Engine selection for Twins - Size FS
 

HI there!
I am considering building a twin engine aircraft, Have Warbird experience, scratch build YAK9, LAG7 and IL2 all were around the 72 - 74 inch wing span and used a SAITO 120 FS engine. I am considering building (Scratch building) a Pe-2 and was thinking building a "HAK" first to get some twin time before I go to my scale project.

So the question is, for specs like:
W/S= 70 - 74 inch
Weight -11 - 13 lbs
Wing area 800 - 1100 sq inch

What type of engine will give good performance? I know that this is hard to determine, fuselage, wing selection etch, etch However I assume that a single engine develops 10 lbs of thrust with 2.1 horsepower, then in order to get the same performance from 2 engines would one devied the horsepower in 2? and select 2 engines? that add up to the original single engine rating??

Any tips are appreciated.

Kindly Regards

Erick!

RE: Engine selection for Twins - Size FS
 

Erick,
I built a very similar plane and you can see my post by searching "obscure russian bomber" in this forum. It is 75" span, 960 in sq wing area and just under 11#. It flys well with OS .52 FS engines. Based on the performance you probably get with the 120's in your existing planes, I would suggest moving up to the .70 or .80 range four strokes for your twin. With a twin you usually don't need two engines half the size of a comparable single; a little less will do. This is because the twin will have more propeller disk area and be a little more efficient.
Chuck

RE: Engine selection for Twins - Size FS
 

Concur with that. My "ROT" ("Rule of Thumb") is to figure what engine I would use were I going to fly the model with a single engine, and then use two engines of approximately 1/3 that displacement. In other words, if a .60 would fly the airframe well, then two .20s would be about right; if a single .90 would fly it, then two .29s would work instead. If a .40 would work by itself, then 2 x .09s will suffice. More twins have died due to being overpowered and overweight than any other cause. Jim

RE: Engine selection for Twins - Size FS
 

Jim:

Almost total agreement, except I go 1/3 the other way. If a 60 flies it as a single I'll use a pair of 40s or 46s.

You like 1/3 of the single size, I just go down 1/3 from the original. But anything in the area of 50% single engine size will work very well.

Bill.

RE: Engine selection for Twins - Size FS
 

Two Y.S. .63s would be perfect for an airframe that size. Fuel tank placement and fuel draw would not be an issue with these engines.

RE: Engine selection for Twins - Size FS
 

Jim OHaver is in fact dead on in what he said. A factor most do not take into consideration with multis is the simple fact that when changing from a single engine to two or more that are of similar total thrust to the single then there will be a substancial performance gain. This gain is caused by the reduction in propellor disc loading when going to two or more engines. A multi will always have more disc area even though the power is comparable to a single engine.

This performance gain will manifest itself in shorter T/O, faster climbs and generally better verticle performance but not neccessarily in top speed.

Reducing the propellor disc loading is what the 3D guys are in effect doing by using unusually long and flat propellors that are typical of that activity.

John