I have a question on prop size for os .40
 

I recently picked up a old altech tamcat. it has never been flown or the motor even started. it is a big trainer plane it has a os .40 la installed. that is the smallest size engine they suggest. what size pro do you guys think i shoul use... i want to get as much power out of this motor i can .

That's a pretty weak engine, even as .40 engines go. A 10x5 is probably all it can turn without bogging down too much.

Agreed, the LA 40 was a dog but it seems to be on every trainer I have ever been given. The LA .46 is much better and OS dropped the .40 a few years back. A 10 inch prop with the smallest pitch you can find works pretty well. As under powered as they are it will still haul around most trainers and are pretty good in 40 size fun fly planes like the Up-Roars.

i have os .46 la i could throw in there

11 x 3 [ APC] or 10 x 5 has worked for me. As others have have said not the strongest of engines.

The problem with a weak engine and a low pitch prop is that you won't be able to power through the wind. That can be a fatal flaw for a beginning pilot. If you only fly in 5 mph wind or less, the weak engine will pull the trainer around just fine. But where I fly, that's a pretty rare thing. It's pretty standard in windy places to put a good ball bearing .46 on trainers just so you can get your airspeed up enough to not get tossed around all over the sky.

I had a 12 year old student flying in a 28 mph wind with a 40 LA on the trainer. I took it off and gave it over to him on the down leg, as soon as he turned it into the wind he was yelling dead stick!!!:D I didn't take it over but just told him to point it down and as soon as it got more air speed all was well. By the end of the day the kid understood wind flying.:cool:
I do like the .46 LA a bunch better, it has plenty of power for a 40 size trainer but I never bothered to remove the .40 until they were completely worn out. By the way, there is a sticky prop chart you can look at for an idea of what prop for what engine plane combo. A longer prop with a lower pitch will give you more pull but less speed and a smaller prop with a bigger pitch will give you more top speed. That's pretty basic but it works out.

OK the Tamecat is not a trainer! It is old sport design intended to look sorta like a navel jet fighter. It is a cleaner design than a trainer and perhaps a bit heavy. Not a great combination with a weaker engne however since this airplane is going to need to fly fast (in the manner of some other famous dogs called duraplanes) and to do that with the LA I would suggest a shorter prop and experiment with APC 9/5 or perhaps 9/6.

John

APC sells a specialty prop designed for maximum thrust that will perform well on your FP .40. It is a 10.5x4.5; Try one, and I think you will be impressed.

i think i will try the .46 what size prop do you suggest for that ?

Same prop. Also might try the APC 12x4. Lots of thrust and 50 to 60 mph speed.

try an 11x7

An 11x7 will severely overload an LA-46 and unlikely even get a tamecat into the air!

I actively use a 10-5 on a slightly overweight airplane with a heavy wing loading similar to the tamecat and it is an ideal size for the LA-46. If you use your LA-40 on the tamecat I would use no more than a 9x5 or six.

John

I used the 46 LA for all my fun fly planes I competed with. Everyone ended up with an 11X6 APC but I did a lot of testing to find out what prop I liked best. I like a slower plane with more thrust and you may like something different so testing is the only way to know for sure. All props mentioned will work. I never tried a 12 inch of any pitch so can't say, seems a bit big for the little LA but who knows until you try it.

Lifer- I think you're being a little optimistic on the prediction for that 12x4. If you do the math, a 4 pitch prop would need to turn 15,800 and change to do 60 mph, and that's assuming 100% efficiency. No prop actually makes its theoretical speed, so we'd probably need 18k or better to get that much speed from that prop. I'll buy a round for every member on RCU if there has ever been a .46 LA that could do that.

A much more believable figure is around 35 mph with that engine and prop. For that combo he'll be in the situation I described above as having a nice flying plane in really calm conditions, but getting tossed around like crazy in the wind. IMO, a trainer really needs to be able to go about 50 mph to be able to fly comfortably in 10 mph wind, and faster than that in more.

thanks guys

My recommendations are based on first-hand experience. Try them if you like.

I used Master Airscrew 10x6s on both my Tower 40s and my old Fox 40. Never had a problem with them.

I use an APC 11/5 on my os 46 sf
APC is my choice for power production
More vertical performance , better braking and control on final
Vertical performance is how I judge the planes overall performance
Strong vertical performance , will pull you thru all other maneuvers
Be sure you are running full bore at the engines recommended RPM
You need a tach to do that

I have had quite a few aircraft with .40 FP'S on them. I always used a 10 x 6 MA prop but always felt that I could get a little more out of that engine. One of the oldtimers in the club suggested that I try a 9.5 x 6 MA prop and it transformed that little dog of an engine into a respectalble little power plant. The smaller diameter prop allowed it to spin up faster and get into that little dog's power band. I've kept all of my .40 fp's because they are so easy handling and running engines. No bearings to worry about. Put'em up wet and they don't care a bit. Now I put them in smaller 40 sized airframes and have a ball. One of my favorite combos was a Great Planes Fun One with one of my old .40 FPs on it. It would fit in the trunk of my vehicle without disassembling it.

