Performance report 447

[Rick Martin]

Sorry Udi.

I've been here so long I forget.

But I guess it's all relative. I'm in heaven when it finally gets cool enough that I can wear a sweater or a coat and be comfortable. 50's here and that's just perfect for sittin' by the fire.

It's pretty weird to go to the beach when I have to wear a coat, and at the same time see northern folks in bathing suits actually swimming in 65 degree water.

I don't get in the water and wade fish until it comes up to about 78 or so.

Damn..........now I almost feel guilty that I get to fly year round in an open gyro. Hmmmmmm.......My homeowner's insurance bill just took away any remaining guilt!

 

[MMorgan]

Rick,

In response to your early posts and gyroRon's comment......when I first got my Dom it was turning 6800 on climbout and had great performance (it's a 618).
Me and gyromike decided one day to change pitch to bring rpm's down a little.....now it turns 6500 and climbs even better and cruises faster at 5500-5700 where I normally fly.
So...more rpm's in my case is not better.

 

[Rick Martin]

Hey Mike,

Thanks for the info. That's very interesting. What prop are you using?

 

[MMorgan]

60" 3-blade warp drive...3 to 1 reduction.

I have never done a thrust test for a comparison but me and gyromike both agreed that it climbed better after the pitch change to lower rpm's.

 

[Rick Martin]

That's what I wanted to know Mike. If you had said IVO, then we might conclude that the IVO's are just so flexible that we had crossed the line where too much rpm induced resistance/pressure caused them to kind of "feather".

But since it is a Warp, it's even more interesting because they seem so stiff. I wonder if all props have this kind of threshold where they start to lose thrust?

So in theory, it it better to just set it for max thrust and disregard the highest usable rpm the engine has?

Maybe Chuck or others can shed some light?

 

[Mac]

An analogy to prop pitch would be like climbing a 20 degree slope with a 447 powered atv. It climbs the hill good only in first gear and only goes 12 mph.

Now take the 618 powered atv and start up the hill in first gear, it winds out at 15 mph.

This next time, take the same 618 powered atv, put it second gear and give it the gas and it climbs the hill at 25 mph but try starting up the hill from a dead standstill in third gear, the 618 stalls out (over pitched prop) without going 5 ft up the hill.

PS The 618 atv will not wind out as tight in second gear as it did in first but it sure goes up the hill faster.

PS Change from a 60 inch prop to a 68 inch prop on the 618, thrust would increase but you would probably have to decrease pitch and search for the highest rpm.

 

[gyromike]

Rick,

When we started messing with Mike's prop, it would top out at ~6800 rpm in straight and level flight, and ~6500 RPM static.

I assumed that if we added pitch those numbers would drop, but they didn't. We made two or three pitch adjustments, adding pitch every time until we got it to show a drop in RPM on the static runup before we flew it.

My suspicion is that the prop alone wasn't keeping the engine from overspeeding, but that the engine was bumping against the upper edge of the powerband.

Two-stroke tuning is mostly affected by exhaust. From what I've read up on, the shape of an exhaust (length, taper, diameter, etc.) are critical, and with the truncated shape of the can (or belly section) you can get a sharp dropoff in power.

This is just my guess, but Chuck B. or one of the others can shed more light on it. I need to experiment a little more.

 

[Rick Martin]

Thanks Mike. Again, that is interesting and confusing at the same time. Because if I'm reading your post correctly, even though you changed the static rpm it still went to 6800 each time at full throttle in the air. I'm certainly no mechanic, but I tend to believe your theory about the top of the power band.

Mac, thanks for the analogy. but does your scenario take into account the flexing of the prop?

 

[MMorgan]

Rick,

No.... it tops out now around 6500 in climbout.

 

[Rick Martin]

Sorry Mike,

I didn't express myself very well. I meant 6500/6800 through those first few adjustments before you finally got it to change.