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New twin design project

Old 07-01-2006, 01:54 PM
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canadagoose
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Default New twin design project

I am currently going to start construction on my first float equipped, low wing, twin engine aircraft once my kit arrives, the plane will have a 63" span, 670 sq in area and will be 49" long w/floats. Estimated weight should be 7 - 8 lbs. I have two engines for it but have been wondering what size of engines other experienced builders of twins would power it with? i definitely prefer 4 strokes.
I have one other question maybe someone could help me with, i have sanded a blue foam plug for laying glass cloth on to make my engine cowls and do not want to do another, i would like to be able to use the same one for both. What would be a method that i could remove the plug from the first layup to use it again?
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Old 07-01-2006, 02:13 PM
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jrf
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Default RE: New twin design project

52 four strokes would give you plenty of power and the weight should allow you to use scale-like proportions for nacelle length.

Jim
Old 07-01-2006, 02:45 PM
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Default RE: New twin design project

Thanks jrf, i did have a Magnum 52, was small and light for a 52 size engine, i guess my two Saito 45's might be OK i was thinking they might be too much.
Old 07-01-2006, 02:55 PM
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Default RE: New twin design project

Is your plug the exact length it needs to be? To be able to reuse it, it'll have to be as smooth as is humanly possible for 1. The glue lines and seams you have in it now will probably not let it release when and how you want it to, so those will have to be dealt with first. You will then need to coat the foam with an epoxy type resin, something that will not eat the foam. Be careful not to mix whatever you use too HOT or it'll melt the foam. Your looking at some work but the more you put into building something, the more you get out of it like a better looking job and the satisfaction. You know that or you wouldn't be undertaking this project more than likely. I'd need to know how much extra did you allow yourself on the open end? Are you intending to do a bunch more sanding and shaping? The way you have this plug made will leave the outside of your nacelles in the rough big time. If you want a smoother product to work with and still just use the one plug you have now, you can finish the one you have by the method above. Then use a good wax to rub the plug very good. In other words polish it good. You can find what you need at any auto body shop or auto parts store. Cover the polished plug the with your glass and resin of course and let sit till it's cured good. Remove the plug and then what you have is a mold that'll leave the outside of the nacelles a lot smoother that the way I assume you were going to do it. A auto body shop could be a big help here.
Good Luck,
Slowjohn
Old 07-01-2006, 03:56 PM
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Default RE: New twin design project

Thanks Slowjohn, i get what you are saying about having it polished smooth so that the plug stands a chance of pulling out and that's what i'll try and do, so i can just you automotive wax as a release agent? I suppose any wax residue left on the inside of the cowl can be cleaned of so more glassing can be done on the inside? I don't care too much if the exterior of the cowls aren't perfect because i'll wet sand any bumps out.
The plug is the exact length i want the cowls, is that OK.
I'd need to know how much extra did you allow yourself on the open end?
I'm not sure what you meant by this.
Old 07-01-2006, 04:20 PM
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Default RE: New twin design project

If you made the plug a little longer, say 1/2" or more than you needed, you wouldn't have to worry about any glass or resin over laying onto the backside of the plug. Room to spare so to speak. You'll need room to trim and sq. it up. I'm thinking that the way it is you'll almost have to go past the end of your plug which means that you'll have excess on the backside of the plug that'll have to be trimmed off before you can remove it.
If you were to make a mold from your plug, then you could make your 2 cowls with the mold and the exterior of the cowls would be a lot smoother to start with.
I'd check with a body shop or call a fiberglass business to see what they recommend for the release agent. It's been 30 yrs. since I did that type of work and I'm afraid I've forgotten a thing or 2. Maybe 3.

Slowjohn
Old 07-01-2006, 04:42 PM
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Default RE: New twin design project

I'll look into this some more, maybe on one of the fiberglass and molding suppliers online.
I just put a coat of epoxy resin on the plug so the length will be staying as is, i do have about 1/4" leeway
on the length of the cowls.
Old 07-01-2006, 06:15 PM
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Default RE: New twin design project

I'm catching this in between naps. I have to work tonight and I've been up all day. that'll make for a long night unless the fish are biting. Nightwatch at the harbor/turning basins. I fish in between rounds. Get pd. to fish. Good job!
Sounds like you have just enough room then. If possible use quality materials wherever you can. I used some cheap Dollar Store paint on my Twin Mustang, after a couple of weeks it was still tacky. I even placed the pieces in the oven with the door open to try and cure it but it didn't work so I've had to go thru I don't know how many sheets of sandpaper trying to get that mess off. I got so disgusted and laid it off to the side, went back a few weeks later and it still just gums up the sandpaper and leaves my hands and clothes sticky. The practice piece (unsealed) did fine but the fuselages had been sanding sealed and filled. Might have been a chemical reaction between the two. I don't know but I did wind up with a big mess and a chore plus some. Have a Safe and Sane 4th of July.

