Inverted Flight with Undercamber Airfoil?

[G.P.]

There is some discussion about how, why and if a Slow Stick can be flown inverted. Many of us have done it, but some still don't believe.

Here's the thread: http://www.rcgroups.com/forums/showthread.php?t=610182&pp=15

Can any of you comment on it?

Thanks,
Greg

[Sparky Paul]

It obviously can be accomplished. It's not all that efficient though.
I've had one of my SAE Lifters inverted... empty... and descending..
Those that opine undercambered airfoils can't be inverted... pay no attention to anything they say about anything.

[G.P.]

I understand that it won't be efficient by any means, but the wing will be producing lift right? It's not just flying on thrust alone, like in high alpha harrier flight?

[JetPlaneFlyer]

Interesting conversation.

I'd have to take something of a middle line between the two arguments... 'Most' airfoils are capable of generating lift while inverted. A Clark-Y certainly is and providing the model can maintain a high enough speed then sustained inverted flight is perfectly possible with such an airfoil.... However the same is not necessarily true for a heavily cambered airfoil. These would need to operate at a very high negative AoA before any negative lift could be produced and some airfoils will stall before adequate AoA is ever reached.

On the attached file I've plotted Cl (coefficient of lift) graph for a Clark-Y and for a heavily cambered BE6306... You can see that the Clark-Y happily operates in negative Cl territory. The BE3606 on the other hand stalls before ever generating 'negative' lift.

[BMatthews]

This is a job for FOILSIM ! ! ! ! ! :D

Go to http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html and you can try it for yourself.

I inputed the span at 3 feet, chord at .45 feet, camber at 6% and thickness at 2.5%

At 5* angle of attack upright and 12 mph it generates .72 lbs of lift. Sounds pretty close to a Slow Stick as I recall.

Start inputing a bigger and bigger negative angle of attack until the lift is negative .72 and there's your angle needed. It ended up being -18*. So there's certainly some truth to the idea that prop thrust is partially lifting the model. Likely to the point you'd only need -15* instead... :D

Jet plane, the chart doesn't show that it stalled from what I can see. Just that it didn't use a wide enough AoA range to continue the curve.

Certainly an undercambered airfoil will produce a nasty separation bubble pretty much instantly when loaded from inverted. But even with that separation bubble you'll still get lift. Not efficient lift but it'll still fly.

[Ollie]

A diamond shaped kite has with crossed sticks and bowed outline strings. In this type kite, it flys with an upside-down camber airfoil. The airfoil flys at a high angle of attack and far beyond the stall. The whole top of the kite is flowing with very high turbulent (some what chaotic?) flow. Still it generates lift with buffetting.

[Sparky Paul]

Putting a simple flat-bottom like a Gentle Lady inverted is easy... Keeping it there isn't. :)

[JetPlaneFlyer]

Ollie , BMathews...

You are of course quite correct... it is possible to generate 'lift' even after the stall. This explains why the wing sections in foilsim keep generating some lift (but not much) all the way to 20 Deg AoA, but only at the expense of huge increase in drag.

Using this argument then I guess 'yes' any airfoil can generate 'some' lift while inverted. However in the case of heavily cambered sections this negative lift will only be generated after the wing has stalled therefore flight, as we know it, would be impractical.

[HELModels]

That other thread shows the video and starts out upright. The camera is pointing rearward and downward relative to flight direction, maybe G.P. can say what the camera angle is. When it goes inverted, the camera angle is still downward, but slightly more downward looking than when upright. This would confirm the higher inverted AOA, but G.P. needs to show the setup to better estimate real world AOA from the video. This might not be the point of discussion, but it would be interesting and provide more proof.

[G.P.]

The video isn't mine, but here's the thread that it came from. There are some pictures of the setup.

http://www.rcgroups.com/forums/showthread.php?t=182935

[Troy]

That was an entertaining thread. I didn't know the Slow Stick was so controversial in it's flying habits. How dare it thinks it can fly inverted..... LOL :D

[vintage1]

Ive flown lots of RET flat bottomed wing stuff upside down..and under reasonable control..I learnt my lesson though when one model steadfastly lost height and it was try and do a half loop or land inverted..caused a fair amount of damage..Ive even done a sort of slow roll..though i takes a lot of doing..never done an outside lop tho.

With enough power anything is possible. Even Zagis..