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Old 02-09-2007, 10:50 AM   #1
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Default Crosby CR-4

This might belong under scale aircraft but I'll give it a shot here...

I just found out tonight that my good friend's grandfather helped design the Crosby CR-4 Racer back in the 1930's. He found a photo album of pictures of the plane before Crosby placed in some of the races in the late 30's. It has a real classic and sleek look. I was wondering if anyone has seen plans, schematics, kits, etc for this plane??

http://www.daisey-designs.com/nx13688/crosby/crosby.htm

I was thinking that it would be a fun project someday and I thought I would start the hunt now. Is it in any vintage plane books that people know about, like Paul Matt's??

Also, how would one go about estimating motor sizes and weight of the plane pre final assembly? How do I know that if I found scale plans for it that once it was done it would fly right?

Thanks for helping on my quest

Andrew aka Lip84
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Old 02-09-2007, 12:37 PM   #2
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Default Crosby CR-4

Hello Andrew, I found quite a few photos and references to the Crosby CR-3 + CR-4 with a "Google" search. Apparently the "4" was in a 1939 film called "Tail Spins". There are 3-views from Bill Hannan here;
http://www.hrunway.com/shop/index.ht...tml&lang=en-us
As far as designing a model, it would be easier to scale the model up to a given motor size, say a "400" motor. In general, this would be from 30" to 40" wingspan (towards the smaller end for a racer), 15-25 ounces ready-to-fly weight and about 150-250 square inches of wing area.
The original had a 16' wingspan, so this would be 2" to the foot scale, or slightly more. On scaled-down full size airdraft, it's very common to enlarge the horizontal and vertical tail surfaces, about 10%-15%. This helps with stability and control. Depending on the original wing airfoil used, a change might be needed there, also.
The "Shoestring" racer is about the same overall size and planform as this aircraft and is widely modeled. If you study the construction of this model it will give you some ideas on construction. There are also many other "Goodyear" and "Bendix" racers in model form, to refer to.
Good Luck!
Ron
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Old 02-09-2007, 08:15 PM   #3
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Default

I found some more pictures that have scale and multiple views. It also has the airfoil on it. Looks like they use a N.A.C.A. 23015 at the root and N.A.C.A. 23009 at the tip. I do not know any Aeronautical Engineering so I do not really know what this means.

Scaling to the motor seems odd to me, but maybe that is only because I have never done it. I always figure out performance requirements based on the weight of the plane and then find a motor/prop combo that will provide the required thrust/watts/speed. The only problem with this is that I wont have a weight to judge with until I have made the plane.

I think that 2" to the foot would provide a nice size model(32" windspan and 43" long). Now, I have a battery of questions to ask

1. You say a 400 size inrunner would work, what would be an equivalent outrunner (Axi/himax)?
2. Do you think a high end inrunner would be fine (i.e. Mega)?
3. What kind of servo eq? (I only have experience with foamies and they usually use micro/feather servos)
4. What grade balsa? In my experiences building balsa models(not airplanes) it is very, very breakable. There must be special balsa for aircraft design.


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Old 02-10-2007, 02:42 AM   #4
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Default Now We're Cookin'!

Hello Andrew, now that you've found the three-views, we've got something to work with. Excellent drawings, by the way. The multiple fuselage cross-sections are vital to duplicating a complex fuselage shape like this.
The reason I mentioned using a standard motor size as the baseline is exactly what you stated; We don't know what the finished weight will be. So, without that important factor, I figured this would give you the widest choice of power outputs, geared or direct drive, inrunner or outrunner, even a selection of physical size of motor (some 400's are bigger, thicker, larger/smaller diameter, etc). If you ended up needing an uncommon motor "size" (power output) you would have very few choices!
It will also help in the design stage, since there are other similar-sized models to "borrow" ideas from. We can't always re-invent the wheel!
The NACA series of airfoils are US Government-designed and tested sections; Lots of info available here;
http://www.ae.uiuc.edu/m-selig/ads.html
http://www.citlink.net/~kelcomp/airfoils5.htm
So, I'd suggest as a first step, taking one of the 3-views to a print shop and asking them to enlarge it to 2" = 1'. Two or more copies would be a good idea.
Question # 1; The AXI 2808-20 or 24 or HiMax 2816 with a 3s 2100mAH LiPo is about the range we're looking at, roughly 120-170 watts output. But I don't know if a motor that large in diameter will fit in that slim nose! You will have a better idea after you get the scaled-up plans, but I'm guessing a planetary geardrive will fit better.
# 2; Yes, a Mega with about 150 watts output would work also.
# 3; Hitec HS 55's should be fine, or other quality-brand "micros".
# 4; I would use "Contest" grade (very light) balsa for the fuselage sheeting and any other large sections. For the ribs, fuselage bulkheads, medium weight standard balsa. Srtingers, spars, thin stock (built-up tails, etc) would be hard, straight-grain balsa.
The wing center airfoil could be thinner (not as "deep" or tall), I'd bet the reason they used a 15% {last two digits} at the center was to fit the retracts. Aircraft (and models) which are designed for speed rarely use sections more than 10%-12% thick.
That rounded fuselage cross-section is going to present some real building challenges! I'd guess the two main ways to construct it would be either a "crutch", a long central (shaped like the top-view of the fuselage) base with slots for each bulkhead, then stringers at key locations, then sheeted.
Or, build a "half-shell" like the Guillows kits, just the left side, remove that from the plans, add the right-side formers, stringers, sheeting.
Anyway, it's a great project, not a 'starter" by any means, but if you've had building experience, certainly possible!
Good Luck!
Ron
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Old 02-10-2007, 06:43 PM   #5
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Default

Ron,
Thanks for all your help so far but I have some more questions for you.

