It has been a while since I built a 1/2a airplane so I decided to make one. My last project was an Adrian Page Gee Bee R2 for a 91fs and that was the largest plane that I had built. So I began looking for a rarely modeled plane to make. I started to design a Globe Swift but decided that I wanted to try a twin. A friend suggested the Grumman XF5F. It is a fairly obscure plane that is not often done. I know that there are Pond plans for this plane in about the size that I am doing but I have lots of time at work to design things. It will be for twin Norvel .074s as I have 2 of them. It will have Robart retracts but I don't think that I will try for flaps just yet. I have A LOT of foam so it will have sheeted blue foam wings and a fiberglassed white foam fuse. The nacelles will be fiberglass and I may try to make a molded glass fuse. Dimensions are 36.25" ws, 25" length, 224 sq in area with a projected weight of 28 oz gives a 18.5 oz/sq ft. The loading is a bit high but the plane is based on the airfoil that my HOB V35 (which is still going strong after almost 3 years) has and it has a 16.5 wing loading with an advertised max of 18.6. This will be my first RC twin and am very open to comments and suggestions from those of you who have done this before. Also the first one will not be very well finished so that it can be used as a trainer. With the foam construction it isn't hard to crank out more. I am not sure what the construction schedule of this thing will be as we are very busy at work and I have been checked out on another plane, so I may not have a lot of time to get this thing done quickly, but I shall keep it updated.
Ben

I would STRONGLY recomend a larger model to hold those two 074's. Your wing loading at this size is very heavy for the Reynolds number range that our models operate at. The landings will be a vicious stall just waiting to bite you.

Scale it up to about 400 sq inches but keep it close to the same 30'ish oz of weight and you'll be far better off and it'll still be a very strong performer.

Beenie,

I've been threatening (myself) to build a Skyrocket for years. Congratulations on taking on this neat airplane! I have drawn up a plan for two TD .010 but I chickened out before cutting any wood when I realized what a handful it would be when one engine quit before the other - unless I tried to flat spin it until it hit the ground or goes deadstick.

Anyway, I am behind you dude. Have you considered a Blackhawk paint scheme?

I came across a set of plans for one a while back. Either Flying Models or Model Builder. I'm pretty sure it was one of Ziroli's 70s pre-giant scale warbirds. You know he did a whole line of .40 size ones. Anyway, it could be scaled down, and at least it's a design that is known to fly. I'll try to find it tonight.

Will a single flameout allow a Auto-Rotate decent ? Or will it just frisbee....
Just for comparison, I ran twin Norvel 074s on a bashed 6' glider that had a final span of close to 80". With MAS 6x4 3blade props clearing the fuze by 3/4", the narrow nacelles were no big deal on a flameout on that wide wide wing. The model you're planning will be a real handfull on 1 eng.... well, that's my opinion.

Looks like a challenging project, hope you get her worked out. Post lot's of pics as you progress.

Flying Models may have plans for a twin 45 powered version,

beenie:
Best of luck with your project, but, to be honest, I am with Bruce. That is a lot of power to tie on your proposed model size. If I were you, then I think I would either increase the size of the model or reduce the engine size. Which ever way you go, good luck.

beenie, I foung the referrence. It's a Flying Models plan #CF386, 56" span, Oct. 75 issue and it was a Ziroli design afterall. It would be easy to scale it down to a size appropriate for .074s. Although I don't know what that would be. Maybe 40"?

Thanks for all of the input. All of my 1/2a planes except for the V35 have been lightly loaded floaters, IC powered parkflyers really. The Bonanza is my only reference for a higher wing loading. I didn't want to put too much airplane on these two engines and have a slow fighter model. I was hoping for something with some speed. But it seems as though I overdid it. The more I think about it, 18 oz/sq ft is a lot of wing loading, and should make for a really interesting engine out situation. The plane started out with a 42" ws and about 300 squares, bringing the loading to 15 oz/sq ft. I think I'll try that size again or maybe a little bigger. One good thing about CAD is that scaling is really easy. I should have some time at work next week to play around with it.
Ben

I can't wait to hear how you will get it to balance at the correct COG for a wing in that configuration.

A Hydrogen filled fuselage will not really help.

The plane is a born Prop Hanger.

Beenie, speed comes from a sleek shape and slippery airfoil with the overall size of the model being of a secondary concern as to speed. 300 is a step in the right direction but it would still offer a lot of problems in your one engine out situation.

