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Model Forum / Radio Controlled / Air Models / November 2006measuring Engine RPM in the air
Strikes me one can measure the RPM of a flying plane by measuring the frequency of the noise it makes (beware of doppler effect) with a frequency counter. If one isn't handy, one could record the sound and look at it with any number of programs on a PC. Plenty of DIY kits and circuits on the net too. > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. Yup. Formula one teams with sound analysis equipment are able to determine using Doppler and the engine note their competitors speed, maxiumum RPM, and gear ratios, to astonishing accuracy. >Strikes me one can measure the RPM of a flying plane by measuring the >frequency of the noise it makes (beware of doppler effect) with a >frequency counter. If one isn't handy, one could record the sound and >look at it with any number of programs on a PC. Plenty of DIY kits and >circuits on the net too. Hmm. If you also recorded air temperature and altitude along with the sound of the plane, maybe you could calculate the airspeed from the Doppler effect. Or are there too many unknowns? It may be that you'd have to know the RPMs during the recording session to determine airspeed from the Doppler effect. You're also going to have some noise in the data (pun intended; you may laugh now), aren't you? The carb and the exhaust make noises, too. And the airframe probably acts as a mixer for every vibration coming from the engine. I think if I really wanted the information, I'd take a tachometer apart, put the sensor up near the prop, and radio the output back down to the ground in real time. It would take more hardware but less calculation. Maybe you could even market the unit. Some kind of thing that could be strapped on to any plane big enough to carry it. Combine it with an airspeed indicator and you'd have a nice test bed for playing with various props. Lemme know when it's ready. I'd pay $100 for a think like that. OH, dang! The pitot tube has to be out of the prop blast, doesn't it? And the reference tube has to be in calm air. The batteries, processor, and TX have to be hung around the CG, and the unit can't induce drag or it'll ruin the measurement of your top speed for bragging rights. <cue wailing and grinding of teeth> Complexity happens. </w.a.g.o.t.> Let us know how your experiment turns out. One observation is worth a thousand expert opinions. ;o) Marty Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. > >Strikes me one can measure the RPM of a flying plane by measuring the > >frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 51 lines] > One observation is worth a thousand expert > opinions. ;o) I have looked at sound waveforms on oscilloscopes and it's actually very easy to tell the frequency if there's not much else going on. With a plane you'd just have to take the frequency/rpm reading when you fly it past yourself. Most people test their props on the ground, which is no accurate indication of how it performs at normal airspeed. As for telling the plane's speed, that's a different ballgame. The police use radar on cars. Simplest is to measure the time it takes between two points. > Marty > > Marty > -- > The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) > are under new management. See http://www.big-8.org for details. >> Let us know how your experiment turns out. >> One observation is worth a thousand expert >> opinions. ;o) >I have looked at sound waveforms on oscilloscopes and it's actually >very easy to tell the frequency if there's not much else going on. > With a plane you'd just have to take the frequency/rpm reading when >you fly it past yourself. >Most people test their props on the ground, which is no accurate >indication of how it performs at normal airspeed. Yes. It sounds like it would be fun and worthwhile to get some information while in the air. >As for telling the plane's speed, that's a different ballgame. The >police use radar on cars. Simplest is to measure the time it takes >between two points. That only measures ground speed. The decisive factor for determining that one prop is more efficient than another is true air speed which I think has to be measured at the airframe. The speed demons among us probably get along OK without the airborne data. It's us pluggers and gearheads who would like to have some more toys to play with. :o) Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. >> Strikes me one can measure the RPM of a flying plane by measuring the >> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 10 lines] > the RPMs during the recording session to determine > airspeed from the Doppler effect. I thought that all cancelled out if you compare approach sound to sound leaving? > You're also going to have some