MikeMc

OK, another outside the box question from your favorite speed n00b.

Anybody ever do twin engines? I was thinking tandam configuration like the Do335 Arrow. In the full scale world twins have been usually twice the engine HP (2 engines), 5x complication with very little if any speed gain. I'm already on the drawing board with one. What are your feelings on this?

thanks,
Mike

ChuckAuger

My Feedback: (12)

I always thought the rear prop would be turning in dirty air and wouldn't get as good a bite as if it were on the wing in clean air. Also, the availability of pusher props is pretty limited.

combatpigg

My Feedback: (3)

You get more pulling power for climbing, but you won't get any more speed. If you could figure out a way to multiply the thrust from the forward prop to the rear prop, then you might be on to something, but now you are talking about some kind of 2 stage fan. Extravagant ideas usually translate to extra weight, and cubic dollars.

MikeMc

Here's what I was thinking. Same exact engines put the rear would have a slightly higher pitch. The air is already accelerated off the front prop so the rea won't have to work as hard. Slightly higher pitch would give you greater speed.

I'm sure there's book and book on this for full scale but I always have to find out on my own.

skyflyer47

My Feedback: (2)

I like the sound of the two stage prop. Give it a try the worst thing that could happen is it won't work.

combatpigg

My Feedback: (3)

Sounds doable, duct the thrust off the front prop into the rear prop, set the rear prop blades at 45 degree angles,[ whatever pitch that is] and let 'er rip! A TURBO BEE type setup would be a kick to see set up like that, with a pair of big ROSSIs! You can't get something from nothing, and only what goes in can come out, so there would have to be an auxillary air inlet to help feed stage 2.

MyWay

If its speed your looking for, one engine with the same displacement as the other two combined would be faster. What are you trying to accomplish?

MikeMc

Fun experimentation.

schmleff

So build a Pond racer. Your not going to get much cleaner than that.

happyal

Hi Mike,

Build it, if it works I'm sure we'll see a lot more planes with the same design. If it doesn't work, at least we'll know.

How about a smaller prop at the front and a larger prop at the back.

I'm sure that in WW2 the Germans built a Push and Pull fighter, can't quite remember the detials, but I think it has a bigger engine at one end for some reason. If you can find any detials on it you can use there research and built on it.

Post a pic of your plans, see if anyone has got anything to add.

skyflyer47

My Feedback: (2)

I'm no expert but I'm pretty sure two engines of half the displacement will produce more thrust than a single engine. because of the greatly increased prop area if you calculate it out and figure the area of two circles as apposed to one slightly bigger one. For what it's worth. I'm currently working on a twin project not necessarily for speed just fun. It has two old os fp 25's on a 60 inch wingspan with a rebuilt 40 size extra 300 fuselage. skyflyer

Rudeboy

Of course two engines with a given displacement and a given prop (say two 40's with 9x7 props) will produce more thrust than a single engine of the same displacement with the same prop (one .40 with a 9x7)... If they are mounted side by side that is.

The point with twins is that you can cut back on the prop diameter, but retain the same prop disc area (thrust) because you have two of them... and step up the prop pitch at the same time without overloading your engine (again, side by side engines).

One engine behind the other... that's a hard one to get working properly... but hey, experimenting is half the fun!

skyflyer47

My Feedback: (2)

You are right expermenting is half the fun I just wish i could build fast enough to try more stuff out.

skyflyer47

My Feedback: (2)

One other thing you might want to check into a Cessna Bird dog I think thats what it is called. Has two engines front and rear twin boom. I saw it in the movie Bat 21. Don't know if anyone has plans.

lloyd a

have a plane with 3 0s46 on it turning 10-6 does not have the pull or the speed as the identical plane with one gms-120 turning a 16-6. remember that the wider the prop is it gives more thrust even compared to same area displacement of 3 small props, larger diameters are more efficient due to more of there surface is giving clean air (not obstructed by engine)

Lomcevak Duck

My Feedback: (14)

Sky flyer: The plane you are thinking of is the Cessna Skymaster (C337). The bird dog was a lanky looking single engine high wing design. I have wanted to build a big skymaster for two O.S. 1.60 FXs for a long time now, but have never found any plans. This won't be a fast plane at all, but definatly cool.
-Steve

crasharama

This is a link to the KMP Cessna 337 ARF...

http://www.kmp.ca/index.htm

They were known as a Super skymaster in the civil world or the O2-A in the military Foward Air Control role.

Tony