Engine size vs plane weight...

[Pat]

Hello all,

 I have a newbie question for the forum. Is there a formula to match a plane with an engine by weight of the plane?

Pat

[Steve Helmick]

There's such a huge difference in "power" between old engines and new engines, it's kind of dubious. My experience with old engines was about 1 oz per .01 Cu. In., so a .35 would be good with 35 oz, .46/46 oz, etc. With more modern engines (and flatter pitch props) then it would go more like 1.3 oz per .01 Cu. In. 

In more practical terms, thrust is what we really use. I have found that with the engine running properly, if I hold the nose straight up (and it leans out) and if it doesn't obviously have enough thrust to more than equal weight, it will be underpowered. In other words, it should be able to climb vertically out of your hand, if you are clumsy enough to let it loose. That's what is called "authority" in stunt engine terms.    Steve

[Brett Buck]

Hello all,

 I have a newbie question for the forum. Is there a formula to match a plane with an engine by weight of the plane?

Pat

   Not a good one. It depends on a whole lot of factors that vary all over the place - for instance, the difference in power between a Fox 15 Slantplug (insert Hurl reference here, it's this weekend..) and a Zalp 15 FAI speed engine is well over a factor of 10. But the weak engine might work better on a Ringmaster Jr. Even tiny variations in how the engines run make a HUGE difference in the kind of airplane you might hang it on.
 
    If you have a candidate engine or candidate airplane, tells us what you have in mind, and I am sure you will get plenty of help. I usually design the airplane around the engine, since it's a lot more difficult to adjust how the engine runs than it is to adjust the airplane to it. But, a lot of competitive stunt fliers have been building roughly the same size/weight models for decades and have gone from 46s to piped 40s to piped 51s to piped 61s and most recently, piped 75s - all in more-or-less the same size/weight airplane.

    Brett

[Dennis Moritz]

Thrust will differ depending on prop. Also, engines can be run differently, the same engine tuned for a lighter weight plane, or a heavier plane. Then there is the issue of the kind of run that you prefer, or the kind of run that mates well with the model you intend to fly. If you are using a familiar design, engine recommendations can be offered, practical experience being the criteria. The desirable tuning of the engine, discussed as well. In addition to weight of a model, the induced drag of different models will vary, a factor separate from weight. A thick airfoil straight wing plane needing more grunt for stunt than a thinner airfoil swept back wing. (I think sweep back is part of the issue.) Where you are flying, the prevailing wind conditions, even the characteristic air density of your home site (elevation) will also be a factor to consider. It's always useful to ask the opinion of fliers who fly well at the field you will be using.

[PJ Rowland]

One critical factor to consider is

Wing area for weight..

For example : A 57 oz model with a .60 is ideal at 640 + Sq Wing Area, Not great at 500 sq Wing area.