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Old Aug 09, 2014, 11:40 AM
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24" 1920's-1930's racer.

So I've decided I'm going to have a crack at making a 24" WS 1930's style racer. As of right now, I don't have any sort of plans or anything drawn up. I can't even decide if I want high, or low wing (I don't want mid...). Here's what I do have:

- HK 1811-2000kv motor: I was running it on a StevensAero Adrenalin Rush, but after some reading of this thread, I decided I could stand to fit a bit larger prop.
- I want to try out making a successful plane using the basic build techniques used by vtdiy in his Simple Shoestring Racer thread.
- As far as the wing goes, I'm currently torn between something inspired by a Gee Bee D and/or Travel Air-R, or something more like a slimmed-down Hall Bulldog.

I'll probably start cutting some foam here in the next hour, so I'm curious as to which airframe I end up with...

EDIT: I am no longer going to use the 1811 motor on this. I'm taking thzman's advice and getting a larger motor. I instead decided to fit a HK 20g motor, which is a bit of a compromise between the 1811 and the 24g motor thzman and I talk about below. It is rated to spin the same 7035DD prop, but has a higher Kv which should get me some additional speed without adding too much weight. Hopefully with this higher power motor (and consequently slightly larger battery) I should be able to keep the weight under 6-7 oz, but will have more speed, and thanks to the amazing and sometimes tyrannical v^2 term in the lift equation, that speed should translate into some good flying qualities.
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Last edited by CaptBojangles; Aug 10, 2014 at 11:43 AM. Reason: New motor
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Old Aug 09, 2014, 11:42 AM
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A note on the motor/prop:
I plan on running a GWS 7035DD, using a 2s 300mAh lipo. Interestingly I was a bit concerned about motor heating with a prop that size, so I spun it up to about 75% throttle for about 30 seconds or so, and used the highly scientific "does it feel hot?" method. It was a bit warm, but nothing too concerning. When I tried the same thing with a GWS 6050SF prop, it was significantly warmer... not enough to burn up, but definitely enough to raise an eyebrow. It felt like there was a bit more prop torque with the 6050 as well...
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Old Aug 09, 2014, 03:51 PM
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Alright, time for some pictures of the wing. It's going to end up being a low wing, based on how I ended up doing the landing gear. The wing is made out of 1/2" thick insulation foam.

So first I cut out a wing piece, 12" by 6". For some reason, I decided to cut out the wing in two pieces, which I didn't even think about until I took the first picture. Oh well, I guess I'll make it work.

So then I cut out the second piece and sketched out a rather rough airfoil on the edge. I used an exacto and a hacksaw to cut away most of the excess.

Once I got rid of the excess and used a belt sander and a fine grit paper to get the wing smooth, I realized that I forgot to add the elliptical like I wanted. This is what happens when I don't have a design sketched out beforehand.
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Old Aug 09, 2014, 03:59 PM
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So with the two wing halves cut out and shaped, I had to figure out how I was going to do the landing gear. For no reason other than I think it looks cool, I decided I wanted to have wing-mounted wheels like the Gee Bee D.

I had tried to do that once before on a wing made out all depron with 6mm for spars, and 3mm for skin. On the plane's first landing, the wheels ripped right off... So I decided to do something a bit different, we'll see how it works.

I have a piece of 1/16" spruce that I'm going to set into the wing, and use as a brace for my single piece of landing gear wire. I guess that wood will also act as a spar, but since my goal for this plane is under 8oz, I really hope I don't need it.

There's a bit of a gap between the two wing halves, which concerns me somewhat. I think it won't be that big of an issue, since I plan on putting some gorilla glue in the gap and the slit for the spar/landing gear strut mount, and I am going to give it a paper mache LE for strength.

Anyway, taking a break for now while the paper dries so I can flip it over and put some glue in the gap
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Old Aug 09, 2014, 06:56 PM
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Do you have any pics of this plane you are modeling? What it's going to look like?
RM
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Old Aug 09, 2014, 07:02 PM
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Ok very nice that's the one u was thinking about . Can't wait to see it fly! I like it.
RM
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Old Aug 09, 2014, 07:18 PM
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Quote:
Originally Posted by RITTMEISTER View Post
Do you have any pics of this plane you are modeling? What it's going to look like?
RM
I don't know if I would say that I'm modeling this particular plane, but I would say that what I'm building is styled after the Travel Air R "Mystery Ship"

I haven't really looked at any 3 views of that plane, so it won't come out exact, (or might not even come out close) but that's the basic style I'm going for at this point.
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Old Aug 09, 2014, 07:52 PM
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That paint scheme I see a lot don't recall other ones. What did you have in mind? On The Realflight simulator we have that plane flies good .
RM
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Old Aug 09, 2014, 07:56 PM
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I have seen balsa ones Pat Trittle design but not foam,. I would like to add this plane to my collection. Any PDF link? Ok I see no plans need to locate some and paste here. I can't with this crappy I phone .
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Old Aug 09, 2014, 08:24 PM
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In regard to your motor combo it seems has been well tested with the 7x3.5 prophttp://www.rcgroups.com/forums/showthread.php?t=1771408 Although an 8oz airframe AUW might fly like a lead brick on that motor. The smaller the scale of the plane the less you can get away with flying on a smaller amount of thrust. In the above link the planes running on that motor are in the 4oz range. I have built and flown the f4 wildcat on that thread using a 10g motor with similar thrust to yours. After crashing and repairing it a few times the weight crept up to 160 grams or 5.6oz and it was getting close to unflyable difficult to gain altitude.
a 10 gram difference in weight makes a noticeable difference in lift in these small 24in planes.
a 24gram motor would be more suitable for an 8oz airframe.

