blhollo2

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Hello, I have a rc guys decathalon with a 40gx and there is no real information on the rated H.P. horse power,static thrust or best prop to run on this engine. anyone know where I can find this info? I'm trying to get more speed out of this engine and good thrust. Evolution engines states it will turn a 20x8 prop at 7700 rpm with my thrust calculator that 26LBS of thrust and a estimated flying speed of 58mph.

blhollo2

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anyone know?

pe reivers

What thrust calculator? prop type?, muffler used?
Here are some data I collected. Not much to go by yet.

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prop	rpm	HP	fuel	exhaust	plug	run	remark	date
20x8 3W	6750	0		3271		0	side muffler, new engine, reivers
20x8 APC	7700	0		3271		0
20x8 APC	8050	0		3268 TP		0
22x10 mej	6400	0		3271		0	factory data
22x10 mej	7000	0		3268 TP		0	factory data
22x12 Seyer	7100	5				0	about 5hp!

blhollo2

My Feedback: (278)

I dont understand your data, is this on a stock 40gx? i have a vess 20B prop on it

aussiesteve

I use a Xoar 20/8 on my MVVS 40 SP (same engine as the 40GX). It turns high 7000's on a J-Tec Pitts muffler (Sold as the "large volume" wrap around for the Evolution 35). With the MVVS "Side dump" muffler it turned similar RPM and was much quieter. I can't give you the current exact top ground RPM as I haven't tach'd it for a while - I don't get anywhere near as hung up on "top RPM" as I do on throttle to airframe response on the plane/engine/prop combo.

I don't have experience with the particular airframe you have but I would think it requires a similar power curve to what mine is fitted to. This combo is fitted in an 82" Extra [link]http://www.ydmodels.com.au/products%20/extra-330l-26/[/link] and it flies it quite well. Not unlimited vertical or unlimited pullout from a hover but more than adequate as a sport flier for teaching people that aerobatics can be done "on the wing".

If you just want more speed. Try some of the following
- a smaller diameter and coarser pitch prop.
- a Tuned pipe
- an airframe designed to go faster.

pe reivers

With a MVVS 50 or 58 that plane would be a rocket.
at 6.5 kilo flight weight the Imac program would just be fine with the 40.

Desertlakesflying

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Is it just me or does everyone leave out altitude when deciding on prop sizes?

The 40GX runs a 20X10 3 blade at 7800 rpm here at 4600 feet. I would guess around 30 pounds of pull, but I don't know for sure.

aussiesteve

You are absolutley right Pe - they are definitely a rocket at that and not a combination that would normally be used. I have been meaning to change the wording of that post for a while - I finally did it.

Like most airframes in the "smaller 50cc" size market (some of the "older style" 50cc airframes that are still around). It builds easily under 6.5kg with an engine like the 40 on it - which in turn makes it even easier to fly. This has been proven over and over so many times.

Anyway - back to the original. Cowboy is right - we tend to not mention altitude. My altitude here is 99 ft ASL at ground level here, so we fly at an official maximum of 550 ft ASL in great weather all year round (except when it rains, like it was doing all of this last weekend - still around 20 deg C though - and it is Winter time here). I am guessing Cowboy has a Tuned pipe on his? (I must Tach' mine again soon just to see exactly what it is turning at nowadays)

the Wasp

My Feedback: (1)

CowboyLifesaver,, I'm with you there,, I have a MVVS 35cc Gasser (inverted Bisson side muffler) and it will spin a wood 3W 20x8 at 7200 tuned for flight and really pull a 15 pound plane well, and it will spin a Zinger 20x8 pro about 400 RPMs higher than the 3W prop,,, so I would think the MVVS 40 should turn a 21x8 or 20x10 somewhere around the same..

Jim

Kweasel

My Feedback: (29)

I never found altitude to matter much with regards to prop selection. The engine spins about the same rpm but makes less power at higher altitudes.

pe reivers

Altitude has no influence on engine rpm. The reduced engine power is almost perfectly balanced out by the reduced power demand of the prop.
That can not be said of the planes however. They need to fly faster at altitude. Combined with the reduced engine power, prop and engine choices will differ if you fly in high altitudes.

blhollo2

My Feedback: (278)

so it sounds like a vess 20B prop is a good fit??
by the way i'm 100ft above sea level

pe reivers

On the Decathlon I would suggest a 20x10 prop. It depends on your muffler. A 20x12 or 22x8 also would be nice.

