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HomeAircraftHelicopterEngines Plugs Mufflers Fuel › MHW C-Spec PS review...
12-07-2004 12:21 AM  13 years agoPost 41
Secret Squirrel

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New Zealander living in Melbourne, Australia

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NO, THE PEAK SHOULD NOT BE AT 14000.
Curtis has advocated running at 14000 at an 8:1 ratio which is 1750.

You do not run your engines bang on the peak power band, you run them just above it, ie 14000 so that when the load comes on, the engine goes INTO it's power band. 500rpm in the scheme of things with these little motors is not 'well above the power band'.

The peak power band for the OS is lower than the YS, that's why people have always advocated running a slightly lower gear ratio for the OS than the YS.

I think Reesy is trying to come to the same point, but I still can't tell .

TO SUMMARISE:
A Nobody with an MPII should be running 1625 with an 8:1 ratio and expect to get peak performance that's stupid.
B What is there to believe? All this article is saying that within the test parameters, this is the results the engine produced and the results were not surprising.
C This article isn't telling us anything we didn't already know!

Si

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Simon Lockington

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12-07-2004 12:29 AM  13 years agoPost 42
Secret Squirrel

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New Zealander living in Melbourne, Australia

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Also Reesy, I just checked your figures.

If we're talking about an 8:1 ratio (which is what I am), then the headspeed at the peak power position which you've stated is 13,450 would actually be 1681 not 1635. So the difference between 1681 and 1750 is a lot less (ie 69rpm vs 115) which highlights the point about running slightly above your peak power band.

Si

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Simon Lockington

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12-07-2004 03:59 AM  13 years agoPost 43
Unbalanced

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Melbourne, Australia

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{I get 1660 at 13,450 rpms with 8.1 btw. 1750 is 14,175 (diff 725rpms on the engine/90rpms on the head) }

Ok - I've been thrown by the gov reviews and recent discussions when it seems important to hold the headspeed diffs to zero, when in fact the c-spec/mp2 with the ratio's above clearly is designed to sag by 90 rpms on the head under max load.

Sounds very reasonable (plus obviously works really well) - the system has a buffer to help the gov etc + the higher gear ratio helps avoid losing more headspeed if the engine drops back to its max power & 13,450.

So - its a good thing for this combo to have the headspeed drop by around 100 when under load - after all it is designed to.

Jeff

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12-07-2004 04:14 AM  13 years agoPost 44
Secret Squirrel

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New Zealander living in Melbourne, Australia

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It's 8.0:1 not 8.1:1

Si

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Simon Lockington

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12-07-2004 05:03 AM  13 years agoPost 45
Unbalanced

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Melbourne, Australia

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Ah - thanks for the correction Si - dang missed that little dot.

Ok - so its only 70 rpm's on the head above the engines peak with 8:1 and a headspeed of1750. I'm running 8.27:1 (which is what I think Reesy based his figures on) which would be a low headspeed (1692) to run with the engine at 14,000. Guess reving up to 14,500 gives a big margin to drop back under load & the gearing is even more suited to digging out of the hole.

Disadvantage being that the max power available at the head will be down considerably. Might have to try 8:1 on my heli.

Jeff

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12-07-2004 09:34 AM  13 years agoPost 46
G.Man

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Bristol

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Reesy

you are making too much of the peak power/rpm figure...

Do you know what the power drop off curve looks like?

The mufler will probably hold the peak power for about 1000-1500 rpm within a fraction of a BHP...

So if Mike says that the peak power is at x rpm it will probably only drop off a fraction of a BHP in the next 1-2,000 rpm...

You cant tell how much HP that curtis's motor is putting out either, but with a serial number on the crankcase (factory motor) you can BET its not putting out 2.87bhp more like 3.87!!!

This article is about retail motors, not the factory specials the sponsored flyers get...

Yamaguchi uses a lower ratio and higher headspeed with a matched tuned pipe, the shorter the pipe the greater the peak rpm/power that engine/pipe combo will bear no resemblence to the tests Mike Billinton has performed here...

Si, Unbalanced, yes the small increase in RPM puts the engine fractionally above its peak power/rpm but you are right, when loading up the power is there to re-increase the headspeed...

Had it been set at the exact engine peak power, loading up would take it below its peak and make the return to correct rpm take longer...

Don't Email me as I wont reply - PM Only (spam countermeasures)

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