Can some of you technical guru's help with a calculation. Is it possible to work out the Thrust lb's for each of these engines from this information.

LASER 200v 300v
Bore mm 27.7 33.5
Stroke mm 26.9 26.9
Weight grm 1160 1660
Propeller 18x6 20x8

I am considering them both for a WM GS Zero that should end up around 17<18lb's and need to know which one would give the best scale performance, (with a three plade prop if possible).

Thanks for the help.

Not without RPM Numbers.

Ross

For calculations you will need the following data first: prop diameter, prop pitch, number of blades, air density and RPM. Prop efficiency is a factor too.

For an 18 lbs plane flying scale only you are best running an engine that will turn something like a 20x8 at about 8,000 RPM. An 18x8 turning at only 8,500 RPM will give you about 19-21 lbs thrust and that is just nice for a 18 lbs plane. It will fly nice but you do not have much more in reserve. I think the 20x8 turning at about 8,000 RPM will give you no less than 24-26 lbs thrust. If its scale flying that you want then its okay.

A general idea of engines vs. plane weight (we are taling almost maximum weight for the plane to fly decently and not 3D etc), does not apply for planes like Piper Cubs and 3D or Bipes. I got all this form trial ... its not definitive but a general idea of where to start with. Nothing beats people who have flown the plane before.

46 2C - 6.5 lbs
50 2C - 7 lbs
61 2C - 7.5-8 lbs
91 2C - 8.5-9 lbs
120 2C - 12 lbs

72/82 4C - 7 lbs
91 4C - 8.5 lbs
100 4C - 10 lbs
110 YS - 12 lbs

Smoggyboy..

Download from this site : http://freespace.virgin.net/barry.ho.../thrusthp.html

You can calculate from propeller size and rpm in the ThrustHP programme and get answer in thrust, hp, and speed... You need tachometer to measure rpm before you take calculate...

Jens Eirik

20x8 @ 7500 rpm generates about 25 lb. of thrust. I am not sure which Laser V will pull that. But I'd go for the 300. You will need the extra weight to balance.

Thank's all,

I have downloaded the convertion chart so I can have a go now, thanks Motorboy.

Just one last thing , is there a genral rule of thumb to approximate the lbs Thrust to wieght ratio for certain types of flying. eg;

1.2 X The dry wieght of the aircraft= approximate lbs thrust required for sport flying.
1.5 X The dry wieght of the aircraft= approximate lbs thrust required for 3A flying.

Thank's for all the help.

1.0 X The dry wieght of the aircraft= approximate lbs thrust required for scale flying.
1.8 X The dry wieght of the aircraft= approximate lbs thrust required for 3D flying.

1.8 for 3d? I have heard around 1.5, but I am new to 3d, so I am not too sure.

1.5 for 3D is like the minimum (if possible) ... x2 is heck of a lot of fun, then we are talking about being able to punch out of a prop hang with a lot of authority. I am gonna test the amount of thrust on my F90 with a YS110 but I suspect that I am getting about 16-17 lbs thrust. The plane is about 9.5 lbs. Hence I have about 1.7:1 power to weight ratio. The plane hovers at 55% throttle but I would love to see her punch out much faster. If only I had like 1.8 or even 2 times. For me its about the thrust for uplines ... love to see a plane climb forever and able to pull nice big loops with ease.