot,.... I bet you were giggling thru the entire build.

[Chuck Clemans]

I asked Eric Sanders of Compufoil the question regarding Ideal Angle of Attack and this was his response:

"The ideal angle of attack is relative to the chord line and is the angle that provides the least amount of resistance. The angle that you mount the wing at will depend on how heavy the plane is and how much lift the wing provides. At cruising speed, you want the wing to provide enough lift to keep the plane at a constant altitude with the fuselage in a streamlined straight ahead attitude. Three degrees is a very common angle of attack for the wing."

That pretty much summarizes what others have said.

For those that enjoyed reading the Orb build thread, here is the link to the first of such designs by the very creative Mike Evans. I had a ball flying my Beast while on vacation last summer. Got some great stills and video using an inexpensive camera.

http://www.rcgroups.com/forums/showthread.php?t=700572&highlight=beast

Chuck

[BMatthews]

Any ideal angle of attack where the drag coefficient is minimized is far more applicable for full sized long range airplanes than for us modelers. We seldom fly at a fixed speed for more than a short time. Even for a photo plane the time spent at a fixed speed cruise is minimal and typically fuel consumption, be it liquid or electrical, is not a big concern.

Also it's not so much that there is a single fixed angle of attack as there is a range where the change is minimal. Stay within that range and the model will fly at speed with less power be it in the climb, at cruise or in the glide. If the airspeed stays the same the angle of attack will be constant. The only thing that changes then is you add power to climb at that speed, reduce it to cruise at that speed or take it away and trim the elevator to glide at that speed.

The trick is to realise that the angle of attack can stay the same even when the model is climbing or gliding. And when it does that means the airspeed is constant.

Now consider that there's a range of speeds rather than one single optimum speed. For some airfoils the range may well be very narrow and for some others it's quite wide. Which is what can be seen by looking at a chart of lift to drag coefficients. The typical airfoil polars that we often see. For a common chart showing Cl on the vertical axis and Cd on the horizontal one the airfoil with the more open and smooth curve to the left side of the chart will be the one with the broadest optimum speed range.

And just to show that I'm not all numbers and science I have to admit that I had a good chuckle at Chuck's Orb... :D

[nmasters]

Also it's not so much that there is a single fixed angle of attack as there is a range where the change is minimal.

Yes and if you size the wing so that it is operating at minimum drag at cruise speed you'll be able to stay in the low drag range during maneuvers. See it's not just about cruise efficiency. When you bank to turn the weight effectively increases and the wing has to generate more lift to maintain altitude. If the wing stays in its low drag range you don't have to mess with the throttle so much.

[eflightray]

And all this time I had it wrong.

[DT56]

Well, Eflight nailed it.