But of course thanks to those huge engine nacelles and relatively large fuselage you sure don't have the sleek shape. I know I looked at the X5F a couple of years ago and put it on the back burner as far as a glow engine model went. I just could NOT get enough wing area to satisfly my wing loading needs without the cowls being so large that onely the last inch of prop stuck out past the cowl opening..... not the outside diameter but the opening in front of the cowl. At that point I figured that this was a model best used for gear drive props and electric motors.

Bruce,

My "solution" for the nacelles was to "profile" them. I just drug (did I actually use that word?) out my drawings and for the side view I tried to be accurate but from the front view I slimmed them down. If I ever do build mine I expect two things; one, she woun't live long but two, the sound of two TD .010s "singing" will be worth it. Who is it here that says, "so many designs, so little time"?

OK, time to eat crow. I did a quick scale up in CAD of your wing and cowl from the 3view you posted earlier. I don't remember what form of size I was playing with but actually this is quite doable.

At 42 inch span the wing area is 316 sq inches and the cowls are 4.6 round with a 3.2 inch opening. The fuselage is 3.4 inches wide at the trailing edge.

At 44 inch span the wing area is 350 sq inches and the cowls are 4.8 inches round with a 3.4 opening. The fuselage is 3.5 inches wide at the wing trailing edge.

At 46 inch span the wing area is 385 sq inches and the cowls are 5.0 round with a 3.5 inch opening. The fuselage is 3.7 inches wide at the trailing edge.

If you choose to go with the 44 inch or 42 inch option you'll need to keep the weight down. At these sizes the scale effect is hitting hard and the weight and wing loading can't just be linearly reduced. I would suggest that you will not be happly if the 46 incher has a loading of more than 15 oz/sq ft. That equates to 40 oz. To fly the same I would say that the 44 incher would need to be 14 oz/sqft and the 42 incher no more than 12 to 12.5 oz/sqft. Those loadings equate to 34 oz for the 44 incher and 26 to 27.5 for the 42 incher. This is all based on the idea that the Reynolds numbers (that old "scaling effect" thing) are really stacking the deck against us small model flyers.

My own feeling is that if you can build it light enough that the extra wing area of the 44 or 46 inch version is easily worth the extra cowl size. At that size and given what I know about the Norvel power output if you can keep the weight down to about 2.5 lbs (40 oz) you'll have a great flying model that will offer good climb, reasonalble speed and yet still slow down nicely but it won't be a floater by any means.

At any of these sizes I would also suggest that you choose to use the Selig 8036 airfoil. It's a thicker but low camber shape that is picking up a good reputation for having a gentle stall and slightly lower landing speed on heavier wing loading scale typle models.

But if you truly want a fast and zoomy model I'm afraid that while this is a fantastic looking project that you're doomed to failure in the speed department due to the two built in parachutes on the wings of this model. Something with inline engines would be far better.

On a positive note the size of the cowlings and the style of the retract gear would make doing your own design gear quite an easy option with the legs being made from carbon fiber legs and the side brackets to hold the pivots being from simple plywood. There's easily enough room in there to let you do this in a larger but still light method that supports the leg pivots without point loading the structure highly.

The big issue here is the lack of prop diameter that the Norvels can swing. They produce lots of power but won't put it into a larger diameter. Something that occurs to me is that the 46 inch option with two OS 15LA's in it would swing the prop sizes needed while still only putting out the same power as the two Norvels.

I drew up plans for one about 25 years ago, intended for two Cox .09 Medallion R/C's.
I'll drag out my plans and see what I had come up with. I remember that it was designed
to use 2-liter soda-bottle bottoms (the old ones were seperate pieces, and the ones for Diet Rite
Cola were silver!) and I used a rib pattern from a Guillows P38. Also used a carved foam
turtledeck to save weight. Don't know why I never built the silly thing. The 3 view I have
(from the 40's) shows an alternate version with much more streamlined nacelles.
Hey, they only built two of the radial ones, so wy not model one they never made ANY of?

Dave

Dave, there was one built with the Allison inlines as I recall. Saw a pic of it years ago. It was also the inspiration for the Blackhawk's planes in the comic books instead of the radial version. So it's only SEMI wrong.... I doubt there's drawings around for the Allison version but if someone was to graft on the cowls and spinner design of the original P38 prototypes that did not have the large radiator intake on the chin it would be just like the pic I saw. It would also make the noses a little longer since the Allison cowl would start right at the leading edge of the wing. Beanie, if you do that you'll just have to do some sketches to match up the scales of the two planes.

But then you would HAVE to use the comic book Blackhawks color scheme and markings....

