Author Topic: 4 stroke rpm increase in the air. (Read 9870 times)

Andrew Tinsley · « **on:** September 10, 2015, 11:58:50 AM »

I am relatively new to 4strokes. I was flying at the weekend with a Saito 56 powered stunt model and also an older model with a Merco 61. The Merco increased revs in the air as one would expect for a 2 stroke. The penny then dropped, the Saito 56 hardly increased revs at all once airborne. I have never been aware of this, when running the Saito 56. Is this normal behaviour for a 4 stroke or do I have a problem? The flights with the Saito were fine with no problems at all except the pilot!

Regards,

Andrew.

bruce finley · « **Reply #1 on:** September 10, 2015, 12:08:28 PM »

Mine do not increase in revs as the flight goes on. Nice isn't it?

Andrew Tinsley · « **Reply #2 on:** September 10, 2015, 12:44:51 PM »

Hello Bruce,
I have always thought that the prop was stalled when stationary on the ground, surely you would expect the prop to unload and speed up when airborne as per 2 stroke experience? So what is going on here? Even my diesels speed up when airborne! is it just that my senses are dulled by old age and can't hear what is going on these days. I refuse to use my hearing aids because the birdsong drives me crazy!

Regards,

Andrew.

rustler · « **Reply #3 on:** September 10, 2015, 01:36:45 PM »

Hi Andrew.

I am totally new to 4-Strokes. Since the Nats, I have been advised

a) use a tach.
b) lean the motor out to max revs.
c) open the needle to reduce revs by 500.
d) fly.

This should ensure you don't run the engine lean in flight. On my 4-str. flight at the Nats, which I think I set to max revs by ear, it ran consistently the whole tank, but I did wonder if it leaned out a little bit towards the end. However, people who saw the flight thought not.

Brett Buck · « **Reply #4 on:** September 10, 2015, 07:00:16 PM »

Quote from: Andrew Tinsley on September 10, 2015, 12:44:51 PM

Hello Bruce,
I have always thought that the prop was stalled when stationary on the ground, surely you would expect the prop to unload and speed up when airborne as per 2 stroke experience? So what is going on here? Even my diesels speed up when airborne! is it just that my senses are dulled by old age and can't hear what is going on these days. I refuse to use my hearing aids because the birdsong drives me crazy!

I can't quite tell if you are seeing no unload from ground to air, or no speed increase throughout the flight. I am very surprised/skeptical about no unload. How to you determine the in-flight RPM in this case?

No speed increase in-flight depends on the tank venting. 2-strokes don't speed up during flight, either, unless you let them (by using a suction tank). If I fly level long enough for the temperature to stabilize, I get the same lap times at the end as do at the beginning within a few thousands of a second.

Of course you can take a suction tank, and either make the engine speed up over the flight, stay the same, or slow down - sometimes drastically - by setting the needle differently. If it's over the top lean at the beginning, as the setting goes leaner, it will slow down. If you start out rich, it will generally speed up. That's not different with a 4-stroke, although you might not be able to tell whether it is too rich or too lean, at least not easily.

Brett

Andrew Tinsley · « **Reply #5 on:** September 11, 2015, 07:38:36 AM »

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.

Thanks for the inputs,

Andrew.

Brett Buck · « **Reply #6 on:** September 11, 2015, 08:19:05 AM »

Quote from: Andrew Tinsley on September 11, 2015, 07:38:36 AM

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

RandySmith · « **Reply #7 on:** September 11, 2015, 10:48:02 PM »

You can record the flight from ground to air, then run the audio thru an oscilloscope , this will show you the RPM change, and even RPM change in maneuvers.

Randy

Bob Reeves · « **Reply #8 on:** September 12, 2015, 07:26:55 AM »

Quote from: RandySmith on September 11, 2015, 10:48:02 PM

You can record the flight from ground to air, then run the audio thru an oscilloscope , this will show you the RPM change, and even RPM change in maneuvers.

Randy

Been wanting to do exactly that for quite some time but never got around to it. Have always suspected 4 strokes (at least the way I run them) do not vary RPM by a whole lot even in maneuvers. Would be interesting to see what they actually do, hard to tell just by ear.

My Saitos do speed up during the flight by about 0.2 seconds, conventionally plumbed clunk tank on muffler pressure. Never been a problem and like the tad more boost for the first loop of the clover especially when it's really windy.

Andrew Tinsley · « **Reply #9 on:** September 13, 2015, 12:57:56 PM »

Thank you everyone for some interesting information. I shall try recording my next 4 stroke flight and doing the analysis with a scope. I will report back when I have the results. I am strongly inclined to put this down to faulty hearing!