noise in the data > (pun intended; you may laugh now), aren't you? [quoted text clipped - 43 lines] > The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) > are under new management. See http://www.big-8.org for details. >> ... If you also recorded air temperature and altitude along >> with the sound of the plane, maybe you could calculate >> the airspeed from the Doppler effect. Or are there >> too many unknowns? It may be that you'd have to know >> the RPMs during the recording session to determine >> airspeed from the Doppler effect. >I thought that all cancelled out if you compare approach sound to sound >leaving? Beats me. I've never done any Doppler measurements or calculations, so I'm not sure what the requirements are for making good observations. I suppose if you had a tape, you could analyze it on both sides of the peak. Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. >>> Strikes me one can measure the RPM of a flying plane by measuring the >>> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 13 lines] > I thought that all cancelled out if you compare approach sound to sound > leaving? AFAIK, yes. The average would give you RPM, the difference would give you airspeed (after much calculation). All this assuming a straight-line flight path & no RPM changes during the run.  SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html | Maybe you could even market the unit. Some kind | of thing that could be strapped on to any plane [quoted text clipped - 4 lines] | Lemme know when it's ready. I'd pay $100 for | a think like that. If all you need to know is the RPM of your prop ... http://www.eagletreesystems.com/MicroPower/micro.htm about $60, plus a $10 sensor. Though the optical sensor is designed to have something alternate black and white, so I don't know how well it would work with a prop, and the magnetic sensor requires that you glue magnets on your prop, which might not be wise at 20K RPM. If you need to know airspeed, http://www.eagletreesystems.com/Sailplane/sailplane.html though that's a bit more expensive (because it does more.) Definately fun stuff. SignatureDoug McLaren, dougmc@frenzied.us If Java had true garbage collection, most programs would delete themselves upon execution. - Robert Sewell >| Maybe you could even market the unit. Some kind >| of thing that could be strapped on to any plane >| big enough to carry it. Combine it with an >| airspeed indicator and you'd have a nice test >| bed for playing with various props. >| Lemme know when it's ready. I'd pay $100 for >| a thing like that. ... >If you need to know airspeed, > http://www.eagletreesystems.com/Sailplane/sailplane.html >though that's a bit more expensive (because it does more.) >Definately fun stuff. Yes--that looks like the complete deal: airspeed, RPMs, telemetry. The only problem is the price--and I'm sure it's worth what they're asking, just outside my "mad money" range. I may be able to talk my buddy into a 4M glider with the Eagle Tree system on it one of these days ... Thanks for the link. Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. > | Maybe you could even market the unit. Some kind > | of thing that could be strapped on to any plane [quoted text clipped - 21 lines] > > Definately fun stuff. Splendid, but I'm happier saving the dosh and doing it a cheaper way. > -- > Doug McLaren, dougmc@frenzied.us > If Java had true garbage collection, most programs would delete > themselves upon execution. - Robert Sewell | Path: be1.texas.rr.com!cyclone.austin.rr.com!news.rr.com!border2.nntp.dca.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!postnews.google.com!e3g2000cwe.googlegroups.com!not-for-mail | From: markzoom@digiverse.net | Newsgroups: rec.models.rc.air .. You should fix your newsreader. | > http://www.eagletreesystems.com/Sailplane/sailplane.html | > [quoted text clipped - 3 lines] | | Splendid, but I'm happier saving the dosh and doing it a cheaper way. I didn't say this was the only way of doing it. Martin said he'd buy such a device for less than $100, so ... The funniest part of the page on determining this by recording the sound is that if you can't figure it out, you can call him up (he gives his number) and play the recording on his machine, and he'll get back to you with the results :) What the Eagle Tree micrologger is really good for is measuring your amp draw and voltage in flight, but knowing RPM and temperature is useful as well. It's what was used to give you the dashboard on this video, for example -- http://www.tppacks.com/video/Raptor%2050%20Z50-600%2011s-3000%20eMoli%204-29-06% 20HD.wmv (note that this video is worth watching even without that -- there's a big surprise about what happens when you don't secure your battery pack properly in the middle.) The more expensive Eagle Tree device can keep track of