How do I know what my thrust to weight ratio is on any given plane.Is there a way to measure it or can I calculate it.....???

I believe there is a thrust calculator out there, but it is not as accurate as strapping a digital fish scale or similar to the tail and hitting the throttle, and that is not as accurate as just plain flying the plane and seeing how it pulls, feels, and accelerates out of a hover. I had a 50cc Yak that weighed 15 pounds and would show 24 pounds of thrust on a fish scale. You'd think that would be awesome, right? Wrong. The plane had barely enough power to pull out of a hover...
I can tell you that if you do rely on the fish scale method, expect 2:1 thrust to weight to have a decent-powered plane.

Bodywerks is looking for "Foamy" performance from his I.C. planes. I agree that 2:1 measured thrust is a good power level, but it is very hard to achieve with a scale nitro plane. What some think is "rocket" performance is only minimal to others. Long story short--There is no such thing as too much power. There is, though, such thing as too heavy of a motor or airframe.

What motor and airframe are you looking at. I bet you the folks here can get you close.

My OS1.6FX puts out a 18 pounds of measured thrust(with the right prop) and is on a 11.7 pound plane fully fueled-- So my thrust to weight on take off is
18:11.7 or a dismal 1.53:1 --performance is good for me with 1,100 sq in of wing area (UL Giles) Would I like more--sure!

Looking for but never achieved (got pretty close with my 6.5 pound Edge and Saito 125)! You are right. Let us know what size plane you are looking at and someone here can help:
YS110 = good for up to 9.5 pounds
YS160DZ = probably 'round 13 pounds
OS160 = 12 pounds
Saito 180 = 11 pounds
DA-50 = 16 pounds
OS46 = 5.25 pounds
Evolution 100 = 7 pounds
These are my guestimations for various size engines performance at sealevel...

Thanks for the replies but how do i measure the thrust of the motor when i change props.

Warren - if you wanna be so darn certain then you need to get some weighing scales and attach it to the plane and give her all she has got. Another way is to use ThrustHP as a mere guide and discount say perhaps 15%. I use that as a guide and it seems to be about there when I remove 15%. What Bodywerks has done for you is pretty accurate (its through experience) if you were to use the most preferred prop on the motors he has stated. Most preferred prop can be had by going to the Tach forum or perhaps asking around here.

Fly it. There is a difference between static thrust and dynamic thrust. Putting a fish scale on the tail and taking measurements isn't going to tell you how the plane is going to pull out of a hover or how fast the vertical speed (upline and downline) is going to be. To give you another idea, a turbine engine might put out 30 pounds of thrust, but it won't accelerate as quickly as a propped engine that also has 30 pounds of thrust. This is because the turbine is moving less air but at a higher speed to get it's thrust, while a propped engine is moving more air at a slower speed.
Like I said, buy a few props that your engine is capable of swinging and just fly the plane and see which one suites you better. I have gone through 7 different props on my 35%er and am still looking for the perfect prop, although I am currently only using one of those 7 props because, through actual flight-testing, I have found it to be the best of them for my flying.

Thanks Ill just fly it then...

STG,
You said 'performance is good for me with 11,000 sq in of wing area (UL Giles) Would I like more--sure! ' Should that be 1100 sq in?

[sm=lol.gif] Yes 1,100 sq in[sm=lol.gif]

I mentioned wing area because that is a factor. The lower the wing loading (and better the Reynolds number) the less Thrust to weight you need for 3D flying with some "get out of trouble" power.

So if my plane hovers at 1/2 throttle does that mean my thrust to weight is 2:1.Also,can someone explain what wing loading is.

No, 1/2 throttle is not 1/2 power. For instance, I have a plane the hovers about 1/3 throttle(ends up being about 11 pounds of thrust for hovering and full power is 15 pounds of thrust) but my thrust to weight is only 15/11 or 1.36. On a good day for power pull out is O.K. by my standards(but some guys at the club think it is a rocket)--It could use more go go.

Do a search on wing loading and Reynolds numbers -- some good reading.

throttle stick and RPM are far from linier, RPM and power are also far from linier!!!

Also, power to weight depends on altitude, humidity, all sorts of things! on big cowls like the yaks have, its important to have a good distance between the front of the cowl and the prop, you loose sugnificant thrust if its too close. a smaller, streamlined, extra style cowl has have a smaller gap. This isnt just apparent on RC models, but also on full size (see a pic of a full scale yak!!)

and whats quite important, and what most people forget, is DRAG! if your going at 0mph, say in a hover, drag is no big problem, but in a strait line it is. If you double the drag, you have to x4 the power. 3 times the drag, 9 times the power, its just squares.

Don't forget Coreolis effect. It can be a killer.

TJ

I set up my throttle to be linear. Half throttle = 50% of maximum static RPM. 25% throttle = 25% maximum static RPM, etc.