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Airfoil suggestion.

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Airfoil suggestion.

Old 10-25-2002, 01:31 PM
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aubej
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Hi guys,
I'm wondering what would be a good airfoil for a prop jet I would like to scratch build. I'm looking for something fast with good flight characteristics. This wing will be swept back. Any suggestions?
Thanks,
Jason
Old 10-25-2002, 02:02 PM
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C5galaxy`
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Do you have any background in aerodynamics? Dont mean to insult if I have.
Old 10-25-2002, 02:09 PM
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aubej
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No insult taken.
I'm a Mechanical Engineer. I studied Fluid Dynamics when I did my degree. Although that's been a while now.
Jason
Old 10-25-2002, 04:01 PM
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Ollie
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A fast airfoil has low drag at low lift coefficients of zero to 0.2. This usually calls for a thin airfoil. The airfoil has to have a high enough maximum lift coefficient to fly slowly for landings and takeoffs. How slowly and at what wing loading? The maximum lift coefficient will also determine how tightly the plane can turn at high speed without stalling.

Could you be more specific about what you mean by good flight characteristics? Stall resistant or easy stalling for spins and snap rolls? Inverted performance?

Why a swept wing planform since you won't be aproaching the speed of sound? Very few full scale planes use very much sweep back except to avoid compressibility problems (which models don't encounter). How much sweep did you have in mind?

Airfoil selection involves structural considerations too. The airfoil has to be thick enough to contain a spar that is strong and stiff enough. How many G's do you want to be able to pull? What aspect ratio did you have in mind?

It would help resolve conflicting objectives if you would prioritized your specific performance and handling goals.

Do you feel like you have opened Pandora's box?
Old 10-25-2002, 04:12 PM
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aubej
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Hi Ollie,
Pandora's Box....yes.

The swept back design is based purely on looks. I don't expect this thing to be super aerobatic. Spins and snap rolls are not an issue. I'd like a sleek looking fast plane that looks similar to the BVM Super Bandit pictured below.

Obviously, everyone would love a plane that would fly 200 and land at 5. Since there is a give and take when it comes to airfoil design, this is not what I expect.

An airfoil similar to the GP Patriot or DC Tiger Shark would be ideal.

Any suggestions? Anybody know what airfoil these planes used?

Thanks,
Jason


Old 10-25-2002, 05:42 PM
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Ollie
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Sorry, I don't have any information about the airfoils used on the two models you mentioned.
Old 10-25-2002, 05:44 PM
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aubej
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Any suggestions?
Old 10-25-2002, 07:32 PM
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Ollie
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The S6063 with a slight reflex in the trailing edge of the wind tunnel test model has the lowest drag coefficient of any airfoil that I have seen reliable test data for. It is similar to the MH33 which is used on pylon racers. It is only about 7% thick so it would require special consideration of the wing spar design and torsional rigidity of the sweptback wing structure. The S6063 has a low drag range from a coefficient of lift of zero to 0.4 and stalls at a lift coefficient of about 0.8. This airfoil will be very fast but, the practicality of its landing and takeoff speeds will depend on the wing loading you design and build to and possible use of flaps.
Old 10-26-2002, 01:34 AM
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Default Airfoil suggestion.

I like Ollie's suggestion for the S6063 but you're shy on a lot of detail here.

How big are you planning on making this model? And by "prop jet" and the pic of the Super Bandit I gather you're looking at something that would fit into the "extreme speed prop planes" forum.

If so then you could do a lot worse than to use any very thin and low camber model specific airfoil. Options that come to mind from a quick scan though my airfoil plotting software are the Quabek HQ 1.0/08, the Martin Hepperle MH22 designed for F3D plylon racing, the Rolf Gersberger RG14 or RG14a or finally the Selig S2055, or a thinned to 8% SD6060. These are all thin low camber foils that would work well. But then so would a smoothly constructed semi symetrical "that's about right" as long as it has a sharp trailing edge for low drag. You're going to be depending on the engine for moving this puppy anyway and it's not going to be operating at the high lift coeficients much of the time. All this is not to take away from serious aerodynamics but more an admission that you're not trying to design a contest F3B glider but rather you're going to be shooting around the sky like a madman with the model operating at very small lift coefficients except during high G pulls.

As for the landings a set of flaps and drooping ailerons will help you a lot. Just take a page from the full size fighters and when you droop the ailerons don't drop them more than about 20 degrees and the flaps about 45 to 50 degrees. This will give you decent lift but still maintain normal aileron control. At 20 degrees you may find that you still need some rudder coordinated in to ensure the adverse yaw doesn't play up but from my time with a flaperon thermal glider I found that 20 was safe. For takeoffs you could go with 10 degrees on the aileron and 25 or so on the flaps. And to help during the high G pulls you could couple a little flap action into the elevator. Here again much like a modern fighter. Did you know that when an F18 pilot simply pulls back on the stick every surface on the plane moves, including the canted rudders?

This difference in angles between the flaps and ailerons ensures that you've got LOTS of washout in the wing during flapped operation to avoid tip stalls.

How you fit the separate servos into that thin wing is up to you. I suspect you'd need to put them in the center section and use pushrods and crank arms. Keep the linkages tight to avoid slop and flutter and all that.

So you'll be looking for that 7 or 8 channel all mixing, all dancing radio now I guess.......
Old 10-26-2002, 03:34 AM
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aubej
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Very interesting info BMatthews,

Like you said, I'm not going for a competition aircraft, but rather, a fast jet like plane.
I appreciate your comments.

Jason

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