I'm sure I'll be dodging flaming cannonballs on this subject, but I have to ask. Since most of us probably wouldn't try to fly a scale twin very long on one engine, why do we power them with enough engine to do so? Wouldn't they fly better with lighter engines and smaller fuel tanks?

Good point!

Hi!
Your'e absolutely right!
Inexperience is the key word why people still do so (use to large and to heavy engines that is)

Regards!
Jan Karlsson
Sweden

I have to disagee!

The reason some of us fly with larger engines is to swing a more scale looking prop. With scale twin warbirds, you will need a ton of weight in the nose to balance them. Having a larger engine helps rather than just adding dead weight. If it was a case of inexperience then having too much power up front would get you into trouble very quickly. If you have ever tried to fly a Mosquito or any single tailed, tail dragger you would find out in a matter of a few feet what it means to have too much power and not enough experience. The plane wouldn't get in the air because you would be ground looping.

Yes, I do have experience with several twins. Yes I have flown some of them back safely with one engine. As a matter of fact my twin trainer was a Magnum 80 with a pair of YS 45's for power. This was an 80" single tailed constant chord twin. I set up the engines so that I could get the left engine to quit . I would bring it in low and slow then kill the engine and perform a go around. It teaches you to use your rudder and to manage your power while being very, very patient.

If you are wondering which twins I own:
Magnum 80, 80" w/ YS 45's
Aerotech P38, 72" w/ same YS 45's as above
Aerotech Mosquito, 72" w/ Saito 56's
Blown up BT Mosquito, 108" w/G38's
Ziroli P38, 114" w/G62's

Cary

First I'm from the Ed Heineman school of design, if you can't make it smaller, lighter or cheaper...go away! I also believe that there should be enough single engine power to safely obtain a slight climb. I'd really like to try to save my airplane by being able to fly it to the runway vs dumping it where ever the first engine failed.

bash-ace has probly never heard the expression, "enough power to steer to the site of the crash."

A good flier and good engine man can fly with lower power. A good flier knows how to use rudder to hold the plane straight in the event of an engine out. If you read back thrugh this forum, you'll find several reports of people losing an engine and having the plane roll over and die. If you can't hold rudder in and at the same time adjust power, you need to get a sport twin that has good engine-out characteristics and learn to fly it, practicing engine out flying. Do this before you try a scale twin. It will save you a lot of grief.

I would also recommend a good powerful rudder servo and stiff controls. I prefer a digital servo on my twins. You also need to test your rudder linkage. Hold full rudder and try to straighten out your rudder. If you can, you need to fix the pushrods, linkages or change servos. Do this both directions. I've seen some scale planes that I could move the rudder back to neutral with 1 finger.

You also need to learn how to run engines for twins. It must be the dual vibration causing more fuel foaming or something, but an engine that may run well on a single can go sour on a twin. It is human nature to try to tweak a little more power out of your engine, especially if you are running smaller engines. On a twin, this is a single engine landing. Learn on a sport twin before trying a scale plane with little engines.

You guys can have your planes with undersized engines. Me, I want the biggest engines, big props that clear the cowl and a big digital rudder servo with pull-pull cables. I've been there and got the T-shirt.

Ed the situation you mention is exactly where "just the right amount of power" is critical. There is a point with only a single engine running that the airplane cannot over come the additional drag of the rudder being deflected a large amount, and the airplane will slow till it goes below VMC and spins out of control. The airplane should have enough power to climb on one engine with full rudder deflection.

FWIW, I have found that one can reduce the power needed to maintain level flight in a "one-engine out" situation considerably by banking slightly toward the operating engine.
My understanding is the following: When the rudder is deflected to keep the aircraft flying straight with level wings, a side force is generated by the tail that will cause the aircraft to sideslip. This is a very efficient method to reduce performance a lot!
Banking slightly toward the operating engine will incline the lift vector (from the live engine), creating a side force that offsets the one produced by the fin. Once these two forces are equal and opposite the aircraft will no longer slip. Drag will be reduced less engine power is needed.
The explanation may be "hand-waving" but it works for me and I suppose some aerodynamically versed person could explain why it works a lot better than I can.

/Red B.

Some very good points, how's the flying been with the Eglin Aeromodellers Ed? I sure miss flying with the crew down there. This is what brought up the question, I flew one of the TF DC-3's when they first came out, it had a pair of .46 2-strokes on it, I was not happy with the scale flight speed of the aircraft, it was very stall happy at scale speeds for a DC-3. Recently, I test flew one for a buddy of mine, it has a pair of .25FP's on it, took a nice long take-off roll but flew so much better (scale flight speeds were easier to obtain without a snap tendency) than the first one. Both aircraft were built by superb builders, I'm not sure what the weight difference was between them. Another design, I'm sure all have seen the RCM Twin Otter plans (80" WS), the fuselage on this plane is the size of a small coffin (about as big as my 31% edge fuselage), the wing chord is only 8", the plane calls for a pair of .30's (not 30cc). The flight report claims that it can fly on 1 engine and maintain a slight climb. Maybe it's the high aspect ratio wing and the huge tail fin, but what about the 9 to 10lbs of weight?

(quote) That may be true on the Mosquito and many other twins. However, my 117" span, .91 four stroke powered Commander twin, balances well with no nose weight added. A flying weight of 26,5 lbs. makes for scale performance and a convincing "scale" speed.

Bigger engines also means a heavier structure and a higher wing loading. If I had beefed up the structure of my plane during construction and installed G-38´s, I guess the flying weight would have exceeded 40 lbs.
(I am referring to the Ziroli twins of a similar size)
I believe that a twin with "power to spare" and scale sized props is more likely to roll over if one engine quits.

As having no single-time on my Commander yet, I´m not sure whether the plane will maintain altitude on one engine. According to a full scale Commander web site, the plane is "quite happy to be trimmed for straight and level hands-off on one fan"
I think I would just cut the throttle and make a dead stick belly landing. Fortunately, my local flying field offers big grass areas surrounding the runway

The plane is electric powered at the moment, but I am thinking of going back to IC power as I am missing the sound of the four strokes.