Andrew.

Mike Alimov · « **Reply #10 on:** October 11, 2015, 10:05:07 PM »

Some interesting (puzzling) data on this topic, as I just put a couple of flights on my Strega ARF + Saito 72 this weekend.
I measured ground RPM at about 8400-8500. The APC prop has 6" nominal pitch, measuring closer to 5.25" - 5.5" on a Prather pitch gauge. Lap time was 4.8 sec (brisk!..) on full 70' lines, averaged over 4 laps multiple times, so pretty accurate.
If you add about 2' to the circle radius to account for my arm length and do the math, the in-flight RPM had to be 11,300 (!!!???) in order to return these lap times. That's a gain of almost 3000 RPM from ground to air! Alternatively, if we assume that the engine unloads and gains a more reasonable 500 RPM, then the effective pitch of the prop in the air has to be close to 7.5". Anyone heard of an APC prop picking up extra 2" of pitch in the air?

I find either scenario hard to believe, but not sure what gives.

Brett Buck · « **Reply #11 on:** October 12, 2015, 10:27:26 AM »

Quote from: Mike Alimov on October 11, 2015, 10:05:07 PM

Some interesting (puzzling) data on this topic, as I just put a couple of flights on my Strega ARF + Saito 72 this weekend.
I measured ground RPM at about 8400-8500. The APC prop has 6" nominal pitch, measuring closer to 5.25" - 5.5" on a Prather pitch gauge. Lap time was 4.8 sec (brisk!..) on full 70' lines, averaged over 4 laps multiple times, so pretty accurate.
If you add about 2' to the circle radius to account for my arm length and do the math, the in-flight RPM had to be 11,300 (!!!???) in order to return these lap times. That's a gain of almost 3000 RPM from ground to air! Alternatively, if we assume that the engine unloads and gains a more reasonable 500 RPM, then the effective pitch of the prop in the air has to be close to 7.5". Anyone heard of an APC prop picking up extra 2" of pitch in the air?

I find either scenario hard to believe, but not sure what gives.

The pitch you measure is not the "experimental" pitch. The prop has camber to it, so the angle of the zero lift is substantially higher than what you measure. It's not spinning 11300 rpm in the air, it's probably more like 9500.

The "experimental" pitch is the point where the airspeed/rpm yields zero thrust. For a flat-backed prop, you measure the AoA of the blade. But the airfoil has substantial lift at that AoA, so you have to go faster than the pitch*rpm to get zero lift. Look on Stuart Spurlock's site for a full explanation and a way to determine it.

I did some calculations for SSW a long time ago and found that, for example, the experimental pitch for an Eather undercambered prop measured 3.75 at the tips and had, effectively, something like 7" of pitch due to the very high camber.

BTW, it's worse than you think, because you are really only going maybe 65% of pitchxrpm. At a speed equal to pitch*rpm, assuming you use the experimental or effective pitch, you have *zero* thrust. You need something like 1.2 lb of thrust to fly around level, so you have to be traveling at a speed much slower than the pitch*rpm, meaning the pitch is probably even higher than you think.

Next time, get someone with a throttle-equipped engine in the pits to fire it up during your flight, and have them rev it up or down until you hear it go into sync from the center of the circle. Then measure the rpm of the engine in the pits. The 4-stroke will unload a lot due to the relatively weak power, high pitch, and relatively flat HP curve at the speeds you are running, so maybe you will get about 1000-1200 rpm worth of unload.

We usually get something like 500-800 with much stronger piped engines with stiff regulation and very light propellor loads. Last time I did it, I got 10000 on the ground and 10800 in the air with mine. Subsequent ground tests also showed that the regulation effect was very strong and there is almost no amount of reduced load that will permit it to go above about 11,200.

Brett

Tim Wescott · « **Reply #12 on:** October 12, 2015, 10:37:34 AM »

Quote from: Brett Buck on October 12, 2015, 10:27:26 AM

The pitch you measure is not the "experimental" pitch. The prop has camber to it, so the angle of the zero lift is substantially higher than what you measure. It's not spinning 11300 rpm in the air, it's probably more like 9500.

The "experimental" pitch is the point where the airspeed/rpm yields zero thrust. For a flat-backed prop, you measure the AoA of the blade. But the airfoil has substantial lift at that AoA, so you have to go faster than the pitch*rpm to get zero lift. Look on Stuart Spurlock's site for a full explanation and a way to determine it.