where your plane is, servo positions, airspeed, voltage, amperage, temperature, G-forces ... and can even beam this to you if you buy that upgrade, while storing the information for later perusal. If all you care about is airspeed and RPMs, it's certainly overkill, but it does much more. SignatureDoug McLaren, dougmc@frenzied.us Somebody set us up the bomb! > ... The more expensive Eagle Tree device can keep track of where your >plane is, servo positions, airspeed, voltage, amperage, temperature, >G-forces ... and can even beam this to you if you buy that upgrade, >while storing the information for later perusal. It even seems to have an extra channel that you can use for a device of your own creation ($99 extra). >If all you care about is airspeed and RPMs, it's certainly overkill, >but it does much more. I do plan to discuss this with Uncle Dan some day. He's got to finish renovating his house first. Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. a few years ago a circuit was published in one of the mags that was a variable oscillator feeding a small earpiece speaker, and all you did was tune the note of the speaker to match the note from the engine then read the RPM from the calibrated scale on the adjuster ,IIRC it was a 555 timer project > > Strikes me one can measure the RPM of a flying plane by measuring the > > frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 7 lines] > read the RPM from the calibrated scale on the adjuster ,IIRC it was a > 555 timer project Yup, seems another simple way of doing it. >>> Strikes me one can measure the RPM of a flying plane by measuring the >>> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 9 lines] > > Yup, seems another simple way of doing it. You can even go to a music store and buy a 'Korg Mini tuner' which will ID the fundamantal or harmonic ... >> > Strikes me one can measure the RPM of a flying plane by measuring the >> > frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 9 lines] > > Yup, seems another simple way of doing it. Earlier than that (Fifties - that I know of) someone made a bank of reeds that were tuned to different frequencies. The appropriate reed (tuned closest to the engine's running rpm/frequency) would become mechanically excited and could be read to reveal the rpm. There were no electronics/electrical power involved. The reeds were acoustically coupled to the engine. Ed Cregger >>>> Strikes me one can measure the RPM of a flying plane by measuring the >>>> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 15 lines] > electronics/electrical power involved. The reeds were acoustically coupled > to the engine. Since the reed banks were used to control the various channels, you could always tell when your model went into a dive at any given frequency ;-) > Ed Cregger refer to Engine Noise Reduction & Engine - Propeller combination test results Doppler Effect - RPM on Ground and in Air Noise reduction - How to by Flying Sites. Noise Reduction - keep your flying sites Noise test results & procedures - Blackridge & District MFC and more on my web page Regards Alan T. Alan's Hobby Model & RC Web Links http://homepages.ihug.co.nz/~atong/ > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. >refer to >Engine Noise Reduction & Engine - Propeller combination test results Great stuff! This guy figured it all out: <http://www.holdfastmac.com.au/Doppler1.html> So, bottom line, it looks like the answer to the original question is "yes." Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. >Strikes me one can measure the RPM of a flying plane by measuring the >frequency of the noise it makes (beware of doppler effect) with a >frequency counter. If one isn't handy, one could record the sound and >look at it with any number of programs on a PC. Plenty of DIY kits and >circuits on the net too. The Nitro boat guys have been using an 'Audio' for quite some time. The ones I have seen measure RPM's in the 40-50 k range. They just point it at the boat and it reads the RPM. How does it do it .....?.......thats the tach's business. :-) Actually , I do know how it works , but I have no idea as to what it takes in the way of electronics to make it work. They really are a neat piece of equipment. I run Nitro boats some myself and it's a task to get the engine dialed in without one. Just can't rev it up and point a tach like we do with the aircraft. I go 'watch' some of the hot drivers down south race their boats but although I have a couple good runnning boats but can't run with these guys .Just sit and watch. Someone there will always have an 'Audio' tach. Ken With model planes it would be nice to know what prop "on the ground" would present the similar load as on a flying engine. So, for example and 18x8 prop on the ground going full wack might be the same as an 18x10 (??) going