Jester you are way off on your thoughts. first off pitch speed calculators are way off. I have ran a APC 12x4 on a WM T-34 at 17,000 rpm and recorded 96 mph in that plane in flight. most props will fly a plane faster than the props calculated pitch speed by most of the calulators out there. What you are missing is the airfoil on the prop contributing to the thrust.

I dont agree with your thoughts on needing more power to fly in the wind. the airplane doesnt see the effects of the wind only you do. more power will let it travel across the ground faster which to you looks like it flies better.

Also for this plane i think this engine will be way to small as i think that plane had a 65-70 inch wing. I dont know your piloting skills but it could be doable with that engine and if plane to fly the 40 in it, i think the 10.5x4.5 or a 10x5 would be best. if the LA46 then a APC 11x5 would be best IMO.

a good invest ment for the 46 would to add a Jett Red tuned muffler from Jett Engineering. this should ad around 1500 rpm on a 10x6 prop or 1,100 on the 11x5.

The LA 40 will do fine with a MA 10x6, dont go any larger than that pitch or diameter. The FP 40 is a much better engine than the LA 40 IMO. I spin 2 10 x 6 APC's on my Twinstar and it flys like a rocket with the FP 40's. The APC's are a little heavier so I would'nt use them on the LA 40. If I were you, I's put the 46 in there!!

just for info the old FP 40s with steel pistons will spin 15x4 APC also but I wouldnt try to fly it. only about 5000 rpm but needles well....

You can never have too many props.

While true that the FP 40 has about the same power as a good 30 or average .35, it can produce enough if the proper prop is selected. The right prop allows the engine to turn up to the 14,000 to 16,000 range in the air. Since most people don't have the equipment or knowledge to accurately measure rpm in the air, we sort props on the ground with the understanding that a typical engine/prop combination unloads from the ground reading to a higher rpm in the air. The amount of unload depends on many factors such as engine timing and aircraft drag, but in general sport airplanes tend to unload between 1500 to 3000. Very clean (pylon racers) with racing engines can unload as much as 5000 rpm.

So the first thing with any engine is at what rpm does it make the most power. The lowly 40 FP claims 1 Hp at 15,000. Thus for best power on a sport airplane we want a prop that turns between 11,000 (14,000-3,000) to 14,500 (16,000-1,500). Personally I would shoot for 12 to 13K on the ground when peaked out. This of course is much leaner than an actual setting for flying, because when the engine unloads in the air it needs additional fuel.

After testing props for peak rpm you will find a group of 4 or 5 that allows the engine to make power. The next step is to match the airframe with the prop that works best for the flight conditions. Typically (also drag dependent) the speed of the air from the prop is between 8 to 15% faster than the speed of the prop blast in flight. Once again, the cleaner airplane has a smaller differential between the speed of the prop blast and the airframe speed.

Now there is no one best prop brand for airplanes. Everybody that makes props makes them with some fairly loose assumptions. Most props are not constant pitch down the length of the blade, and generally those that are tend to be the worst props. This is because of several factors, again the design of the airplane has a strong influence with what works best. Since the air closest to the cowl and fuselage is moving the slowest, the pitch at the hub should be lower than the main part of the blade. Further out towards the tip, some props are washed out slightly which tends to unload the engine slightly ( it is a rotating wing after all). This also makes a prop work slightly better at take-off where you really need the engine to unload quicker.

Finally if you have a pitch gauge and you measure a bunch of commercially available props you will find them all over the place. Hopefully the blades of a prop will match each other, but this is uncertain depending on materials and manufacturing process. Wood props have stresses relieved when cut, and slight errors in hub thickness can cause differences in pitch. Easy to fix by scraping the backside of the hub. Plastic props may have molding flash on the leading edge which can mess up the airfoil, and they too may have other problems. While it is good to balance a prop, you can fly unbalanced props as long as the heavy blade is aligned with the counterbalance on the crankshaft (single cylinder glow engines). Some engines actually run much smoother with a slightly unbalanced prop, your mileage may differ.

Run an APC 10X5 prop, 15% nitro 20% oil fuel ( you may only find 18% oil but can add a little castor available from sig ) and an OS #8 plug. Check to see if there is a baffle in the muffler, if there is remove it. Work with what you have make sure the fuel system is 100%. Always start with making sure what you have is functioning the best it can before considering hop ups. That engine should be enough to enable you to have a stylish trainer which is what the Tamecat was designed to be.