Hi!
Britbrat...The Magnum XL is like any other sport engine, it develops its max power at around 16500-17500rpm. OS FX .40 ,Super Tigre .40, MVVS GFS 6,5cc, TT. 40 SEetc ... all of these engines develope their max power at about 17000rpm. There is no difference!
But saying that just because an engine develops its max rpm at 17000prm doesn't mean you should prop your engines in your high winged trainer or low winged sport airplane at these rpm.
To the contrary, the right way to prop a high winged trainer is for good trust at low to moderate flying speeds and not for high rpm and max speed (there really isn't much difference in speed though, whatever prop you choose for a plane like that).
For all .40 to .46 sport engines this mean using a 11x5- 11x6 prop. This prop size will give nice and reliable flying characteristics (good trust, low noice, steady and calm engine behavoir with less aggressive behavior when giving full throttle in an emergency situation and good reliable idle) suitable for a newcomer at all ages.

Regards!
Jan K
Sweden

Hi Jaka. Actually they don't all develop peak power up there -- the OS 40 & 46 LA being good examples (~13,500). I'm using dyno data -- not the manufacturer's phony advertising (OS being particularly bad in that regard). However, when a question is asked "how do I get the most out of my ---blah, blah blah engine?", I look at the airframe application & the engine characteristics, then offer a solution. There is no problem at all in running an engine in the 12,000 -13,000 RPM range to get decent power from it --which you pretty much have to do with those feeble LA 40/46's. If he has a sport flier, then there is nothing wrong with winding an engine up to 16,000 if that's where it likes to go & he wants real performance.

However, you have never seen me suggest that a trainer be operated at 16,000 - 17,000 rpm engine speeds

The Magnum is an interesting case, down in the 12,000RPM zone it is only slightly more powerfull than a 46 LA, but let it hit 17,000 & it is a particular monster in that class. It is not nearly as well suited to a trainer as a TT 40 Pro for example, or an OS 40 FX -- both of which peak at high revs, but which also have substantial grunt at lower revs.

There are horses-for-courses & no one-prop-fits-all answers.

Hi!
Well, my opinion is that all .40-.46 sport two stroke engines (except the non ballbearinged engines) are more or less the same powerwise when you use them in a trainer with a 10x6-11x6 prop.
I know there are differences..like for instance over here in Sweden where we run Q-500 pylon using standard sport .40 engines (like OS .40 FX and Webra GT .40) and here the Webra .40 has got the edge.It swings a 10x6 prop (Graupner) at around 14000rpm (80/20 fuel)which is more than all other .40 engines do (we don't allow Nelson and Jett).

Regards!
Jan K

That is your opinion Jaka & you are perfectly entitled to it. My opinion is different.


Dave Geirke did exhaustive dyno tests on nine different 40 size engines, including five BB types & four plain-bearing types. The BB engines were:Enya SS 40 BB, Magnum XL .40 A2, MDS .38 FS Pro, Supertigre GS .40 & Thunder Tiger .40 Pro. The plain bearing engines were: Enya SS .40, OS Max .40 LA, Thunder Tiger .42 GP and Tower .40 R/C ABC.

In dyno tests, turning an APC 11-6, the five BB engines varied in RPM from 10,400 (MDS .38) to 13,000 RPM (TT 40 Pro) -- a 25% variation, & HP figures varied from 0.55 (MDS 38) to 0.95 (TT 40 Pro) -- a stunning 72% difference.

Similarly, for the same BB engines with a 10-6 prop, the RPM's varied from 12,900 (MDS 38) to 14,900 (TT 40 pro) & the HP figures varied from 0.79 (MDS 38) to 1.22 (TT 40 Pro) -- a similarly enormous 54% difference.

If you take the four plain bearing engines with an 11-6, the RPM's varied from 10,400 (OS 40 LA) to 10,900 (TT 42 GP) -- 4.8% difference, & HP varied from 0.53 (OS 40 LA) to 0.62 (TT 42 GP) -- again closer, but still a 17% difference.

With a 10-6 those same engines had RPM variations from 12,100 (OS 40 LA) to 13,000 (TT 42 GP) -- 7.4%, & HP varied from 0.72 (OS 40 LA) to 0.85 (TT 42 GP) -- a 12% difference.