full wack moving in the air. That way one could tune the engine on the ground flat out with the smaller prop to get the right performance in the air with the flying prop. > >Strikes me one can measure the RPM of a flying plane by measuring the > >frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 18 lines] > > Ken > With model planes it would be nice to know what prop "on the ground" > would present the similar load as on a flying engine. > So, for example and 18x8 prop on the ground going full wack might be > the same as an 18x10 (??) going full wack moving in the air. That way > one could tune the engine on the ground flat out with the smaller prop > to get the right performance in the air with the flying prop. I know that this hobby is all about applying various scientific principles to toy machines for the purpose of having fun, but this discussion about airspeed and RPM data is getting a little bit ridiculous. If you want to know which prop works the best, try 4 or 5 of them on the plane in flight and see which one works the best. If your data disagrees with your flight performance you would disregard the data anyway. The good news is that there aren't more than a half dozen propellers to choose from in the first place for a given application. > > With model planes it would be nice to know what prop "on the ground" > > would present the similar load as on a flying engine. [quoted text clipped - 11 lines] > data anyway. The good news is that there aren't more than a half dozen > propellers to choose from in the first place for a given application. Trouble is large props are bloody expensive. I got two 16x8 props for £ 10 each special offer (about $19 each). 4 large props would be more than the brand new petrol strimmer engine cost me. | I know that this hobby is all about applying various scientific | principles to toy machines for the purpose of having fun, For some people, yes ... | but this discussion about airspeed and RPM data is getting a little | bit ridiculous. Why is it ridiculous? I don't see anything ridiculous about it. | If you want to know which prop works the best, try 4 or 5 | of them on the plane in flight and see which one works the best. If | your data disagrees with your flight performance you would disregard the | data anyway. Maybe. You might also see if you can track down the source of the discrepancy. | The good news is that there aren't more than a half dozen propellers | to choose from in the first place for a given application. Seems to me like there's usually more than a half dozen *brands* of propellers, let alone different possible sizes ... SignatureDoug McLaren, dougmc@frenzied.us If you can't learn to do something well, learn to enjoy doing it poorly. >| I know that this hobby is all about applying various scientific >| principles to toy machines for the purpose of having fun, > >For some people, yes ... >| but this discussion about airspeed and RPM data is getting a little >| bit ridiculous. >Why is it ridiculous? I don't see anything ridiculous about it. I see it as fun--IF I can persuade someone else to invest in the $500-$600 setup. :o) >| If you want to know which prop works the best, try 4 or 5 >| of them on the plane in flight and see which one works the best. If >| your data disagrees with your flight performance you would disregard the >| data anyway. >Maybe. You might also see if you can track down the source of the >discrepancy. If I were into air racing, I'd want to have the data on RPM and true airspeed for each prop, along with some sense of the air temperature and humidity at the time of the test run. I would think that would give me more satisfaction than trying to evaluate performance solely on the testimony of eye and ear. >| The good news is that there aren't more than a half dozen propellers >| to choose from in the first place for a given application. >Seems to me like there's usually more than a half dozen *brands* of >propellers, let alone different possible sizes ... There are amazing prop gurus out there. We may have been visited by one from Australia a few days back. Some of the pattern guys heat their APCs and repitch them. Gives me the shivers just to think about it. I think some of the props are set by the racing classes, so that wouldn't be an option. Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. Look at this page and scroll about 19 pages down and you will see a reference to an audio tach from 1951 by some engineers from GE. http://www.lymanslack.com/ModelAviationBiography/temp.html A flying friend of mine tried this in the 1960's and it was not sucessfull. After he passed I got some of his junk box stuff and found the audio tach. So being an individual that wanted to experiment and having technology to use that did exist in the 60's I applied microprocessors to the job. This was about 2002 and it did not work any better now than it did way back then. This is a technique that works well in theory but not in practice. The narrow band pass (zero beating) of the human ear is too narrow to compensate for the doppler effect and other noise made while a plane is in flight. With the plane held on the ground you could make it work. My $.02 >> Strikes me one can measure the RPM of a flying plane by measuring the >> frequency of the noise it makes (beware of doppler effect) with a >> frequency counter. If one isn't handy, one could record the sound and >> look at it with any number of programs on a PC. Plenty of DIY kits and >> circuits on the net too. >Look at this page and scroll about 19 pages down and you will see a >reference to an audio tach from 1951 by some engineers from GE. >http://www.lymanslack.com/ModelAviationBiography/temp.html >A flying friend of mine tried this in the 1960's and it was not >sucessfull. After he passed I got some of his junk box stuff and found >the audio tach. So being an individual that wanted to experiment and >having technology to use that did exist in the 60's I applied >microprocessors to the job. This was about 2002 and it did not work any >better now than it did way back then. >This is a technique that works well in theory but not in practice. The >narrow band pass (zero beating) of the human ear is too narrow to >compensate for the doppler effect and other noise made while a plane is >in flight. With the plane held on the ground you could make it work. >My $.02 Seems to me that trying to use it in the air would be incredibly hard (I don't know nothin' 'bout narrow band pass). Doesn't the frequency heard vary continuously as the plane approaches (rising note) and departs (falling note)? Maybe it'd work if you were flying CL and the plane was at a constant distance from the pilot, or if you flew the RC plane in a circle around the pilot. We try to discourage that at our field, but some of the oldtimers did fly that way when I started at the field in 1995. Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details. > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. OK, I have managed to get the 28cc strimmer engine running on an 18x8 prop (stationary mount). Now comes the running in and tweaking. This is where it would be nice to know the equivalent prop size to put the engine under the same load stationary as it would be moving in the air. Any suggestions? >> Strikes me one can measure the RPM of a flying plane by measuring the >> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 10 lines] > > Any suggestions? It doesn't work that way. The airframe has influence over how much the engine will unload in the air. This is why we usually have to take a few props to the field with us in order to find out which one loads the engine properly on a given airframe. There is still a bit of "art" involved in model airplanes. Ed Cregger > >> Strikes me one can measure the RPM of a flying plane by measuring the > >> frequency of the noise it makes (beware of doppler effect) with a [quoted text clipped - 13 lines] > It doesn't work that way. The airframe has influence over how much the > engine will unload in the air. I am talking about maximum revs, not power at various throttle settings. If the load is X and the plane doing Y mph horizontally, then that is the approximate load I want to simulate on the ground. Obviously the engine would have virtually no load on it in a dive, and the same as stationary when it is prop hanging. If I build an electronic ignition unit with timing advance, it would take some trial and horror guesswork out of it if I tested the various advance timings at simulated air loading. So, If I am swinging an 18x8 prop stationary, what prop would present the same load as 30mph ? > This is why we usually have to take a few > props to the field with us in order to find out which one loads the engine > properly on a given airframe. There is still a bit of "art" involved in > model airplanes. Well, you can sure get *some* better idea of max air rpm by using a smaller prop in stationary testing. It would also be better for running the engine in. Big props are expensive here and to buy a range of them would cost more than the strimmer engine did. In fact I already bought two props at a reduced price, if I buy a third, they will have cost the same as the engine. > Ed Cregger | > It doesn't work that way. The airframe has influence over how much the | > engine will unload in the air. | | I am talking about maximum revs, not power at various throttle | settings. If the load is X and the plane doing Y mph horizontally, then | that is the approximate load I want to simulate on the ground. Ok then. Then determine your RPMs in the air, and then try props on the ground until you reach