Keep us posted with lots of pics,

Slowjohn
Old 07-01-2006, 11:49 PM
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Default RE: New twin design project

That's a real bummer with the paint on you're Mustang Slowjohn[:@] hope you can get it fixed up.
So far i've have given the plug a second coat of epoxy after wet sanding the first. After this one cures i'll try wet sanding it to a real smooth finish.
I found some info and products that i might just have to order here
http://www.fibreglast.com/
Anyone know of a company that could make a mold from my plug?

Here is couple computer renderings of my project plane in incomplete 3d model form. This helped to verify parts fit. I did find a couple of mistakes that have been corrected. The other pic is the plan side view with one of my Saito 45's. Anyone see anything wrong aero dynamically with this design? I tried to keep the engines as close together as i could and keep the CG fairly far forward to help with yaw stability with floats and assymetrical thrust.



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Old 07-01-2006, 11:56 PM
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Default RE: New twin design project

Wow goose, can't wait to see you build this baby!! [sm=thumbup.gif]
Old 07-02-2006, 12:05 AM
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Default RE: New twin design project

Goose:

Looks good, but I'd both lengthen the after body and greatly increase the area of the vertical fins and rudders.

Bill.
Old 07-02-2006, 04:13 AM
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Default RE: New twin design project


ORIGINAL: canadagoose
Anyone see anything wrong aero dynamically with this design?
As a young teenager around 1950 I built numerous own design hand launched gliders, as balsa became available I used 1/64" sheet for profile models. I read around that time in Aeromodeller mag' that making a paper mock up of the side elevation (using two thicknesses of paper for twin engine or especially twin fins. The mock up was suspended by a thread over the C/G to test the Center of Lateral Area. I must admit I did it mostly by eye but it seemed to work.

I love the appearance of your design, I don't recognise it but it seems evocative of an Italian style aircraft. It also brings to mind the cartoon story aircraft of thirties American boys adventure books.

If it is a scale job I guess you may well have the flying skills I never achieved. I feel as a free flight job it may have a short life.My own skills with the sicks mean that I only dare disturb the flight of a naturally stable model, when I let go the sticks it must recover by itself.


old git - - - - -aka John L.
Old 07-02-2006, 04:39 AM
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Default RE: New twin design project

Thanks Goose, I'll get it done soon as things settle down some. I'm too far into it to stop and I've got 1 fuselage finished sanding the silver off and the other is over half way done. I failed to mention that it was the $3.00 a can shiny silver. I tried spraying the 3rd coat outside in some high wind and it was looking good till a gust caught me twice, once on each fuselage and I got a couple of sags. If I wasn't naming this one after my mother, "Betty Jo" I might not have worried about it but being that I wanted it to be good looking like she was in years past...and the P-51s were a favorite of hers at the airshows we went to each year, well I just couldn't live with it. I wanted to lightly brush the final coat with steel wool for a weathered look but I'm just going to go with a not so shiny silver this time.
Okay enough about my problem. After the second coat is good and set/dry and you think you've sanded it enough. Get some dark spray paint and give it a light coat, let dry and then sand again with a flexable sanding bar. What I use is a piece of 1" X 1" styrofoam block about 4" long. Any low spots or imperfections will show up. When you have it as good as you can get it or want it, you'll be ready to make your own mold that'll make you the 2 you need. Let me know when you get there and I'll tell you how. Just remember that the better job you do on the plug, the better your mold will turn out and then your nacelle's. It's a time consuming chore but worth it if your up to the task. If not, then take the finished plug to a auto body shop or anyplace that works with fiberglass and they can do it for you. Get the plug right and the rest is comparitivly easy.
Looking at the plans, I have to agree with Bill (W.R.) It is a different plane to be sure. The only other thing I see wrong is that the Bandersnatch doesn't appear to be syncronized with the Congonckulater? Can't wait to see this one finished. I can't recall ever seeing one either. Will it be planked or film covered?

Slowjohn
Old 07-02-2006, 07:22 AM
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Default RE: New twin design project

Thanks for the responses, it's too late to change the fuse, AK Models is laser cutting my kit for me now. I could make bigger fins and rudders if need be or i can also slide the tail feathers back almost an inch. I won't be glueing the tail feathers on on this build but will be bolting them in case they do need changing.
I think that the fins and rudders actually have more area than those on a Liberator bomber and are further back as it is drawn now.
Here's a pic of the side view on the plans.
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Old 07-02-2006, 07:41 AM
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Default RE: New twin design project

You have to invert those engines. Can't have ugly heads sticking out of the top of what looks to be a beautiful nacelle.

Micheal
Old 07-02-2006, 02:50 PM
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Default RE: New twin design project

Horizontal mounting would work also, with the bonus that tank positioning with relation to the carb will be better.