When the engines were propped to reach their maximum outputs, the BB engine differences varied from 1.27 HP (Enya SS .40 BB)to 1.42 (Magnun XL 40 A2) -- a 12% difference, & the plain bearing engines varied fron 0.78 HP (OS 40 LA) to 1.11 HP (TT 42 GP) -- a huge 42% difference.

In my opinion, prop-fit & engine characteristics matter very greatly --- even for trainers.

I don't think anybody's mentioned it, so I will. Static thrust is not dynamic thrust, so who knows what goes on in flight. For real airplanes you will never see a thrust measurement on anything but jets. You cannot measure how a prop engine unwinds when its flying.

I agree with kdheath: there is too much emphasis on power these days. If you were flying with scale power, a .40 size trainer would have .25, maybe. Have you ever ridden an airliner where the pilot flips inverted on take-off and climbs to 40,000? I don't think so...

Only once, and I'm pretty sure it was unintentional. May not have been though, since it was a NW pilot.

Hi!
Britbrat...those figures are just in bench figures ...say nothing (or very little) how the engines behave in the air.
My experiences are based how engines react when flying them...in the air.
I agree that there are small differences among those .40-.46 engines mentioned ...but not anything a newcomer with a high winged trainer would notice. But for an experienced flier in a fast low winged airplane ...yes there are differences!
That's why (nearly ) all Q-500 pylonracers over here use the Webra .40 GT
Which I don't like being the Swedish MVVS importer[:@]

Best regards!
Jan K

LOL!

Hi Jaka

I agree that in-flight engine behaviour is different from the static situation. I also agree that if the engines are over-propped, the differences will be muted.

However, when more optimally propped, the differences can be quite significant -- even on a trainer. Those differences can mean either a dull, uninspired performer, or an interesting and spirited plane to fly, despite being a trainer.

I guess that we just have different opinions in this regard.

Cheers

Guys. 3D prop hanging is ALL about static thrust. So is 2 or more engined planes, land and sea.

CYCLOPS, good point. It is for me a matter of curiosity to know what the thrust is, I do a fair amount of scratch building and like my planes to have unlimited vertical. With 1/2A models, that doesn't allow for much margin of error. The THRUST HP CALCULATOR is a handy tool to have and the down load is free.
I built a test stand where the engine is mounted to a set of gliding rails. The engine is positioned vertically, and the idea is to find out how much weight the engine/prop can maintain in a hover. I compared my findings with the theoretical results at THRUST HP and they were very close.

If you really would like to measure Thrust Try a Digital Bow scale should fit the bill nicely. I own one of these and it should work great that purpose. I use mine for Archery. It will remember and display that Highest force that was applied. See this link

http://www.lancasterarchery.com/shop...eea98880c2ef89

I prefer the fishing variety -- if there are any errors, it makes your thrust look bigger

Was the set of engine tests mentioned above the one which ended up thinking the Fox 40 was about the best for power/weight/dollars?

My engine thrust testor is simple, accurate, CHEAP and can handle anything up to 5hp.

electronic 15# fish scale, 1/2" thick piece of EXT. plywood, a larger piece of 1/2" that is 6" X 36" long. Hole in the center for 2 flanged 1/4 " id bearings, backup washers, 1/4 " X 6" bolt and 4 nuts. 2 C clamps.

It has additional holes for different size engines and several scale holes at the other end to give ratios of other than 1 to 1.

It is bolted temporarly to the end of the table saw table. This allows me to wheel it out side for tests and it will not flip as the 2 C clamps limit travel to 2" max.

I always had the bad habit of OVER propping engines. By making the fuel tank free to be moved above or below the engine up to 10", I now know when I am over or under propped.

The Perry pumps are installed AFTER it can suck fuel great from 5" high to 5" low.

You are right if you see compromises in the prop sizes. I find it quicker to test on the saw than in a plane. AND the prop is ALWAYS way in front of me.

I go even simpler! I use a digital fish scale with a cord looped around the tail with the plane sitting on its wheels. Fire up the engine and read the thrust on the scale. There is little drag from the wheels and the vibration negates that. As to what it means, who's to say. Just another bit of data!