gerryndennis

Hello Everyone,

I just spent a most enjoyable afternoon with a good friend of mine running engines from a Mills .75 to a RCV 90 SP. Including running in two OS LA 10's of his.

The 10's are intended for a small twin model, and we are wondering if they will be powerful enough.

The original model was designed for speed 400 motors and flew at 29 oz/ft^2. The model is a reasonably low drag sport design for hand launch (no gear) with the engines aft fuselage pod mounted similar to the full size A 10 warthog.

Dave (my friend) has scaled up the plan for the IC engines and expects an all up weight of around 3 1/2 lb and wing loading of around 24 oz/ft^2 (he's a very good builder so will certainly achieve this).

My understanding is that twin engines will produce more thrust than a single engine of the equivalent capacity, but how much more? Will two 10's equal a 25?

I know that a 20 will fly a 3.5 lb model quite well, so suspect that the 10's will be fine.

What do you all think?

Is there a generally accepted method of comparing the thrust of twin engines to a single?

Maybe I would be better asking in the twin or multi forum?

Thanks in advance

Dave H

Lnewqban

Some references:

http://www.rcmplans.com/index.php?ma...oducts_id=1373

http://www.rcmplans.com/index.php?ma...oducts_id=1747

gerryndennis

Thanks Lnewqban,

I guess the Hornet would be closest to my mates model at 360^2 inches. Pity RCM doesn't mention the weight or wing loading, but 10 to 15 engines on a 360^2 inch scale model would probably mean 10's on a 336^inch sport model might not be too far out of the ball park.

Hopefully someone will be able to give me an equivalent to the two 10's. Then I will try and make sense of the area loading model to get a reasonable weight for a sport model.

Either that or someone will say 'yep two 10's are about the same as a 25 (or 20, or 30) and that will fly a 3 1/2 lb sport model fine (not at all, barely)'

Cheers

Dave H

BMatthews

The engines will do fine in a 3.5lb model. Should be quite sporty in fact.

I'd be more worried about the wing loading though. My own experience with models in the 44 to 50 inch span range is that they fly best at around 17 to 20 oz/sq foot where such a loading gives them excellent low speed manners. A model that I had which came in at 25 oz/sq foot flew OK at higher speeds but the slow side of things was less than ideal with handling being a bit dicey and tip stalls due to using the ailerons at low speeds fairly common. An A10 style of model is the sort of thing which would be fun to use for "low and tight" passes in simulated attack runs. Having a heavier than ideal wing loading affecting the slow flight charactaristics and waiting to sting the pilot during even higher speed with high G load maneuvers may not be ideal. If it's possible to lighten up the tail so that not so much nose weight is needed it would be a good idea. I know that's tough when you've got your engines mounted back behind the wing and don't have that honkin' big cannon up front to balance it. But he should try to do the best he can by altering the construction style in a few spots for lighter alternatives as well as selecting his balsa for lighter stock in the tail and wing.

gerryndennis

Thanks BMatthews,

He's a pretty good builder and flyer. I will tell him he needs to aim for 20 oz/ft^2 maximum, he will need to use his superior building skill to avoid having to use his superior flying skill.

I've found a photo of the original model and will post when I get back home to my computer in a few days.

The original is 40" span and 44" length. It is A 10 style but quite short coupled and with a large stabiliser and mildly swept wing. Engines are only just behind the trailing edge. Nose is reasonably long so hopefully not too much trouble to balance. At this stage Dave is planning a 16% enlargement so 46.4".

We ran his engines on a mock up of the pod and strut to see what the vibration level might be like. It was dead smooth so he may be able to save a bit of weight there. The engines seemed to run for ages on the 2 oz tanks, even tuned rich for running in, so there may potential to save weight with smaller tanks also. Maybe even a mid mounted single tank in the fuselage slightly forward of the engines. The tank would still be quite close to the carburetters.

As you say he needs to keep the weight down at the rear.

Thanks for your reply, You have put our minds at ease regarding the power of the 10's

Dave H

gerryndennis

Ok, I think I worked out how to upload an image.

Hopefully you can all see a picture of the original model.

I played around with WING CUBE LOADING (WCL) by FRANCIS REYNOLDS Model Builder - September 1989. http://homepage.mac.com/kmyersefo/CWL/reynolds.htm He gives a method for comparing wing loadings of large and small models. The theory is that you end up with a number, all models with the same number will fly similarly. Gliders will have a WCL around 4 etc

Glider 4
Trainer 6
Sport Aero 9
Pattern 11
Racer 12
Scale 10-15

So the original model has a WCL of 17-20! (depending on which wing loading you believe, the ad says 25, but the build article 29)

Dave's 16% larger at 3 1/2 lb would be WCL 15.5

If he can get down to 17-20 oz/ft^2 as recommended by B Matthews he will be WCL 11-13.

So far the consensus seems to be;

2 LA 10's will be plenty of power for 3 1/2 lb,
Keep the tail light,
Need to increase wing area or get a lighter build to achieve 20 oz/inch^2 max.

Thanks for your help,

Dave H

BMatthews

Since you guys are drawing the plans up from scratch for this larger verion I'd suggest you make the nose a bit longer and wider/deeper. First off it'll look more scale like and second it'll let you put the heavier radio components and the nose lead needed to balance correctly that much further forward.

It's going to be tricky making those stalks strong enough yet keep the weight under control. Especially since you need to make them long enough to have clearance for a 7 inch prop. He's also going to want to make the pods small enough that they don't interfere with the prop disc more than needed.

As you confirmed for yourselves with the cubic loading and with the surface loading it's going to be a heavily loaded model by any standard. It'll be better with big open style jet maneuvers and don't let it slow down. You'll need to do a lot of playing around with props to find the ones which let the model reach a decent speed while still letting the engines rev up to their best RPM to get the best out of such a model.

gerryndennis

Thanks BMatthews (Bruce?),

The plan certainly is to have the radio equipment well forward. The equipment should fit in the nose as designed.

We ran the engines on a reasonably light mock up of the pylon made from ply with no evidence of vibration or twisting. Hopefully Dave will be able to lighten the pylon even further by shaping to airfoil section and using a built up ply pylon with no loss of strength. Engines will be mounted tractor

Yes we will have to experiment with props to get sufficient acceleration for a hand launch combined with good top speed. A takeoff dolly or bungee launcher may not be out of the question.

Spent a fantastic day yesterday flying Dave's floatplane at a local fly in.

Thanks again for your help.

Dave H