I agree with Bill, a twin needs a lot more yaw stability than a single (and more than a 4-engine also) to overcome asymmetrical thrust. Also, as you know, a float plane needs more vertical tail area than the same plane on wheels, to overcome the more forward side area of the floats. A good (successful) rule of thumb is that the distance from 1/3 wing chord to 1/3 stab chord should be 45 to 50% of the wing span and the total area of the vertical tails should be about 10% of the wing area. Those two measures are inversely proportional, so the shorter the fuselage, the larger the verticals and vise versa. If you keep the product of those two numbers close to .005 your airplane will have good, safe yaw stability.

Jim
Old 07-02-2006, 10:05 PM
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Default RE: New twin design project

Bill, after looking at the plane some more i think you are probably right in it needing more yaw stability, i will make those vertical end plates bigger. More area definitely won't hurt there anyways. I did have them much bigger when i started designing it but thought they looked too big.


Looking at the plans, I have to agree with Bill (W.R.) It is a different plane to be sure. The only other thing I see wrong is that the Bandersnatch doesn't appear to be syncronized with the Congonckulater? Can't wait to see this one finished. I can't recall ever seeing one either. Will it be planked or film covered?
I can never get those dang Bandersnatches sychronized with the Congonckulaters[] I will be planking the fuse and nacelles and fully sheeting the center wing section as well as the horizontal stab. The elevators, rudders and outer wing panels will be covered with fabric.

On this bird the heads will be sticking out the tops of the cowls but if this thing flys good or after getting it to fly good, i want to scale the plans up to build an 84" span bird, just big enough to give two snarling Webra 120's a home. I will plan on having those side mounted or inverted.

jrf thanks for that rule of thumb, i'll have to see now how much bigger they should be.
Old 07-02-2006, 10:34 PM
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Default RE: New twin design project

I just measured the vertical stabs and together (not including the ventral fin) their area equals 13% of the wing area. The distance from 1/3 wing chord to 1/3 stab chord equals 41% wing span at present location or 42% moved as far back as the tail feathers can go. I think i will increase their area a little seeing that the plane does have quite alot of frontal side area ahead of the CG.
Is this rule of thumb for twins or float planes?
Old 07-02-2006, 11:03 PM
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Default RE: New twin design project

Goose:

The SWAG rule of thumb concerns all side area, it doesn’t matter if the area is a float, a nacelle, or the main fuselage.

You need to remember the lateral area is the complete fuselage, TWO nacelles, and TWO floats. It’s not just the projected area that you see in a side view.

I still think it would be best to lengthen the afterbody, the fins will be out of proportion when they are big enough as short coupled as your plane is.

Lengthening it shouldn’t be hard, you are planking the fuselage so the longerons you’re having cut can be scrapped – balsa will be strong enough to hold the more widely spaced formers as you do the planking. And with a gentler curve you’ll find it a lot easier to plank, too.

Bill.
Old 07-03-2006, 08:50 AM
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Default RE: New twin design project

Ok i increased the areas of all fins and moved them back as far as they can go. The area of all fins equals 17.5% of the wing area now.
I will build it like this but I won't change the fuselage now, if it still needs more yaw stability after flight trials
then it'll be back to the drawing board i guess.

What is the SWAG rule?

I did calculate all side areas including both floats and it has approx a 2:1 ratio (area behind cg / area ahead of cg)
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Old 07-03-2006, 09:11 AM
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Default RE: New twin design project

Do you think that perhaps we're looking at an optical illusion here? That still looks small but it is X 2 and laying 90 degress off what we'd think of as normal.

Slowjohn
Old 07-03-2006, 09:59 AM
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Default RE: New twin design project

My Rule of Thumb works well for twins or float planes. I have not applied it to a float twin, but since you are now looking at an area x moment of .00735, I suspect you will be fine.

Your latest plan looks right, and as we all know, "If it looks right, it will fly right"

I must say, I have followed your designs for several years, and they are spectacular. I have lusted after each, and this one is no exception.

Jim
Old 07-03-2006, 10:52 AM
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Default RE: New twin design project

Okay, here's a question I should know the answer to but darned if I can come up with a definite-positive answer. With the stabs and rudders being in the prop wash like they are, will that help or hurt? Too sleepy I guess as I worked last night and no sleep yet.

Slowjohn
Old 07-03-2006, 11:20 AM
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Default RE: New twin design project

Thanks for the encouraging and complimentory words Jim.

On a twin (from what i have read) it is good to have the fins and rudders in the propwash, i think that's why designers started putting smaller dual fins and rudders in line with the wing mounted engines.
I believe that if a twin is designed with a standard tail, it needs to be bigger than usual because of the reduced propwash acting on it. I could be wrong on this but it sounds good anyways.
Old 07-03-2006, 11:23 AM
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Default RE: New twin design project

Go to bed now Slowjohn, my turn for nightshifts starts tonight[&o]

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