Question on changing the airfoil on a Micro plane. (Undercambered to flat bottom) - RC Groups
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Jan 27, 2005, 03:56 PM
Registered User

Question on changing the airfoil on a Micro plane. (Undercambered to flat bottom)

Well it's pretty simple. I am building the Micro Pitts. It starts off with a RET controls and an undercambered wing. I'm looking to make it ARET and sheet the wing flat bottom. I'm thinking this would be good for when I roll or try to fly inverted (if only for short stint).

A) do I need to sheet it? I seem to get the impression from the modelers by me that If I don't she'll fall out of the sky when i go to roll.

B) I'm concerned about the flying speed of the A/C. for a given wing loading X (X being between 4oz/in^2 and 5.5oz/in^2); if I go from undercambered to flat bottom will the stall speed increase. If yes, how dramatic are we talking?

This is a 12" ws Bipe for indoor flight. I'm watching some fly a Tensor 4d around in our gym and i don't see why i can't make this one work. the tensor is a flat foam wing i believe. Which leads me to wonder what to do.

Thanks in advance.
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Jan 27, 2005, 06:14 PM
Ascended Master
Sparky Paul's Avatar
Add the ailerons, forget about the extra sheeting.
It won't "fall out of the sky".... it might be somewhat sluggish though.
Jan 27, 2005, 07:56 PM
Registered User
That was another road I thought about... at $24 for this little lasercut kit, I can always build another. I figure I will put ailerons on all four surfaces. I figure that would help to combat the sluggish-ness?

Thanks for the info.
Jan 27, 2005, 08:17 PM
Ascended Master
Sparky Paul's Avatar
You'll have to hold the nose up when inverted.. that's about all.
Jan 28, 2005, 03:26 PM
B for Bruce
BMatthews's Avatar
Ah, something to consider. That little 12 incher was intended for the RFFS system and a very light power package. It sounds like you're using the equipment from the next rung up the ladder. I suspect your weight is going to go up to where the airfoil may be the least of your worries. What is your target weight for the model with your gear compared to the original setup?
Jan 28, 2005, 03:50 PM
Registered User
Chris had is in at 29 grams. 4oz wing loading. I'm looking mid to high 30's
His thread has him saying it should fly well up to 6 oz loading... I'm aiming for 5, but may hit 5.5oz/in^2

I haven't picked a motor gear combo yet. I had asked in another thread for recommendations on that. So that's a weight I don't have set yet. That's my main variable right now.

If this doesn't work than I will enlarge to 18" and try again. don't want any bigger than that though as this it for an indoor gym.
Jan 28, 2005, 06:36 PM
B for Bruce
BMatthews's Avatar
OK, sounds like you did your homework. Sorry to pull the fire alarm for nothing...

Arced is definetley going to be pro upright. But flat 1/32 has all the stiffness of a wet noodle. Flat bottom sheeted both sides isn't going to be much more inverted friendly than the arced shape. I'm still not sure what to make of flat plate but there's no doubt that on super light models it works for as much flying as the typical 3D model performs. But it does have a sneaky stall and you can find yourself with severe separation when you don't relize it. The BEST option would be thin built up symetrical wings tissue covered top and bottom. But that's a bit more work. of course.
Jan 28, 2005, 08:33 PM
Registered User
That's not a bad Idea. I can easily make it semi of even fully symetrical. It's just a matter of re-cutting the ribs to form the lower curve. I know this will improve manuverabilty, but will it have any adverse effect on lift? This is all futile if it becomes to fast to fly indoors... I know I'm asking for alot here....
Scale looks, acrobatics and indoor speeds...
Jan 29, 2005, 03:41 PM
B for Bruce
BMatthews's Avatar
Camber in an airfoil lets you fly at higher lift coefficients before a stall occurs. However flat plate style airfoils tend to stall earlier thanks to the separation bubbles from the sharp leading edge.

Light wing loading is the real key for slow flying. Your gear is a fixed commodity but you can control the amount of wing area via the scaling. You're obviously thinking this way with your comment about an 18 inch version. What about splitting the difference and go for a 16 incher with built up symetrical rib and tissue wings? At that size you can get away with a super light construction using 1/8 sq on the diagonal as a leading edge with 1/32 ribs and simple spars of 3/32x1/4 with a light trailing edge. The ailerons would be best built up if they are sizable but for up to, say, 3/4 wide just 1/16 with lightenting holes would be suitable. Then cover with either tissue and dope or one of the new super light films. No sheeting involved in this case but it WOULD be light.
Jan 29, 2005, 09:33 PM
Registered User
Ya know, 16" could be very promising. Here's what I'm thinking.

16"ws = 48in^2 wing area "effective" (assuming Span X MAC X 2 - 25% for rounded edges, underfuse and closeness of wings.)

I found a 10mm geared motor @ small tools inc. 22g of thrust on a 6" prop. 6.6g Keeping 1/32 for the fuse and epanage, and building the wings like you said, I think I can keep the AUW well under 50g... maybe even as low as 42-45 If build it right for CG.

42g = 4.5Oz/Ft^2
45g = 4.82Oz/Ft^2
48g = 5.14Oz/Ft^2
50g = 5.36Oz/Ft^2

All are better than the 5.88Oz/Ft^2 I might well hit on the 12" version. I thin I will build it... If it's too heavy, pull the gear for a 16" version and convert to micro actuators and a JMP 4ch setup on twin 80mah cells. that would shave a few grams.. and then use the 12 for a relaxing flyer and wring out the 16"

Ah this is fun... Thanks for the help.. I'll make sure to post back the results.
Jan 29, 2005, 09:39 PM
Registered User
Just a point of note on the weight thing I've been worried about

talks about a 17" micro stick as an "indoor aerobat" 5.9oz/ft^2 loading...

maybe low 5's on the 12" won't be so bad? Guess we'll see... I hope to fly it on Feb 6 (Should prolly stop typing and build the d/\mn thing

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