Swept Wing Speedster

[GregG]

This thread (http://www.rcgroups.com/forums/showthread.php?t=532529) originally started in the Composites forum, however I think that it's no longer a great match for that forum so it has been restarted here.

This is one of the models that have tickled my fancy from the 1st time I laid eyes on it.

http://www.bvmjets.com/JetKits/BobCat/bc_yellow_orange.jpg

I've also been watching the Pushy Cat thread over in Sports Planes. My rendition of a compilation of the two models is mocked up in cardboard in the picture below. It has provisions for the fuselage to slide foreward or aft 4-6" to help make CG. The horizontal stab looks to be a bit large to me but I'd hate to have it be too small. If it is indeed too large at least the plane should be very pitch stable.


872978

The wing will be vacuum bagged white foam with FG layers top and bottom and CF spars. I like thin airfoils for a speed demon type plane. Looking through the airfoil program Profili the MH 33 looked really good to me. What do you think about this choice? Ollies answer to that is below.

"What do you think about using the MH33 airfoil?"

Depends on:
1.Structure with thinness and strength and stiffness. (How many G's?)
2.Airspeed. (Landing?) (Racing?)
3.Turning. (High speed with maximum lift coefficient?)
4.With flaps of not.
5.Wing size.
6.Wing loading.

Ollie, thanks for jumping in here! http://static.rcgroups.com/forums/images/icons/icon14.gif

1. 7.25% thickness makes it 0.80" (20.37mm) at the root tapering to 0.29" (7.35mm) at the wing tip
2. max est ~85-100MPH (140-165KPH), 26MPH stall speed (42KPH)
3. ????
4. No flaps planned. Do you recommend them? Flaps and Ailerons or just flaperons?
5. 44" WS (1118mm) 319 sq. in. (.206 sq. m.)
6. 21-24 oz./sq. ft. , 46-50 oz. (1450-1600g)

7. 600 watts input swinging an APC 8 X 8 ~15.000RPM :D
Mega 22/20/3, 3S2P TP2100 ~200 watts/lb.! :D

Plans are to use it to fly fairly open high speed large radius turns. This isn't a pylon racer and I don't expect it to turn that way. I've flown a 150+MPH pylon racer that if I remember correctly pulls a tighter turn than what this plane needs to do.

I'd like to get by with no more than 2 layers of .75oz. glass cloth on both the top and the bottom of the wing. There are also plans to install three rectangular carbon rods .032" x .310", in a vertical orientation arranged in a triangle for spar type support. Is this going to hefty enough?

I've found a website that includes a free downloadable MAC program. This is even better than any one that I've used before. :cool: Thanks to Jean-Claude Etiemble for providing this!
http://perso.orange.fr/scherrer/matthieu/english/mce.html

The picture below is the MAC and CG calc.

Last but certainly not least is the question, where to put the main wheels in relation to the CG. How far behind the CG should the center of the wheels be?

:D

[Ollie]

What is the function of sweepback? Just looks? Not for function with sonic speeds for models.

[BMatthews]

In this case it'll be strictly for looks but the amount is not extreme so it shouldn't have much of the bad stuff that comes with sweep.

If it was me I'd make it with a little bit less aspect ratio. You say it won't be used for pylon style turns so a little less aspect ratio will look more like a jet fighter and will still work just fine. The large stab is fine. In fact it ain't that large given the short tail moment arm. Remember that sweeping the wing back moves the MAC back and you have even less tail moment than you think. In fact a touch longer in the tail booms would not be a total waste. Dont' mess up the looks, just add in a little.

If you can figure out how to make an extension shaft then you could keep the nose longer while keeping the motor back closer to the CG for easier balancing. Adding one pretty much means adding a bearing up front to take the shakes out of the system though. The extra complexity may not be worth the trouble.

[GregG]

What is the function of sweepback? Just looks? Not for function with sonic speeds for models.

Just for looks, I like it that way. :rolleyes: :D

What do you think of the wing structure I explained? How about the airfoil choice? Is there a better one that will cut wind resistence?

[capncrunch]

Greg,

I also love the lines of this layout - I'm working up a flat foamy for cdr motor power with forward swept wings.

your mains are going to need to be close enough to the CG that the model sits fairly light on the nose wheel. too far back and it will be hard to rotate. plan the length of the gear to be just enough to let the prop clear at some appropriate rotation angle. With this configuration, you'd need excessively long mains to set it up so the tail touches before the prop does.


Then, with the plane rotated to maximum clearance, the CG should still be forward of the LG. I.E. if you were to push the tail down, the plane should sit back down on the nose wheel, not stay nose-up. if thats hard to visualise I can sketch up a drawing.

-barrett

[Ollie]

"Just for looks, I like it that way."
If you like sweepback, use flying wing configuration.

"How about the airfoil choice? Is there a better one that will cut wind resistence?"

No drag for elevator with flying wing. Check HS522 airfoil.
http://www.aerodesign.de/profile/hs522_px.gif
http://www.aerodesign.de/profile/hs522.txt
http://www.aerodesign.de/english/profile/profile_s.htm
Or:
MH62
http://www.mh-aerotools.de/airfoils/index.htm

Better than airfoil MH33.

See:
http://www.aerodesign.de/modelle/HS/hs40.htm
http://home.cyber.ee/arne/small.html
http://www.aerodesign.de/modelle/HS/hs77v3_01.htm