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HelicopterOff Topics › High speed plank design help...
10-04-2005 01:52 AM  12 years agoPost 1
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Hi guys

Maybe this should be in another section of RR, but here it goes for you plane nuts...

I recently found a Freeflight model I liked, and I ordered plans for it. I then scanned these plans into AutoCAD and drew over top of them, so now I have the plans computerized.

My goal is to scale up the plans (with some structural changes of course) to fit a RC size airplane with an OS 91 FX as power. I'm trying to make this plane's performance similar to that of Great Plane's Shoestring ARF (when powered by the OS91FX)--so my question is sort of an issue of drag versus weight--I'm shooting for a Wingspan of about 61.5 inches, which in turn (on AutoCAD) puts the fuselage length up to 58 inches (does that seem to long?) Also when I import the engine drawing I made, it seems very small compared to the hole plane. I'm wondering that as long as I keep the plane light (around 7 lbs.), does drag matter?

I also noticed that when scaled up the spinner is about 4.6 inches in diameter!!! Is this scaled up too far?

The main goal in this project is speed, so any design suggestions are appreciated.

And incase anyone wants to know, the FF plane I am talking about is pictured in black and white in Model Aviation September 2005, in the Free Flight Sport column.

Tell me what you think, and thanks,
Nathaniel

Nathaniel Rice
Team YS Engines

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10-04-2005 02:58 AM  12 years agoPost 2
mcatech

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Mount Gambier SA​Australia

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Drag is your biggest enemy for speed
If pure speed is what your after then clean up everything make sure things like hinged flight surfaces are a very close fit or cover them over
bury the pipe inside the fuse if possible dont have a canopy on it or if you keep it low and streemlined
reduce all parasitic drag ie wheels horns anything that sticks out
The spinner needs to streemline to the fuse regardless of diameter
however the smaller the better
the length of the fuse is long but not ridiculous you may find its perfomance will be like a patternship with a long moment arm
this design should be precise to fly but they dont perform well
in high G deadstick manouvres and they fly on forever in a straight line
the wing aerodynamic profile is utmost critical if you have scaled it up as well you may well find its far to thick, you will looking for a relatively thin symetrical airfoil or semi symetrical
and last of all keep the weight to an absolute minimum this can be the difference of a great model and a dog to fly
build it strong and use powerfull servos
Cheers Michael

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10-04-2005 04:02 PM  12 years agoPost 3
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Because it is originally a FF design, the wing looks to be flat bottomed or semisymetrical, and it has a LOT of positive incidence--can this be removed for RC? I will measure it's thickness when I get home.

As a 90 size plane, what is the lowest you suggest I scale it down? (i.e. Wingspan and fuse. length.)

The canopy is pretty low-profile, and I plan to have retracts on it (adding weight, but increasing streamlining).

Take a look in MA the picture will help you guys explain stuff to me.

Thanks!

Nathaniel

Nathaniel Rice
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10-09-2005 01:10 AM  12 years agoPost 4
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Okay I've decided ont he scaled size--the nose ends up with a 4.5 inch spinner--this is very large, but It should contour the fuselage nicely, I guess streamlined is what I'm after. Wing will be about 59.5--61.5 inches, and the fuselage is about 55 inches long.

About the retracts--looking at the plans, it looks like the main gear folds straight back--with extrenal covers that look like external fuel tanks--this is a cool idea (I could use nose retracts for this) but it would be complicated and (less aerodynamic, Michael--stuff sticking out?) Should I just go with the standard fold-towards-the-fuselage gear? This would be slightly more areodynamic and probably less weight.

I decided to make this a fully-symetrical wing, and when mirrored in AutoCAD it comes out around 2.2 inches thick and a 13.something chord (tapering to tips). This seems about right to me...

Also for the tail surfaces--I plan on a total thickness of .5 inches--how should I do this? 3/8 framing with 1/16 sheeting? (yes I want sheeting) or 1/4 framing with 1/8 sheeting?

Also, what kind of prop is good for speed--smaller with more pitch or larger with less? I'm worried about this 4.5" spinner.

Thanks!
Nathaniel

Nathaniel Rice
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10-09-2005 01:41 AM  12 years agoPost 5
ShempHoward

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San Francisco, Ca -​too many beggars +​bad drivers

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A lot of work just to replicate a Shoe String Racer. The new GP Cosmic Wind ARF is in the same design league as well but neither is a real speed demon
as they are really fast sport flyer oriented.

Some design ideas you seem to have are inconsistent with high speed and low drag. 4.5 inch spinner is very big. 2.5 inch thick wing seems way too thick of an airfoil.