This flying wing with a 32in wingspan 7-8oz Auw is running a 24 gram motor.http://www.rcgroups.com/forums/showthread.php?t=1542672
your plane (24in wingspan, 8oz's of weight, 10gram motor) has less than half of the wing surface area, the same amount of weight and a motor half the size.

a plane with that much weight and so little wing surface area will need to fly really fast to stay in the air. Faster than your current motor could muster.
So you either need a lighter airframe with your current motor, or a bigger more powerful motor. Depending upon your flying skill you might not want to build a plane that fly's so fast.

In short, for your 1811 motor build an airframe around 4oz, 22-24in wingspan and use the 7x3.5 prop. If you want your 8oz airframe to fly, go with a bigger motor, if your are up for flying a super fast plane.

This is all just my best guess. So, to my guesstimate you need to get the weight down, at best might fly just barely but with little power left for gaining altitude or fighting wind.




Also here is the thrust and prop info for your motorhttp://www.headsuphobby.com/HURC-180...otor-H-560.htmhttp://www.flybrushless.com/motor/view/431
the GWS 6050SF prop may be pushing it. I would go with the 7x3.5
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Old Aug 09, 2014, 09:06 PM
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Thanks for the link thzman, although you're making me nervous with that weight issue...

I pulled the 8oz baseline from WebOCalc, putting in some estimate numbers, which said that it would fly well. Granted, real world experience trumps theoretical math every time...

Was that F4F using a flat plate wing? I have an airfoiled wing, which will help my handling somewhat. So hopefully I'll be a bit less draggy, so that will help me out if I can keep it under 5oz...

So even though I threw out that 8 oz in the beginning, right now with all the components I have, which is the built up wing, and all the motor/servo/battery equipment, I'm sitting at 100g or 3.5 oz.... I'm guessing based on that, I'll end up around 5 ounces, which should fly, in the strictest definition of the word...

I have one of those 24 g motors that I used on a 34" span bobcat that weighed 10oz It may sound a bit heavy, but that thing flew amazing. It was indeed quick, so landing was always fun, but definitely manageable.

I honestly appreciate the advice and the links, but I don't think I'm ready to give up on this guy just yet; I'll just be extra careful about adding weight for now. Maybe if I get further along and the weight creeps up past 5, I'll consider giving up. I really want to get an airplane to fly using this airfoiled insulation foam, instead of a depron or FFF KFm airfoil... The lesson I'm learning though is that building sub 32" planes is hard, and this blue crap is heavy... Worst case, I get to the end, and it's too heavy so I strip out the gear and make it a display piece
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Old Aug 09, 2014, 09:07 PM
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Quote:
Originally Posted by RITTMEISTER View Post
I have seen balsa ones Pat Trittle design but not foam,. I would like to add this plane to my collection. Any PDF link? Ok I see no plans need to locate some and paste here. I can't with this crappy I phone .
I don't have any plans for this, I'm just building mostly off of that picture.
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Old Aug 09, 2014, 10:35 PM
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yes it was a flat plate wing. 4mm thick dollar tree foam board.
so your airfoil will have some advantage.
5 ounces is probably manageable for your motor. 6oz is getting in the range of possibly not being able to gain altitude to well.
In my experience with these small 22"ish wingspan planes the general rules of thumb for thrust to weight ratio don't apply( especially if your flying outside), Air molecules don't scale . Anything less than a 1 to 1 thrust to weight ratio is less than ideal for this size model.

yea, building these small planes weight adds up quick. just be careful on the test flights. make sure the cg is good and trim it out on glide tests. when you run it with the motor, I would defiantly put it on full throttle at take off. and lots of soft green grass to land on.
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Old Aug 10, 2014, 06:35 AM
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Well put thzman . With only 3 post you out wit me who has been here for years!
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Old Aug 10, 2014, 08:01 AM
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Have you considered simply doubling the size?
A 48" span will be a lot easier to build, in terms of of having a lot more "wiggle room" for weight issues. Also, its higher Reynolds number will mitigate the "sensitive-control" issues typical of small models.
Also, planes of that general style definitely benefit from stall/spin avoidance devices like drooping leading edge cuffs for the outboard third of each wing (and others). The effort investment for them is so minimal that it really doesn't pay to forgo them.
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