Deadeye

Hi fellas,

Has anybody seen, or does anybody have; a list of, or graph of model weight vs. engine size? I need to know how big of an engine I'm going to need for a 15 pound (I hope that's as heavy as it gets) scratch build.

Of course, a list or graph showing the full range of models from .25 all the way up to the big gassers would be awesome. A graphic depicting this would be even better *cough Minnflyer cough*. Thanks!

TailDraggin

My Feedback: (5)

I don't think it's that simple. How fast do you want your 15 pounder to fly? What is the wingspan? Wing loading? Fuse frontal area? What kind of airfoil? Do you want scale-like flight, aerobatics or fast passes?
All these factors have a bearing on what size (and type) engine you need.

Deadeye

Yeah, I guess you're right. I was hoping for some magical formula. But it's more complicated than that.

This airplane will be a trainer-type of plane. 8 foot winspan, with a 16" chord. I'm hoping it will weigh in at 15 pounds. It's defintley an expermental type of plane, as it's made out of coroplast. I'm fitting it for a Saito 100, because that is the biggest engine I have; and while this might get it off the ground, I don't think it's enough power to do much else.

I'm hoping to employ bombay doors if my airframe proofs airworthy. And possibly using this as a glider tower, skydiver vehicle, etc. I'm thinking it's gonna need about a 120 2 stroke to accomplish all of that. But like I said, I'm hoping a Saito 100 will at least let me test fly the thing.

Deadeye

Here's a picture for reference sake. It's far from being completed.

TailDraggin

My Feedback: (5)

That's quite a bird! Especially compared to the size of the crew transport on the table!
I don't have param's for anything that big, but I can give you some numbers on some smaller stuff if you want to interpolate...

.10 - 300 sq. "
.15 - 375
.25 - 450
.40 - 600
.60 - 725

If you can run out the function to 1536 sq. " (your wing size), you'll be in the ballpark. If I had to SWAG it, I'd say about a 1.8.

Good luck with it!

Cactus.

My Feedback: (1)

what WOULD be good is a graph that plots an engines size VS its weight, that way the graph points shoud raise, but you'll see some engines lighter than others in that class, or THAT 40 might be lighter than a 36, very handy to know.
could be as simple as coloured dots, or as tricky as a whole web thing which has a little graphic to show the engne, click it and get full specs.

TailDraggin

My Feedback: (5)

Engine HP vs. displacement and weight would be useful too.

bearmech

My Feedback: (1)

Check out the calculators @ www.limeybob.com

bearmech

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sorry wrong link

try http://pages.sbcglobal.net/limeybob/

gubbs3

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I don't think it would be very easy, or accurate, to make that kink of graph. You would need to include factors like: wing area, weight (wing loading), drag, lift, etc.

fly109

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I agree there is no magic formula but there are some simple guidlines that can put you very close. When I design my models I use the following as a target, regardless of the size of the model. While there are exceptions to this rule, the majority of models will fall into this range. You can use a little common sense to adapt to a wide range of other types of models. For instance, if you want to build a high performance racer version of a P -51 Mustang such as Dago Red, give it one horse power for every 4 pounds - it will haul *****!

For single engine WWII fighter types;
Allow 1 HP for every 5 - 8 pounds.

For Twin engine high performance types types;
Allow 1 HP for between 6 - 9 pounds.

Multi-engine types (4 and up);
You can allow 1 HP for up to 12 pounds.

Here are some examples that you can see the numbers. My 109 weighs in at an even 40 pounds. Flys just okay on a G - 62 ( 4.75 HP). With a 3W70i, the model flies very realistically and has good scale performance at about 1/2 throttle - 6.5 HP. This means I have 1 HP for every 6.15 pounds.

A top flight Gold P-51 weighs around 12 pounds finished, while it fly okay on an average .60 (1.5 HP), a 90 is the choice for most modelers - about 2 HP, again 6 pounds per single horse power.

A 10 foot wingspan B - 17 weighs in at around 40 pounds. Four .45 two strokes or .53 four strokes will fly the model like a rocket. At around 1.2 HP each, the total HP is 4.8 carrying 40 pounds - 8.33 pounds per HP.

A full scale P - 51 weighs what, about 9,000 pounds? With about what 1,300 HP? Guess what, 6.92 pounds per HP!

Like I said, there are no magic formulas, but the amount of power to fly weight is a constant that we can pretty much rely on.

Cheers

fly109

My Feedback: (1)

Deadeye,

Your model (trainer type) at 15 pounds will fly great on about 1.5 HP and up. The Saito 100 (The Twin) is a little shy on power and is equal to about a .60 two stroke but if you go with a single cylinder such as a .90 you will be better off. If you want better performance be prepared to up the ponies. Good Luck.

Checklst

My SR Telemaster has a 96" wing span with about 12inch wing cord its weight is 11 1/2 pounds(with motor) It flies on a TT91 FS and has lots extra power. At full throttle she will lift off in about 30 ft (dead calm wind) about half that in a 10 mph headwind. Half throttle take off are the norm for me.

Hope this helps