Take a look at Q40 Pylon Racers and see if you can replicate their designs in your own way. These are planes that routinely reach speeds of 200+ mph with a Nelson or Jett 40 size glow engine.
They do not have retracts and have very thin airfoils.

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10-09-2005 01:20 PM  12 years agoPost 6
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Actually the wing is 2.2, not 2.5...and this is at the root--the wing will taper from LE to TR and also from root to tip.

What do you suggest about the 4.5" spinner?

If I was looking for a true high speed plane, yes, I'd go out and buy plans or a kit for one of the competitive racers--but I like this plane a lot--it is very unique and I and confident that I can get some speed out of it. By the way this is NOT a shoestring, it is the FF design I discussed in my first post--please look at it--it will help!!! I'm aware that this is alot of work but I am up to the challenge of desinging a plane!

Nathaniel

Nathaniel Rice
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10-09-2005 06:08 PM  12 years agoPost 7
rcsoar4fun

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Boise, Idaho

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A free flight plane will have a much larger spinner because of the FF prop hub. You want it about 3" or so.

For speed you want more pitch.

In addition, the tail volumes and dihedral are going to be completely wrong.

Its like trying to take a Hummingbird heli and scale it up to Predator gasser size.

Kristopher

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10-09-2005 06:12 PM  12 years agoPost 8
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Yes but if I change scale the plans to make the spinner 3, than the whole plane ends up being too small for a 90 size powerplant. Or should I just narrow up the nose, and maybe add a sort of air intake under the spinner to make it look like a P-40 warhawk?

Nathaniel

Nathaniel Rice
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10-09-2005 06:26 PM  12 years agoPost 9
rcsoar4fun

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Boise, Idaho

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Scale the entire plane up to fit the 90 sized, then slim the nose down to fit a 3" spinner. The scoop should be big enough to allow the engine to breath, but it will create drag.

Kristopher

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10-10-2005 12:23 AM  12 years agoPost 10
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Forgot to add this will be mounted pitts style, if that changes anything. 3 might be too much taper--so anything less than 4.5 I pressume?

Is there a big reason why a 4.5 inch spinner is not aerodynamic?

Also, what do you guys think about a wood cowling? I mean, I don't really know how to make one out of plastic or fiberglass--but could I frame up a wood one? Would this withstand the vibration and heat?

Nathaniel

Nathaniel Rice
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10-14-2005 02:35 PM  12 years agoPost 11
NewHeli

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Pittsburgh,​Pennsylvania, USA

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I've been messing with the airfoil, the thickness is around 18% of the chord length...I'm thinking its a bit too thick. Does anyone know where I can find "plans" or airfoil section to get a starting point for the correct thickness?

Thanks,
Nathaniel

Nathaniel Rice
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10-17-2005 01:04 PM  12 years agoPost 12
Hawk4flyer

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Deland,Florida

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18% is way to thick. 10% to 12% is more resonable for a sport plane. 8% is where you'd go for a true racer. 8% would require carbon for strength.

Don't change the spinner size. It will just look weird. I assume you know the original dimensions of the matterials will not work for your design. There are two schools of thought for designing when it comes to matterial thickness.

First, design to be crashed. (thicker materials for stronger structure)

Second, design to fly. ( just enough material to hold up to flight loads.)

The first is heavy, but will last a long time. The second will fly much better than originally designed.

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10-17-2005 08:52 PM  12 years agoPost 13
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Thanks for the reply

Since my last post I have been doing some research on Airfoils, and I found the NACA 0012 which is supposed to be low-drag, and it has a thickness of I believe 11%--I scaled it to the correct size and it gives me a thinkness of about 1.6 inches--this sounds much better.

I am aware that material sizes will be incorrect after scaling, I am changing them accordingly--so far I've decided on 5/16 spars, a 1/16 (or maybe 1/8) shear web, and 1/8 wing ribs (with a 1/4 root). I am starting on the wing, I have not yet begun to structually change the fuselage or tail surfaces...

If anyone knows any pros/cons about the 0012 that would be great (low drag obviously being a pro) such as stall, low speed, and other parameters.

Also in doing the research I read that a swept back wing also serves as dihedral--this plane has a swept leading edge, but the TE is pretty much straight (it has a 1 degree taper towards the front). Does this count as a swept wing?

Thanks alot,

Nathaniel

Nathaniel Rice
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10-17-2005 09:30 PM  12 years agoPost 14
Rafael23cc

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Junction City, KS

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Airfoil coordinates

Check out this page for a _FEW_ suggestions on airfoils. They are free to download.

http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

My suggestion is to select a scale, and then adjust everything to fit. There is no way you can scale a spinner to the scale of the airplane. Just move the engine forward on the nose taper until you have a useable spinner. Now you have a problem with balance because you moved the engine forward. Maybe you need to extend the tail.

As far as drag, cover everything, seal the hinge line and make the controls internal if possible. Do not leave square joints on the fuselage. Build fairings for the wing and the tail surfaces. And last suggestion is to reserch this thing a LOT more, seems that you came up short on your homework.

Rafael

Keep your feet on the ground, but your eyes on the sky.
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10-17-2005 11:30 PM  12 years agoPost 15
NewHeli

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Pittsburgh,​Pennsylvania, USA

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I don't understand what you mean by moving the engine forward...

What do I need to research more?

Thanks for the link and tips!

Nathaniel

Nathaniel Rice
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10-18-2005 01:06 AM  12 years agoPost 16
cudaboy_71

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sacramento, ca, u.s.

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I don't understand what you mean by moving the engine forward...
that means you wont be able to go to the store and find a spinner that's exactly scale to your plans. so, just grab a spinner that's close, and modify your model so that the profile and length of the plane is still scale.

for example, if "scale" requires you to have a 4" long spinner, and the closest one you can find in a similar shape is 3", then you'll have to adjust your motor mount 1" forward, modify your cowling to take up the extra inch, and rebalance the airframe due to the moved CG.

if it ain't broke, break it.

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10-18-2005 01:49 AM  12 years agoPost 17
Hawk4flyer

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Deland,Florida

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The NACA0012 is a 12% airfoil. Hence the numbering. 0012.

It is a 0 camber 12% t/c airfoil. It is a subsonic airfoil.

This is the standard for airfoils. Good all around performance. It has a solid break at stall. Meaning the nose will drop abruptly when it stalls. But, the angle of recovery is small. Giving it excellent control through a stall.

Sweeping the wings will provide a bit of dihedral. Yes, sweeping the LE counts as dihedral.

5/16 spuce would be good for spars if you make an "I-beam" spar. Commonly refered to as a "D-channel" wing. Sheeted LE with an I-beam spar.

1/8' ribs will do fine depending on the chord of the wing. No need to use 1/4' for the root. Two 1/8' ribs butted together will be better.

No need to use plywood for the fuse sides. Balsa is strong enough for this. Just be sure to use 1/16' plywood doublers for the high stress areas like the wing mounting.

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10-18-2005 02:30 AM  12 years agoPost 18
NewHeli

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Pittsburgh,​Pennsylvania, USA

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I scaled the plans so that it will have EXACTLY a 4.5 inch spinner--this gives me a 59 or so WS and 55 for fuselage length.

Hawk4flyer-- Thanks! That's what I was looking for.

By I-beem, do you mean an actaully "I"? I was going to put a shear web on the leading edge of the spars, so a cross-section would resemble a "C". So do you think I'll be okay without ANY dihedral?
And the LE--I was going to have a subleading edge piece, with a balsa block glued in front of that...what do you mean by sheeted LE?

And about the balsa fuse sides--The fuselage is curved (nose to tail), but it also "twists" looking from the back--would balsa be able to conform to compound curves like this?

Thanks,
Nathaniel

Nathaniel Rice
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10-18-2005 05:20 PM  12 years agoPost 19
Rafael23cc

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Junction City, KS

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See what I mean by doing some more research? You do not even know what an I beam is.

You will be working on a very fast somewhat heavy aircraft, and you do not have the slightest idea on how to even build one.

Go to the hobby shop, buy a wood airplane kit, preferaby one with a round fuselage and you will learn most of the techniques that we are suggesting to you here. THEN and ONLY then you MIGHT want to _TRY_ what you are suggesting.

If there is one thing that I advocate here is SAFETY. Your idea is great, but you need much more research and experience before tackling a project like this.

Buy one or two of these books they are good: http://www.rcstore.com/rs/general/l...tid=8&catego=BO
I have read the "Building Techniques" and the "Aircraft Design" books and they are great for this kind of stuff.

Keep asking questions, we may or not answer. Just tell me when and where you plan to fly this thing for the first time.

Rafael

Keep your feet on the ground, but your eyes on the sky.
Team Heliproz.com

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10-18-2005 09:25 PM  12 years agoPost 20
NewHeli

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Pittsburgh,​Pennsylvania, USA

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Just for you information, I have built a kit before, see my gallery--it turned out quite nicely and it flys great...I know exactly what I'm going to do for the fuselage, just I'm only working on the wing at this moment in time. Much of my designing ideas will come from the kit I built with some changes to suit this aircraft.

Will be flown at my field in State College Pennsylvania, but not for some time...

I will continue the research,
Nathaniel

Nathaniel Rice
Team YS Engines

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