Just looking at the frontal you can see that a dead engine nacelle is a major % of the total frontage, and there aint alot of plane drag to one side of the good engine with a ton of plane drag on the other side of the good engine.
Spells massive yaw in my book. You may want to test it with some safety Training Rudders: huge goofy yaw killers.

These kinds of concerns are why I like the A20 Havok, good amount of wing since it had that bombload, good lines for air slicing, spacious radial cowls

ok, it's really because the Havok was the only plane I could get a dive bomb run to hit anything in Aces High online sim

Wow,
This project has generated a lot more interest than I expected.
Bruce, I know that wing loading has little to do with speed, what I was referring to is having too much airframe for those little engines to pull around. I am very much a fan of light wing loadings. I also realize that speed will not be too high with two parachutes on the wings. I don't think that they ever built the Allison version, only the 2 radial ones. The pic I attached is not a scale 3-view, it is as best as I can figure a publicity poster. A lot of the lines are very different than the prototypes. It does show the streamlined nacelles though. My initial attraction to the airplane was the two large radial engines, so the model will have them too.
One of the things that I am thinking of doing to get more wing and still keep the nacelles down is to enlarge the whole plane 20%. That gives 41.5. Then just enlarge the wing 10% more to get it into the 46" range. It is not exactly scale but it has worked well for me in the past. It would also let the 7" props at least peek out from the cowlings. I think that I would be able to keep the weight down without much trouble.
Ben

Too bad Lockheed got the P-38 done first. It would have been interesting to see this thing in combat with the two Allisons. I just don't think I believe 450MPH with this design, since the Lightning wouldn't hit that either

You could get around the giant radial cowling problem by making a fantasy model of the plane with turbine power instead of radial, as is done with the Dehaviland Otters- may solve any potential CG problems too by moving the engines foward..

Here's a page with the otter's conversion from radial to sleek turbine power..

http://www.bluewateraviation.ca/walter.cfm

Beanie, your idea of slightly enlarging the wing beyond scale is an excellent one if you're after a stand off scale model that gives up a little fidelity in favour of flying. You'll want to enlarge the tail section over scale by the same amount as the wing and cheat maybe 1/2 as much on the tail moment as well. Even with those alterations it should still retain the pugish look nicely.

Your solid scale plan in the last post is way off scale in terms of fuselage cross section. The original has more of a round top trapezoid look to it. If you haven't googled on it yet do some searches on "grumman skyrocket" as well as the X5F designation. It should turn up a few extra drawings.

I seem to recall that I may have some very old control line scale plans for the Skyrocket in about the 45 inch span range. The construction will be crazy heavy as all good control line scale models were back in those days but the sections and outlines may provide you something to work with. Let me look around for them and I'll get back to this.

Bruce,
I am not really after a 100% scale plane, I accepeted a long time ago that if I wanted good flying 1/2 scale airplanes, they would have to be fudged a bit. I did neglect to list the tail area being enlarged with the wing. I have found several photos and 3-views of the plane, and they all differ. The pic in the last post seems to be more of a conceptual drawing, I found that it is a little more attractive than the plane turned out. It is more rounded in the fuse and the nacelles. It is not the drawing I used for the plane.
As for the turbine version, I think that it would look a lot like the proposed Allison version. There is just something about the two large radials out there forward of the wing and fuse.
Here is a link to the most complete history I have been able to find on the web concerning the 5F http://home.att.net/~jbaugher1/p50.html
Ben

It does look more cute and unusual with the round radials and the snubbed fuse grabbing the wing like it is on the navy version..
I found this
http://www.rcgroups.com/forums/showt...?t=80645&pp=15

Beenie:

Consider the long nosed version, the XF5F-2 or XP-50. That will, if nothing else, give you a place to put balance weight if needed. And a lot better looking plane.

There's a picture of the LN version in the link Clint poisted.

Bill.

Is there any reason why NARROW bladed, larger props could not be used ? E props are spun as fast or faster than glows.

That would put the effectice end area of the props beyond the cowls edges.

I wound up some of the Windsor Black E blades no sweat or vibration.

Cyclops:

Use Eprops on a single cylinder engine at your peril. An electric motor has a pretty constant rpm, while a piston engine speeds up and slows down on every revolution. Remember the torque peaking with the cylinder's firing? Then the prop speeds, for the rest of the engine's operating cycle the prop is slowing down, greatest is when the engine cones against compression again.

This is constantly bending the blades, and stressing the hub to blade joint. And is the greatest reason for throwing a blade.

The Eprop may well stand up to it for a while, but they aren't designed to have to.

An Eprop may last forever, but at the same time it may break the next time you start the engine.

Bill.