jeffie8696

This is a rather simplistic way of thinking about a very complicated issue but here goes.
When choosing an engine is there a graph or chart that can be used to match all up weight and wing loading to the correct size plane?
I know guys that bolt .40 size engines on .20 size planes without issue , that is why I ask.

ace4-40

Not really and it all depends on the airplane. Like you mentioned, wing loading makes all the difference in the world. A Cub will fly well with an engine that won't get some planes of the ground. The SIG Kadett Senior is another example. I had one with an old Ohlsson 60 with an OS 4B carb on it. It was rated at 1/5 HP and flew that big light loaded wing airplane very well.

As for using a 40 on a 20 sized airplane. It seems that everyone is into winged rockets today. The old Astrohog won the nationals in 1957 with a K&B Greenhead 35. Today everyone uses a 60 or bigger and I've seen them with 90 four cycles.

jeffie8696

I have seen some really strong 32-36 size engines on the market and was wondering if there would be an advantage to using one on a .40 size trainer given the weight savings. I think you could also decrease the fuel load . And with modern lightweight receivers and batteries. See where I'm going .

rainedave

My Feedback: (1)

You can use Pe's spreadsheet to calculate static thrust:

http://mvvs.nl/prop-power-calculator.xls

If you simply want a good sport model a 1:1 thrust/weight ratio is all you need.

You can also divide the engine's HP by the model's weight in ounces to get the HP per ounce. Current 2M pattern planes typically have a hp per ounce ratio of around 0.021, or so. A 5lb. model is 80ozs. An average .46 produces 1.5hp. That's 0.01875hp per ounce which is adequate for sport flying. Put a 2hp .61 on the plane and the hp per ounce goes up to 0.025. I would say a hp per ounce figure of 0.02 is more than enough for sport flying.

David

ProBroJoe

Well, frankly I wouldn't put a .40 on a ".40 size" trainer! I'd go for a ball bearing .45 to .53 for better grass field take offs and power to gain altitude quickly. The weight savings between a "powerful" .32/.36 and it's "lighter fuel load" isn't going to make that much difference on a typical ARF trainer, which is rather light in the wing loading department anyway. With the high-power ".30" class engine you'll be forced to swing a smaller prop at higher RPMs - which is something you want for a slippery, faster model, not a trainer!

Worried that a typical ARF trainer is a bit heavy for you (e.g. too fast) then build a Kadet Seniorita or Kadet Senior... those things are practically powered kites and idle around the sky on the minimum manufacture's recommended engine!

Speaking of which, it's been my experience that the kit/ARF manufacturers typically give recommended engine sizes that are on the low end of the scale, so if you go with the larger engine of their suggestion, you'll be just OK and probably want more "ummph" for that added comfort level. You'll probably do alright on an asphalt strip, but flying off of grass will most likely be frustrating...

FWIW, I agree with rainedave's suggestion of a 1:1 thrust to weight ratio for most sport planes... but as I've just eluded to, there are always going to be exceptions to the rule!

-Joe

w8ye

My Feedback: (16)

A friend has an old Kadet Senior that's as old as the OS 35FP that powers it. Takes forever to get off the grass but flies along rather well up in the air.

microsprint9

I have a TT PRO.36 on my Hobbico Avistar ( .40 sized ), the plane comes in under 5lbs and spinning an APC 11x5 it takes off very quickly and has lots of power to fly it, some guys bolt on a .46 on this plane but the .36 IMHO is perfect for this plane.

For most of my planes i use ThrustHP and as above, i go for at least a 1:1 thrust ratio and at least a 50MPH pitch speed. Even when i did my 10lb 10 foot B2 foamy bomber i knew my TT PRO.46 would get close to 11lbs thrust and 50mph pitch speed and even at 10lbs this plane flew quite good, but also as mentioned above the wing loading was quite light.

Welcome back ace4-40, i like the new name

Here's a clip of the mighty .46 VS 10lbs of foam and wood http://www.rcuvideos.com/video/10FootB22-wmv

w8ye

My Feedback: (16)

The Avistar is a wonderful plane but is smaller than the LT40 & Kadet Senior type planes

I bet that 36 took it for a whirl.

huck1199

I had an Avistar with an LA .40 and it was good for power. I used on a grass field. The LA .40 is like a BB .36 I would guess.

microsprint9

I have the TT PRO.36 and a .40LA and i found the .36 to be stronger by about 3-400rpm and it also weighs 2oz less.

LuvBipes

My Feedback: (4)

You can never have too much power, only too much weight. As long as good wing loading is maintained and the CG stays where it's supposed to be, adding power only enhances performance, provided you know what the left stick is for.

jeffie8696

I have heard that my Tower Trainer 40 mKII ARF is best with a light engine. I have several .40-46 size candidates and they vary in weight by several ounces. However the power output also varies (but none as wimpy as the LA40).
One of you is guilty of putting a .40 on his .20 and he knows who he is. How's the skiing?

microsprint9

Well i've never put a .40 on a .20 sized plane, but i have put a 1.28 (21cc) on a .40 sized lanier explorer and the funny thing was the CG was perfect, and yes it had lots o power. This plane now resides with a friend of mine, he has a stihl 26cc on it now.

rainedave

My Feedback: (1)

There's something to be said for limited engine displacement. My favorite r/c models are the classic pattern ships of the '70s and early '80s. They were restricted by the rules to using 10ccm (or less) engines. This rule is what determined their size, weight and streamlined appearance. The models of that era would all look totally different if larger engines were allowed (and dimensions were limited). Those modelers really pushed the design and performance limits, using retracts, sleek and slender fuselages and tuned pipes to achieve the high speeds necessary for smooth, large maneuvers. To my eye they are the most elegant and beautiful r/c models ever made. And, it was the 10ccm limit that was responsible for that style of model and flying. Sure, you can shoehorn a YS 110 into a UFO. But, for me it just misses the whole point.

David

BillS

A rule of thumb and a starting point to think about the weight engine size relationship.

.40 to .50 engine – 5 to 6 1/2 pound airplane
.60 engine – 7 to 9 pound airplane
10 to 12 pound airplane – needs a .90 engine
14 to 16 pound airplane – needs a .120 or a 25cc to 30cc engine
Gas engines - weight times 2 = cc

Bill

ProBroJoe

Yea, it's me - so what? I also have a piped .45 on a Sig Kavalier, and a .15 on Lil Extra... and your point is?

jeffie8696

Joe, you be hopeless.
The Extra was supposed to be powered by an OS 10. Goof.
Wish I had picked one up last week, ,,,well you know. [:@]
And the T-20 looks like all engine.

DMorrisPE

I have seen these questions for years, did the research, and have tried to assemble the info into a single document with a couple common examples. In reality, all three methods will result in the same answer - the measurement conversions are baked into the factors. If you are more comfortable using metric, be my guest, the answers will be valid for any size plane.

Mr Cox

That's an old thread, I hope they have sorted it out by know.

Your link looks like it is for electric motors with the power given as the battery drain etc., i.e not so useful for engines.

For planes with engines there are much better and very illustrative graphs: