Andre

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Hi just wanting to pick someone's brain with twin engine plane experinence.I've being in the hobby for 25 or so years building various scale planes.For the past two years I've being scratch building a 415 Canadair 96".I'm in the painting stage and can see the plane will be in the 22-25lb range.Iwas looking at using a pair of Revolution 100's two strokes and wonder if this is enough power to fly this plane.Thanks in advance for any replies "RADAR"

William Robison

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[b]Andre:

You didn't give the wing area, but as a guess you'll be 30 to 35 ounces per foot loading. If that's in the ball park I'd think your 100 engines might fly it, but with marginal performance.

I'd try 180s.

Bill.

jrf

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I have not flown a 25# twin so I must defer to Bill, who probably has, but a pair of Evolution 100s with the right props should be able to generate 25# of static thrust. A static thrust/weight ratio of 1:1 should result in excellent flight performance.

Why is that not so?

Jim

William Robison

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[b]Jim:

While the 100s can give the thrust, they also run out of breath before the plane is going very fast. I just think he'd be happier with more power, but as I said, it will probably fly with them.

Consider. You can pick up a 100 pound bag of potatos, but how far can you run carrying it?

Bill.

jrf

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Andre:

I cannot disagree with Bill. More power is always better as long as the gain in engine and fuel weight doesn't cause wing loading problems. You can always throttle back.

But in the absence of someone who has tried it and can give us the real world results, let me try to quantify this a little bit; Clarence Lee's review of the Evolution 100 in the June 2005 RCM says the engine will turn a Zinger 15x6 at 10350 rpm on the ground. My thrust calculator says that equals 15.35 pounds of thrust, or over 30 pounds for two. The pitch speed of those props at that rpm is a bit over 60 mph, which you should see in level flight at full throttle.

So if you can go with 180s (I assume Bill is talking about 4-strokes) and the attendant fuel load without getting the wing loading too high, (or blowing the budget), by all means do. If not, I would think the Evolution 100s would fly the airplane very well.

Jim

William Robison

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[b]Jim:

You are making my point for me.

When I say "Runs out of breath" I'm talking about reaching pitch speed.

At 10350 rpm. pitch speed for a 6" pitch is 58.81 mph. But this supposes no prop slippage. That big draggy plane will probably limit us to about 70% efficiency, our air speed then drops to 41.16 mph. OK, so the engine unloads, let's sat it reaches 12K rpm in the air. At our 70% number we're still only flying, barely, at 47.73 mph. Or even if the prop grabs better and goes to 85% at 12K we've only managed to get to 57.95 mph. Now if the all up weight were 12-15 pounds it would most likely fly nicely at 45-50 mph, with my estimated 1700 sq inch area.

The plane at this weight just needs more speed which means more pitch, and that will take more engine.

Hope I've made my reasoning clear.

Bill.

Leo

I got scale performance on my 117" Aero Commander powered by .91 four strokes.
Wing area 1708 sq.in. Flying weight 26,5 lbs. APC 14x7 props
The plane is now electric powered. 1600 watts of input power. Flying weight 28,5 lbs.
APC-E 17x10 props. Static thrust 16,5 lbs at 6200 RPM. The performance is similar
to the I/C powered plane.

Hope this helps,
/Leo

SaviCatses

Where do you get these numbers? Someone here spoke of a thrust calculator, anywhere I can find one of these?

Leo

I measured the static thrust on my plane, by using a digital fish scale
Here´s a thrust calculator:
http://freespace.virgin.net/barry.ho.../thrusthp.html

William Robison

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[b]All:

A thrust calculator is a good tool, but it only gives numbers valid at zero air speed.

I have an Excel worksheet that works for in flight calculations, attached picture is two reports on the 15x6 props at 10350 rpm and 12000 rpm. The first at 70% efficiency and the second at 85%.

Some other numbers too, note particularly the true tip speed. This gives an idea how loud the propellor will be.

The numbers in blue are user entered, the rest is calculated for you.

The sheet has a second section that gives blade angle at user selected diameter points, lets you check a commercial prop to see if the pitch is what they claim, alternately it lets you carve your own props with complete accuracy. This part is not shown on the picture attached.

If you would like a copy of this sheet send me your email address. Has to be email, RCU PM system will not allow a machine code file to pass. You will need either Microsoft Excel or a spreadsheet that can import an Excel sheet. I also have a text file of instructions for using it.

Bill.

BillS

Any comment on gas engines at 25 pounds, 93� span and 1550sq. in.?

Bill

William Robison

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[b]BillS:

"Classic" would be G-38s, modern ZDZ 40s or similar.

Bill.

jrf

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I use ThrustHP, which is a "freeware" program I downloaded several years ago. It is still available at http://www.bmaps.net/ under the "Goodies" tab.

Jim

Edit: This is the same program as Leo posted.

multiflyer

Regarding engine size, here is a good thumb rule: You want your static thrust to be about 70% of the takeoff weight or more. Planes will fly at 60% or sometimes less but climb performance is getting really bad. For static thrust use actual numbers (strap a good fishing scale behind the tail and rev it up). So for a 25 lb plane, at least 18 lbs of static thrust. 1.20 size glow 4 stroke is about 9 pound of static pull. I estimate you don't want any smaller.

For example, I just put a bunch of flights on a KMP P38 at 19 lb. Was a real slug with 2 stroke .70 engines (static pull about 6 lb each). 4 stroke 1.20 were fitted and much better. Light wing loading helps too. P38 was right at 3 pounds per square foot. Pretty heavily loaded. CL415 at size and weight you mentioned should be similar wing load. I think the Revo 100s you mentioned would get it into the air but be disappointing.

Look at it this way - how well would one engine fly a similar plane half that weight? What engine would you choose for a 12.5 pound single engine plane with a 3 pound wing loading?? That means about 4 square feet of wing area (a third of 12.5), so span would be about 60 inches. Think of it like a typical 40/60 size ARF trainer (usually about 6 lb) with a 6 lb brick strapped on. If you still have your trainer (and a brick) you can check out the needed engine performance for yourself. Note that above info is practical, based on actual experience. Always yields better predictions.

multiflyer

multiflyer

One more note on measuring static thrust using a fishing scale tied to the tail. Obviously the airplane needs to be able to roll freely and on a flat hard surface. If you want to be super accurate, floating the plane on a pond for the measurement is the best, absolutely flat and no friction. If it's not a sea plane just set it on top of an inner tube or similar to float it. Or hang the plane from an overhead support (the wing does not need to be attached) for the test.

The important thing is to use the prop you would fly with. You can put a tractor prop on for the ground test and get larger readings but this is misleading. A tractor prop will rev out as soon as the plane starts moving. A high wing loading plane will need to move relatively fast through the air to fly. So the optimum prop for flying must have adequate pitch. For 10,000 rpm engine at least 6 pitch and probably more like 8. Don't use a 4 pitch for the test and fool yourself.

My 75% minimum static thrust to weight thumb rule is based on using the right prop for flying. And remember it's a "thumb" rule to get you into the right ballpark.

When in doubt, more thrust is better than less. You can always throttle back. I recommend the 4 stroke 1.20s as minimum. Two 1.50 4 strokes should be nice too. I think 1.80 4 strokes are overkill, unnecessary vibration and fuel consumption for your application.

Seriously consider the example of a trainer with brick strapped on that I mentioned above. What engine would make you happy flying that? I would actually do that experiment. Spent the money to by and ARF trainer with 4 square feet of wing if you have to. They are cheep and it is well worth it. Add one pound at a time and gradually build up to flying at 12.5 lbs. Gradually get familiar with flying the heavy wing loading while testing / breaking in / getting familiar with operating (prop, fuel, etc.) while evaluating your potential engine selection for yourself.

multiflyer