Reverse flow-symetric airfoils

[PaulPounds]

Greetings all,

Ever since seeing that CRW 'Dragonfly' (by Boeing, I think) in the tip-jet thread, I've been thinking about reverse-flow airfoils. These sorts chord-symetric aifoils might be what you'd use in a stopped-rotor VTOL craft like the Dragonfly. By chord-symetric, I mean that the leading edge and trailing edge are the same shape, mirrored about the half-chord.

Obviously, if the trailing edge is shaped like a leading edge, there will be a big seperation region and heaps of drag.

However, if the leading edge is shaped like a trailing edge, there will be little dynamic pressure produced and the thing will hardly lift.

It seems to me that some middle ground may be reached - a compromise between lift and drag... just like every other aero-engineering problem in the universe.

So, aside from low lift or high drag, can anyone think of a reason why chord-symetric airfoils would be especially bad?

Thoughts?

-Paul

 

[Intermesher]

Paul,

A comment and a thought;

1/ Sikorsky's Reverse Velocity Rotor Concept [

http://www.unicopter.com/1281.html

] proposed a profile that is similar to that of the Dragonfly's except it's thickness appears to be slightly less. Also, the profile is close to being chord symmetric, but not quite.

2/ I have previously wondered if a 'conventional' airfoil might work just as well. This is because the airfoil will be optimized for 'forward velocity' and the drag resulting from reverse velocity will contribute to the rotation of the rotors.

Dave J

 

[PaulPounds]

That would make sense if you were using the wing as a rotor during forward flight - would that work if you were using stopped rotors?

One disadvantage I thought of recently was low stall angle. A sharp leading edge would make the flow seperate right at the front of the airfoil, much like a flat plate. Mind you, a stopped-rotor or slow-rotor hybrid sort of thing may not really need to operate at high AoAs.

-Paul

 

[Intermesher]

Paul,

You're correct. I was not thinking in term of a stopped rotor.

Subject to correction by others, I understand that the initial intent with the Dragonfly was to stop the 'rotor' when it was normal to the airflow. Later it was decided to stop the 'rotor' when it was pointing for and aft, and thereby reduce the drag. This sort of makes sense, since if the fixed airfoils can support the craft during transition they should certainly be able to support the craft during fast forward flight.

Dave J.

 

[PaulPounds]

Right, so the Dragonfly doesn't have chord-symetric airfoils, or even reverse-flow airfoils then?

Since the dragonfly is intended for high speed flight, it makes me wonder if it would be more practical to use stopped-rotor reverse flow airfoils for something like a glider. Of course, it would make lots more sense for a glider to simply have rotor/wing blades that could pitch 180 and capitalise on the high aspect ratio with more traditional airfoils.

-Paul

 

[GraviMan]

"I have previously wondered if a 'conventional' airfoil might work just as well. This is because the airfoil will be optimized for 'forward velocity' and the drag resulting from reverse velocity will contribute to the rotation of the rotors."

Strikes me that the best mode for this to work in, would be to disconnect the main rotor in high speed flight. The "drag" would simply provide the power to autorotate the rotor. The retreating tip would provide lift, although serious blade twist would be required. Obvoiusly a pusher prop is required...

If the design was a jet tip rotor, couldn't the jet be bled over the now trailing (originally leading) edge to avoid separation? Admittedly this involves the complication of a valve actuating cyclically, but gotta be worth it for drag...

Mart