Joe Trinkley

I've read some about this and there's an equation out there that tells you the thrust, but it doesn't take into account for the pitch of the prop...so does anyone know an easy way to measure the thrust of an engine so I can get the most ideal prop on my plane, i have an exactly 5 pound plane with a webra 50 on her...a fish scale maybe but really how accurate could that be...maybe tie a 5 pound weight to the tail of my plane and see if it will still pull out of my hands when held in hover position? that would be a 2 to 1 thrust ratio i guess, what's an easy way here? I want to write down numbers and such, something similar to a fish scale but better anyone and affordable?

Thanks

BotleRocketWar

How about a spring scale? I used that about a year ago to measure the thrust on my Pro .46...I can't remember what the result was though...

I can tell you that you will get the most thrust with an APC 12.25x3.75. Anything bigger is probably too big for the engine. That prop will give you over 5 lbs of thrust. Probably around 6 or 7 lbs of thrust.

If the airplane will fly out of your hands, that does not mean you've got a 2:1 thrust to weight ratio.

For it to be 2:1 you would need an engine that produces 10 lbs for a 5 lb airplane.

Besides that, the prop that gives you the maximum thrust possible is not going to be the best for things other than vertical manuevers. It all depends on the airplane, the engine, and how you want to fly it. The best thing to do is to experiment with a few different props.

Joe Trinkley

5 pound weight and a 5 pound plane doesn't equal 10 pounds, which would be 2 to 1 thrust?

ChuckAuger

My Feedback: (12)

Rest assured that a Webra 50 with the 12.25 X 3.75 APC will have plenty of excess thrust to fly a 5 lb plane.

Don't get too involved in trying to measure thrust on the ground. Try a few different props and see which ones fly the plane best.

Cheaper than a set of fish scales and it's a lot more fun to fly with different props than measure a plane tied to a post.

This is just my take on the matter....but if you really want to know, I have a Irvine 53 with a UT muffler, turns the 12.25X 3.75 at mid 14's. It has just over 10 1/2 lbs static thrust, measured with fish scales.

The program you mentioned is Thrust HP and it said I should be getting 14+ lbs. I was sceptical, so I measured it. I fish also, so I have scales. I find that the Thrust HP numbers X .7 are fairly close.

But I still went out and bought a couple different props and tried them.

Joe Trinkley

well i'm turning a 12x4 at 14200, good thrust? what is needed for great pullout of hover?

ChuckAuger

My Feedback: (12)

That'll be just fine! Go give it a whirl, I think you'll be very pleasantly surprised.

BotleRocketWar

Huh?

To have a 2:1 thrust to weight ratio your engine has to have twice as much thrust as the weight of the airplane.

With just about ANY prop, that Webra is going to be more than enough to pull you out...Just get a couple 11 and 12 inch props and see what you like the most.

Chuck, I remember my Pro .46 had over 9 lbs of static thrust with the 12.25x3.75 and Tower muffler. I never did understand that. Never seemed like it when hovering the 4 lb POS...

ChuckAuger

My Feedback: (12)

Yeah..you have to take away the airframe drag, and also remember that as soon as the plane moves forward, the static thrust will go down, as it is no longer static.

Was the 9lb figure from measurement ot Thrust HP?? If Thrust HP...then about 6 1/2lbs would be my guess. Sounds about what you were getting.

BotleRocketWar

It was a spring scale that went up to 5 lbs...

We held the aircraft vertical with the spring scale attached to the tail wheel and went to full throttle. It pulled it past where the scale would register...Not accurate, but over 9 lbs!

Just seems like about half that while hovering. I guess it's probably true, but accelerating out of a hover REALLY takes some power...

If these little guys are giving 10 lbs of thrust, I'll bet my YS .63 would have come out to be about 15 lbs of static thrust...

Something ain't right...

bugsiegel

My Feedback: (2)

I aslo have a webra 50 and a plane that weight in at exactly 5 lbs( sig something extra).
I've been experimenting with different props in the air looking for the right prop for the plane/engine combo.
after some flights with some of the smaller sized props in the 10/7-11/7 range I felt the combo was in need of a larger prop.
I was at the hobby shop the other day and bought a 12/4 apc and two 12/5 wood props. between flying days I made and adjustment to the position of the arm on the servo so on take off the trim was way off. I bounced the plane a little and broke the 12/4 prop before I had a chance to put it in the air. I went back to the box and put on a 12/5 wood which is alot thicker than the apc. I flew for the day with that prop but liked the sound of the 12/4 better. it seemed to spool up faster.
tomorrow I will try one of the 12.25/3/34 apc I have
we will both probably end up with a 12/4 12/5 or 12.25/3.34 apc.
Chuck is right though, do your testing in the air, it's more fun.

Joe Trinkley

so how did the webra pull that little plane around? good vertical? I think I'm going to stick with the 12x4 and try out the 12.25x3.75 at the field just cause I have a couple in my box

bugsiegel

My Feedback: (2)

it pulls it good but I feel with the big 12/5 prop it's a little overloaded for this plane. keep in mind that this plane will fly on a 10/6 prop.

akahoverit

My Feedback: (2)

The other thing to keep in mind is the larger the diameter the prop the slower it spools up, so if your new to hovering you may want to go with an 11x5 or 11x4 to give you quik revs when you need it.

Do what you think is best for you basically.

tetherite

Use a wood prop for the best transition!