If you're enough of an airplane geek to have airfoil section data lying around, you can look at the C_L vs. pitch angle curve for just about any cambered airfoil (like a Clark Y -- that must be germane to the discussion because it was designed for propellers, right?). That'll illustrate what Brett is saying about the angle of the bottom of the curve vs. the zero-lift angle

Quote from: Brett Buck on October 12, 2015, 10:27:26 AM

Next time, get someone with a throttle-equipped engine in the pits to fire it up during your flight, and have them rev it up or down until you hear it go into sync from the center of the circle. Then measure the rpm of the engine in the pits. The 4-stroke will unload a lot due to the relatively weak power, high pitch, and relatively flat HP curve at the speeds you are running, so maybe you will get about 1000-1200 rpm worth of unload.

We usually get something like 500-800 with much stronger piped engines with stiff regulation and very light propellor loads. Last time I did it, I got 10000 on the ground and 10800 in the air with mine. Subsequent ground tests also showed that the regulation effect was very strong and there is almost no amount of reduced load that will permit it to go above about 11,200.

If you have a smart phone you can get pitch generators and audible tachometers. I'm not sure if either one would work better than Brett's suggestion, but it's something you could try. An audible tach will be fooled to some extent by Doppler shift, so it'll "hear" a varying RPM. A pitch generator might be hard to match up to an engine sound -- you'd want to select a "raspy" sound to get the best chance at a match.

If I could buy half a dozen different engines just for Science I'd mount them on my TUT-equipped airplane and do RPM measurements in flight. But -- money, time, etc.

Brett Buck · « **Reply #13 on:** October 13, 2015, 01:52:34 AM »

Quote from: Tim Wescott on October 12, 2015, 10:37:34 AM

If you're enough of an airplane geek to have airfoil section data lying around, you can look at the C_L vs. pitch angle curve for just about any cambered airfoil (like a Clark Y -- that must be germane to the discussion because it was designed for propellers, right?). That'll illustrate what Brett is saying about the angle of the bottom of the curve vs. the zero-lift angle

   OR, alternately, take advantage of Stuart Spurlock having already done the same thing:

http://www.supercoolprops.com/articles/camber_and_zero_lift_angle.php

   This was entirely satisfactory to explain what we were seeing, and the prop doesn't know it's a 4-stroke. Note also this thread from SSW:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=87530&mesg_id=87530&listing_type=search#87542

  where I calculated it for an Eather UC prop, and found it worked out very closely to expectations.

   Brett

Bob Reeves · « **Reply #14 on:** October 14, 2015, 05:56:01 AM »

Way back in 2009 Alan Hahn recorded a Fox 35 flight and posted in-flight RPM data and graphs.
http://stunthanger.com/smf/index.php/topic,14276.0.html
Anyone know if Alan is still around. He hasn't posted in quite some time and looks like he is no longer a member of this forum.

I am going to (hopefully) record a Saito 62 flight this weekend with a camera that records video with audio. I know how to get the audio exported into a .wav file but not sure I have the proper tools to analyze it as well as Alan did with the Fox.

Tim Wescott · « **Reply #15 on:** October 14, 2015, 02:39:24 PM »

Quote from: Bob Reeves on October 14, 2015, 05:56:01 AM

I know how to get the audio exported into a .wav file but not sure I have the proper tools to analyze it as well as Alan did with the Fox.

I'll be able to figure it out if you don't have the tools. I'm sure I have a few cepstra lying around here someplace to do analysis with.

Bob Reeves · « **Reply #16 on:** October 14, 2015, 03:42:16 PM »

Was a really nice day, hauled the SomethinXtra and camera out to the circle and put up a flight from my stooge. Got off a tad rich, didn't reset the needle from the flights last spring but wanted to see what the camera would record as more of a test than anything. Dealing with everything, starting, stooge, camera and flying was about all I wanted to handle on the flight and just went to the handle when it started.

Came back and saved the audio file from the recording but not sure it is going to tell us much. I opened it with a wav file editor to see what I had. The bangs are down in the noise and very difficult to distinguish from what I'm guessing is prop noise. The ground RPM will probably be fairly easy to distinguish but once it gets in the air everything seems to run together.

Can't take the muffler off as it is needed for tank pressure and just not sure how to make it better.

I'll upload the wave file in the morning if anyone would like to play with it.

Tim Wescott · « **Reply #17 on:** October 14, 2015, 04:36:17 PM »

Camera on the plane?

Brett Buck · « **Reply #18 on:** October 14, 2015, 09:41:13 PM »

Quote from: Bob Reeves on October 14, 2015, 05:56:01 AM

Way back in 2009 Alan Hahn recorded a Fox 35 flight and posted in-flight RPM data and graphs.
http://stunthanger.com/smf/index.php/topic,14276.0.html
Anyone know if Alan is still around. He hasn't posted in quite some time and looks like he is no longer a member of this forum.