Originally Posted by Sky Sharkster View Post
I'm guessing a planetary geardrive will fit better.
-I am not sure what you mean by "planetary"

Originally Posted by Sky Sharkster View Post
I would use "Contest" grade (very light) balsa for the fuselage sheeting and any other large sections. For the ribs, fuselage bulkheads, medium weight standard balsa. Srtingers, spars, thin stock (built-up tails, etc) would be hard, straight-grain balsa.
-If I go to my LHS will they know what I mean when I say these things to them? Also, how would you recommend estimating the amounts of balsa needed?
-I have a friend who used fiberglass to cover his F4U a few years back instead of balsa. Is this lighter? Stronger? Harder to work with? not recommended?


Originally Posted by Sky Sharkster View Post
Or, build a "half-shell" like the Guillows kits, just the left side, remove that from the plans, add the right-side formers, stringers, sheeting.
-Do you mean half shell like most foamie fuse's come in 2 parts (split down the middle)?

Originally Posted by Sky Sharkster View Post
Anyway, it's a great project, not a 'starter" by any means, but if you've had building experience, certainly possible!
Good Luck!
Ron
-You are exactly right!! I have no immeadiate plans for building this plane. Like I said before, I only have foamie experience. I am planing on building an all balsa sailplane kit sometime this spring/summer though. I think doing that will at least start to give me an idea of what I would be getting into. I was also thinking of finding some plans for balsa planes from friends or on the forums so I can see how the layout generally goes. On that note, do you have any example pages you could scan or email me for my perusal??

Thanks again Ron, you "old guys" are what keep us noobies in the air
Andrew
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Old 02-10-2007, 10:19 PM   #6
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Default "And The Answer Is..."

Hello Andrew, regarding the planetary geardrive, there are two types of geardrives available for electric motors. Here's photos of both types;
http://hobby-lobby.com/gear400.htm
The Planetary (top) is "inline", the motor mounts directly behind the gearbox (more like a tube, actually!) and the motor shaft has a small pinion gear mounted on it. Inside the tube are 3 small rotating gears that revolve around the inside, in turn they drive the output shaft, which then becomes the prop shaft. Since the 3 small gears revolve around a central gear like "planets' around a "sun", it's called a planetary geardrive. Advantages are; More direct alignment, with an odd number of planetary gears the prop rotation is the same as the motor rotation, very little slippage. Disadvantages; Noisy (all metal gears) heavy (metal tube)and fairly expensive to machine. Planetarys are usually used in powered sailplanes because of their slim, streamlined shape.
The second type is much more common (Lower photo), called "Offset" for obvious reasons. The motor shaft is not in line with the main gear. Advantages; Since the main (Spur) gear is usually a plastic compound, they are quieter. Cheaper to make, housing is also usually plastic. Lighter, less metal and fewer gears. Disadvantages; More slippage, plastic moving against metal pinion wears out, requires a wider or deeper fuselage to fit inside. Prop rotation is opposite of motor rotation (two gears, pinion + Spur).
About the balsa, if your local hobby shop is well-stocked, they will have contest grade balsa as well as regular balsa. If not, you will have to order it onlne. Under the "Builders Techniques" Forum there is a "Links" sticky with listings of balsa suppliers.
As far as amount and size, once you get a set of plans drawn (traced over the blown-up ones) you can start figuring out what size each piece will be. Say you've drawn in two spars, one upper, one lower, both are 1/8" x 1/4". Balsa comes in 36" lengths, the wingspan is 32", so you need two pieces. For sheeting, just figure a rough estimate and add 25% for "extras". Balsa sheets (regardless of thickness) usually comes in 3' wide x 36" long pieces. If you figure you'll need 6, buy 8.
Fiberglassing a compound-curve fuselage is pretty advanced stuff and would require, in addition to years of practice, more specialty equipment that a single project would justify. If you plan to build several models requiring this technique, it might be worth it, but for a "one-off" project, I'd suggest sticking with balsa.
The half shell technique isn't exactly like a foam fuselage split in half, but when it's finished, it will look like one! I'd suggest you buy a small (18" wingspan) Guillows, Dare or Dumas rubber-powered kit and see how they're constructed. Pick one that has a "round" fuselage, not flat-sided. This will turn out to be $20.00 well-spent, because you'll get ideas about structural component size, strength vs weight, gluing techniques and just working with the plans will give you some ideas on designing your model from "scratch".
Good luck!
Ron
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Old 02-11-2007, 01:42 AM   #7
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Default More Ideas?

Hello Andrew, After I posted above, I had a memory flash of a plane that looks a lot like the Crosby, but couldn't come up with the name. I checked some of the sites that mentioned the CR-4 and sure enough (on the "Air-racing site) there it was! The Folkerts SK-3! It is very similar in layout, size and even used the same motor. http://www.air-racing-history.com/ai...sby%20CR-3.htm
Here's a photo of a model; http://www.carstens-publications.com...os/CD0120P.jpg
And a link to the plan set;
http://www.carstens-publications.com...&returnpage=10
And the Keith Rider designs, The Jackrabbit, Firecracker + 8-Ball. Here's a company that makes ARF models of the Rider Racing planes in a "400" electric size; http://www.tdlmodels.com/keithrider.php
Look Familiar?
Hopefully, these will help you in designing the Crosby model. Also, on Builders Techniques, I have a page of kit vendors and plan sources. Many of the plans are free, so you can study the construction methods.
http://www.wattflyer.com/forums/showthread.php?t=14868
Have Fun!
Ron
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