Desertlakesflying

My Feedback: (28)

Well when you run a 14X7 3 blade on an OS 120 at sea level and they run the 14X7 3 blade at 4600 feet you suck a valve in your engine and have to go to the 15X7 3 blade instead.

pe reivers

Why?

Desertlakesflying

My Feedback: (28)

Overspeed I assume since it sucked a valve.......Just like when I run my 15X7 3 blade on my Husky float flying at the lake at 6000 feet it makes a little difference in rpm, but not much...If I went to fly at 8000' I'd have to go to a 16X7 3 blade, a more pitch 3 blade, or a huge 2 blade. My Mustang which is the one that sucked a valve with the 14X7 at higher rpm was slow compared to when the 15X7 was put on it with lower rpm.

pe reivers

Engine rotational speed does not change with altitude if you keep the same prop on and tune the carb well. That is why I asked "why".
You may mistake a happenstance for fact.

Desertlakesflying

My Feedback: (28)

Interesting since my engines are always tuned perfectly. I'm glad I'm not the expert on this since I only deal with it on a constant basis.

Why can I turn a 22X10 4 blade at 4600' with the same rpm as a 22X10 2 blade at sea level, but I cannot get the same rpm on the 22X10 4 blade at sea level as the 2 blade? But I can turn a 24X10 2 blade with the same pm as the 4 blade, but the 22X10 2 blade over speeds at 4600'.

Mind you my engine tuning capabilities are far beyond what your assumption is.

aussiesteve

Easy reply to that
"SCOTT"

as in "S'got me confused"

(say it out loud )

Ok, factual part of it
Tuning is one part of the equation and for the engine performance only, Air density and Moisture content are other parts that not only affect the tuning but also the load on the enigne (prop).

When you did those testes, were the same 2 blade and 4 blade used in all tests, were the atmospheric conditions (apart from the elevation) the same on all tests?
Also what were the 4 blade and the 2 blade brands / blade models? Some brands of prop are reknowned for spinning higher than other brands (I am purposely leaving the thrust produced out of that statement - it is a huge can of worms).

I know how I choose props and it only relies on a tacho to make sure I don't overspeed the engine. What I do is start at the suggested sizes from the manufacturers and from others experiences (a quick search of these forums shows up the good selections for most brands and models), then I fly the plane and decide form there if a change is needed. "Feel the response - Be the plane".

blhollo2

My Feedback: (278)

so lets get back to the reason i started this forum, so i have a Vess 20B prop on the evo 40gx and its taching 7100 rpm and steady. I need to know if a xoar 20x10 prop would be good or a 20x12 xoar..I am a fan of Vess props but I need to know of other possible good props for this engine...i was thinking about buying a zdz 20x10 prop but was unsure. also does anyone know how much horse power this engine evo 40gx is putting out?

apalsson

My Feedback: (1)

Sounds like your engine tuning capabilities are only overshadowed by your modesty!

pe reivers

I do not assume! I try to analize. If your tuning skills are beyond your talking skills, that is just fine with me, so let's go on.
Have you recorded the rpm of the same props both at sea level and at altitude? Could you share those data with us?
also, please define "sucking a valve"

Desertlakesflying

My Feedback: (28)

Only when people make false assumptions.

Guess that's what happens when ones findings are immediately dismissed by those who are "much smarter" than I.

Although not understanding sucking a valve leaves much room for question. It's the same sucking a valve that happens on any 4 stroke engine that has "sucked a valve".

Really? Maybe we should get back to the OP's post instead of trying to make me look bad.

Go ahead and get your plane and head up to 5000 feet or more and get back to me in a new post. Leave this one alone.

skillet92

Now that is funny!! Being that I only have a full scale pilot to rely on for his take on this(I called because I was curious after reading this thread) I will say this! He fly's a single engine cessna! And fly's it all over the country. His take is that from reading the RPM gauge in the dash there is no change in the RPM's at sea level(where we live) and lets say Colorado where he has been. The difference is the thrust that is produced he said. I guess he can tell this because the take offs and climb out are slower out west! Glad that I don't fly in real ones just for fun!! LOL

blhollo2

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lol gentlemen, can we answer my lil ole question about the props?