Hello Brett and all,
I have not defined what I intended to convey at all well so apologies for that. I am simply talking of the rpm on the ground and the rpm when the plane has taken off.
All my two stroke engines speed up by maybe 1000 rpm in flight compared to the ground revs. It is quite an audible increase and it certainly occurs! The Saito by comparison, hardly seems to increase its rpm as it takes off and goes into the first few laps.
OK for what its worth, I usually tach my engines before take off (just to ensure consistency). Obviously I am dependent on my hearing to say if the revs increase in flight and that isn't a 100% accurate!.
I am not talking here about the engine speeding up towards the end of the flight. I must try my piped OS40VF, if memory is accurate, this doesn't speed up audibly at take off, but then the pipe is controlling the constant rpm. Is there some other form of speed control happening on a 4 stroke, if so that would explain my observations. But what form of control would that be?
If it is normal for a 4 stroke to speed up in the air from the launch revs, then either I have a hearing problem or I am not used to the distinctive 4 stroke sound, sufficiently to detect an rpm change.
Andrew.
Probably getting fooled by the sound. The 40VF, using Paul's setup, will pick up around 800 rpm from ground to air. Note this is despite the fact that the fuel pressure actually goes down in flight - leaning it out. That's about the same as any of them. It also varies a lot in flight, in fact, one of the biggest issues we have with tuned pipe systems is that they tend to overcompensate for airspeed/load changes. Adjusting the degree of boost and brake is the primary indication for changing the pipe length, once you have everything else right. 3/8" difference on a VF can take it from too "flat" to too aggressive.
I can't recall actually measuring it inflight on a 4-stroke. Perhaps some of the experts have, but I would be stunned to find that it doesn't unload, when set correctly. Particularly with low-rev setups like Igor Panchenko pioneered (very similar to what Bob Reeves is doing, to the extent I understand it).
Run conventionally they had the same sort of regulation effects as a piped system, with the rapid fall-off with RPM doing the regulating, and RPM very similar to PA/RO-Jett systems. Thats how we first measured the in-flight RPM on the PA - get the airplane flying level, have someone fire up a 4-stroke in the pits, and then have them adjust the needle at pilot direction until it was in-sync, then measure. We used a 4-stroke (Jim's OS52 Surpass, I think) because *you could hear it over the sound of the PA61 in-flight*. There are other more sophisticated methods using PSDs of the sound, for example, but this was the hillbilly method (Napa Valley hillbillies, of course).
In that case it was about 800 RPM in that case, ~10000 on the ground and 10800 in the air. That's pretty close to what we got in other cases and consistent with other people's results. It would be interesting to hear what it does in the low-rev case.
One of the things the locals found with 40strokes is that it was very difficult to tell, in-flight, whether it was going "rich" or "lean" in flight, because it sounded about the same either way. The engine would go soft in some cases, like overhead and the top of verticals. Eventually, we guessed that it was probably going lean, but didn't know what to do about it. At some NATs, Jim was having all sorts of issues and was having to fly like a bat out of hell just to have adequate overhead tension. Brad Walker put him on to clunk tanks, and the entire issue went away, and Jim ended up able to slow the airplane down by something like *1/2 a second a lap* and still have plenty of vertical performance. This was an OS 52 Surpass, but the clunk tank, for all its faults, was like magic for this problem,
That's what put David and I onto the fuel flow/fuel restriction/fuel viscosity issue and since then almost the only significant work we have done is on the fuel delivery side. Just because you can get enough fuel through it in steady-state to get the desired setting doesn't mean it's adequate for transient conditions.
Brett