erazz

Hello,

A friend gave me a used O.S. max .77 Fan engine that he found somewhere. I'd like to use it but I don't have the manual.

Here are the things I need to know:
1) I have a pipe but no header. Should there be a header? What RPM should I expect?

2) What prop should I put on it. It's going to go on a very small and light airframe (we are talking about extreme speed here, arn't we? ).

3) What fuel/plug/oil should I use?

4) Any other advice on how to run the engine?


Thanks for the help.

r-c-guy

My Feedback: (37)

Hi There!
I have used the 77 OS Max on various airframes. We used to race the 'larger planes.' The best header is nade by MAC. There are 2 configurations... fuse top andstraight. The straight is the least constricting. I used to get 25000-26000 in the air with 20% fuel with an 11-8 narrow. I imagine you could go to 10-9 or others... must make sure its apc and that the hub is substantial so the blades don't separate. I lost an engine doing this when a prop broke in flight and the bearings partially dis-assembled! We clocked the ship by radar over 170mph. A nice theoretical formula (NO Drag & 100% efficiency) is

speed(mph)=((Pitch in inches)*RPM)/1056

unfortunately, drag and slippage ranges from 15-20%

if you know any 2 of the pitch, rpm or desired speed, you can calculate the rest

randy

erazz

Thanks!

That really sounds scary... Running a 11x8 prop to 25,000 RPM.

Can't wait to try it out

JeffH

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what kind of airframe, preferrably scale like, would you recommend for and OS 77 DF?

r-c-guy

My Feedback: (37)

HI
I use an ez Dago Red 40 size mustang with an 11x7 or 11x8. On 15% fuel with pipe pressure, we hit 180's level and over 200 in a dive. We added carbon fiber to the wing joiner, eliminated the belly scoop and streamlined the cowling. The plastic stuff ripped off! Do use a Tru Turn metal spinner, plastic doesn't make it - it ripped off..... one other thing - aileron servos in the wings and cut down the ailerons to 6 inch length.

randy

JeffH

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How would a World Models P-51 (.40 size) do with the 77df?

r-c-guy

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It would haul quite nicely. However do beef up the wing joiner(s) with carbon fiber and use mylar tape on the lower side of the wing!

randy

Dustflyer

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rc guy,

What kind of static rpm would it turn with that prop?

r-c-guy

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I got 20 on the ground...
randy

Dustflyer

My Feedback: (13)

rc guy,

Those are pretty amazing rpm numbers. Jett says his 76 FIRE turns an 11X7 at 14,800 and his 95 FIRE turns an 11X7 at 15,800. An 11X8? That's a lot of prop. To turn one of those at 20.000 rpm on the ground is quite a feat.

Where did you find the narrow blade 11X8, I didn't see it on the APC prop list? How much more rpm does a narrow blade give over a standard sport prop.

Is the engine able to turn those rpms reliably without burning up pistons and liners every other flight?

20,000 rpm with an 11X8 would probably make that engine the single most powerful motor per inch of displacement that I've ever heard of.

I will have to look into getting one!

r-c-guy

My Feedback: (37)

My first 77 had over 100 flights and 2 years when the prop separated in flight and destroyed the bearings, liner.... The second is still going strong.... Although since i'm older now, i put a 65 vrdf into the plane.... still goes real fast 21k in the air.

randy