Engine Selection for a twin
 

Hi, I am new to this website, and have been flying RC for about three years.

I am currently building a Twin Commanche from a Jack Stafford Kit. I estimate the weight to be between 9 and 12 lbs. It will be fiberglassed.

The specs are:
Wingspan: 72"
Length: 48"
Wing area: 775 sq. in.

I was wondering what engines would be the best for this airplane - it will use 12" props. I am at an altitude of 6000 ft, and wanted to get two Saito four-strokes for it, but I don't know what size. I would like a fairly high top speed. Is a 72 or 80 good? I am afraid that a 91 would bee too much. And if I got a 91, would I be able to use a 12 x 12 prop on it?

Please help!

Also - I was wondering what torque servos I needed for this airplane. It will have an all-moving tail. It will also have one servo for both the ailerons - each aileron is 2" x 14"

Engine Selection for a twin
 

NJD:

In two-strokes, I have a slightly smaller twin that maxes right at 100 mphg with a pair of Magnum .46 engines, on 10x8 three blade props. And a slightly larger twin with K&B .61 engines and 12x8 three blade props, similar speed.

While I have several twins, none are running 4-stroke engines. As a rule of thumb, though, divide the 2-stroke size by 0.6 to get the equivalent power in a 4-stroke. But don't forget the weight penalty with the 4-strokers, and your density altitude.

With all this in mind, I'd say the .91 engines were preferred, don't really think they will be that much heavier than the .80 engines.

Servos? For normal flying standard servos work very well on that plane.

But do watch the weight. At 12 pounds it will have to be pretty fast, which you wanted, or it will be a pig. Another reason for the bigger engines. And 12x8 three blade props.

Have a good time, build it your way.
. But power gets it off the runway.

Bill.

Engine Selection for a twin
 

Well with 2 stroke i'd go for a couple of TT46 pros or OS46FX's, which means about a couple of 60 4 strokes would be ok BUT then they add extra weight sooo i'd go with something in the 68-72 range, for servos something like Hitec 422's should be planty

Engine Selection for a twin
 

Tanks - I will probably go with the Saito 91's because of the altitude. What would be the differences between a 12 x 12 two blade prop and a 12 x 8 three blade prop? I was leaning towards the two blade since the actual aircraft has two blade props.

I also wanted to ask what torque I needed for the flap servo. The total area of the flaps is about 2.5" x 36".

Thanks - NJD

Prop pitch
 

NJD:

With the four-strokes the 12" pitch would be a lot better, provided the engines will pull the RPM.

At 9000 RPM the 12" pitch will give you approx 90 mph in level flight, with the 8" pitch yopu'd have to crank the engines at 13500 rpm for the same speed. Simple choice, no?

I was wrong earlier saying 12x8 three blade, on a two-stroke they'd be fine. But not on a four stroke.

Servos again? When I know a standard servo wont do I'll go for a stronger servo, otherwise it gets a Futaba S148. And if it needs minimum free play the S148 gets a ball bearing cover.

Side note - I'm not saying Futaba only, I've planes with the Airtronics 94102, Hitec HS-300, FMA S300, even some Tower TS series servos. These are all "Standard" servos, and I'he had no problem with any listed. I have had problems with the FMA S301, the ball bearing version of the S300.

If, after a test flight I find the standard servo doesn't work as well as I would like I'll install a stronger one.

Two reasons for this. 1) I bought several dozen S-148 servos in a batch (got a super deal) and I still have a lot left. 2) A standard "Name Brand" servo is $15-$20, next step stronger gets into the $50 range for a "Name brand." Go "Off brand" you're still at $35. And since I'm really cheap...

Standard servo not strong enough?
. Change it, but paying might be tough.

Bill.