I am going to (hopefully) record a Saito 62 flight this weekend with a camera that records video with audio. I know how to get the audio exported into a .wav file but not sure I have the proper tools to analyze it as well as Alan did with the Fox.

   If you can get the data digitized, it should be a relatively simple matter to do an FFT or Power Spectral Density on it and get the RPM. It will do a pretty good job of picking the signal out of the noise. I think you can do that pretty quickly with SciLab (or MATLAB if you have access - SciLab is free...).

   I don't know how he got the 4/2 break measurement.

   Brett

Bob Reeves · « **Reply #19 on:** October 15, 2015, 04:35:06 AM »

I uploaded the audio file to my web site in case someone wants to play with it.
http://www.somethinxtra.com/Public/100_0258.zip

Douglas Ames · « **Reply #20 on:** October 30, 2015, 11:30:40 PM »

Quote from: Brett Buck on September 11, 2015, 08:19:05 AM

<snip> 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).

Your dealing with a 4 stroke engine that has more inertia than a 2 stroke, also it has more mid-range torque to swing a higher pitch prop at a lesser rpm. -Doug

<snip> 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. [/b]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.

This is why Bob Reeves enlarged the spray bar orifice (along with 20% nitro).
Bob's 4 strokes are tuned to run at or very near the torque peak.
This is when an engine flows the most volume of air/fuel through the cyl. head. -Doug

Brett

Andrew Tinsley · « **Reply #21 on:** November 05, 2015, 08:55:34 AM »

I am recovering from surgery, but a friend did the necessary flying an recording. The four stroke noise was pretty easy to discern on the scope. This showed that the engine was gaining near enough 550 rpm in flight compared to the ground revs. So That means that I have cloth ears and can't distinguish the difference. I suppose having been brought up on two strokes, the ear is deceived by the 4 stroke sound.
The same exercise with an OS VF 40 on pipe was inconclusive, very difficult indeed to hear the engine above all the other noise. Now that seems to be the wrong way around, however so be it! I will try to digitise the sound and try that approach, however I am a bit of an analogue freak, so I will probably get it wrong!

Thanks everyone,

Andrew.

P.S. Sorry to have caused the discussion when it was my faulty ears giving the problem!

Brett Buck · « **Reply #22 on:** November 05, 2015, 10:19:35 AM »

Quote from: Andrew Tinsley on November 05, 2015, 08:55:34 AM

I am recovering from surgery, but a friend did the necessary flying an recording. The four stroke noise was pretty easy to discern on the scope. This showed that the engine was gaining near enough 550 rpm in flight compared to the ground revs. So That means that I have cloth ears and can't distinguish the difference. I suppose having been brought up on two strokes, the ear is deceived by the 4 stroke sound.
The same exercise with an OS VF 40 on pipe was inconclusive, very difficult indeed to hear the engine above all the other noise. Now that seems to be the wrong way around, however so be it! I will try to digitise the sound and try that approach, however I am a bit of an analogue freak, so I will probably get it wrong!

We found exactly the same thing. Despite sounding much more mellow, in most cases, we found it very easy to hear 4-strokes running off to the side during our speed tests, and they tended to drown out the PA61 or RO-Jett 61 in-flight noise.

550 rpm seems pretty reasonable. In most cases, when the engines were running properly, the unload was between about 500 and 1000 rpm.

Don't apologize for asking, it was a very interesting discussion and I don't recall seeing anyone else having measured it. There are people who still believe that the engines *slow down* after you launch them, because they go from a 2-stroke to a 4-stroke.

Brett

pmackenzie · « **Reply #23 on:** November 07, 2015, 11:32:43 PM »

FWIW, there is a free program called Audacity for doing sound mixing.
It includes a spectrum analyzer that can be used to determine engine RPM.

http://sourceforge.net/projects/audacity/

There is also a German program that a lot of guys use to measure speed by doppler shift that could also be used

http://www.sprut.de/electronic/soft/scope.htm

Best place to do the recording from would be the centre to avoid any doppler shift issues.

Andrew Tinsley · « **Reply #24 on:** November 08, 2015, 08:00:45 AM »

Indeed the recordings were done from the centre of the circle, you could even discern the footsteps of the pilot!

Andrew.

Wolfgang Nieuwkamp · « **Reply #25 on:** November 11, 2015, 11:03:45 AM »

For the iPhone there is the app "RacjngTach", you can set it for 0,5 events per revolution for a 4 stroke.

Regards,

Wolfgang
Germany