the same RPMs. | Obviously the engine would have virtually no load on it in a dive That is not so obvious -- that will depend a lot on the prop and the airframe. Some high drag planes won't go signfigantly faster in a dive, for example, and just because the pitch speed of the prop matches the airspeed of the plane, that doesn't mean there's no load on the engine from the prop. | and the same as stationary when it is prop hanging. That much is approximately true. (But not exactly, as the conditions around the prop will affect it's performance to some small degree.) | So, If I am swinging an 18x8 prop stationary, what prop would present | the same load as 30mph ? There's way more variables involved than just that. | Big props are expensive here and to buy a range of them would cost more | than the strimmer engine did. In fact I already bought two props at a | reduced price, if I buy a third, they will have cost the same as the | engine. If you want it cheap(er), try smaller engines. Or go electric -- for engines, this stuff isn't so easy to calculate, but for motors it can be calculated to a high degree of accuracy. SignatureDoug McLaren, dougmc@frenzied.us ..And Scotty beamed them to the Klingon ship, where there would be no tribble at all. > | > It doesn't work that way. The airframe has influence over how much the > | > engine will unload in the air. [quoted text clipped - 5 lines] > Ok then. Then determine your RPMs in the air, and then try props on > the ground until you reach the same RPMs. The whole point is not to buy and try a bunch of expensive big props and then test them all. > | Obviously the engine would have virtually no load on it in a dive > [quoted text clipped - 13 lines] > > There's way more variables involved than just that. What are they? The only thing the prop sees is air stationary, or air moving at 30mph. OK different props have their own slightly different drags and efficiencies but I'm only looking for an approximate equivalent so I can test my converted strimmer engine at roughly "air" loading. So If I fly a 16x8 prop at 30mph, the rough equivalent stationary might be 16x5 or 15x6, yes? I have a 13x8 but that might be a tad too small even if I use it back to front. > | Big props are expensive here and to buy a range of them would cost more > | than the strimmer engine did. In fact I already bought two props at a [quoted text clipped - 4 lines] > engines, this stuff isn't so easy to calculate, but for motors it can > be calculated to a high degree of accuracy. I have plenty of smaller planes and engines, I am building a camera plane/s for video and want to tune a converted strimmer engine for the job. I also want to build some electronic ignition ignition sytems, including timing advance, so I want to have the same load on the engine as it would (approximately) have in the air. All I'm looking for is a guesstimate of how much to reduce the Diameter/pitch for stationary testing. > -- > Doug McLaren, dougmc@frenzied.us > ..And Scotty beamed them to the Klingon ship, where there would be no > tribble at all. ... | > Ok then. Then determine your RPMs in the air, and then try props on | > the ground until you reach the same RPMs. | | The whole point is not to buy and try a bunch of expensive big props | and then test them all. Yes, and my point is that this is a pipe dream. | > | So, If I am swinging a 16x8 prop stationary, what prop would present | > | the same load as 30mph ? [quoted text clipped - 3 lines] | What are they? The only thing the prop sees is air stationary, or air | moving at 30mph. Ok, I'll start by listing a few of them ... -- at what RPM? (this is the biggest one.) -- (ok, you've picked diameter and pitch) -- what shape exactly is the prop? -- what is behind/around the prop? (the cowling/engine/etc will affect this, as will your test bench) -- how about the cooling of your engine? Your engine will cool a lot better in flight than it will on the ground or hovering, and this will affect how it performs. -- at what altitude above sea level? -- where did this 30 mph figure come from? | OK different props have their own slightly different drags and | efficiencies but I'm only looking for an approximate equivalent so I | can test my converted strimmer engine at roughly "air" loading. So | If I fly a 16x8 prop at 30mph, the rough equivalent stationary might | be 16x5 or 15x6, yes? Maybe if you can pick a specific RPM speed target ... SignatureDoug McLaren, dougmc@frenzied.us Game over, man! Game over! > ... > | > Ok then. Then determine your RPMs in the air, and then try props on [quoted text clipped - 16 lines] > > -- at what RPM? (this is the biggest one.) That is what will be determined from tuning the engine and spark timing. The expected *maximum* rpm is about 7-8000. > -- (ok, you've picked diameter and pitch) > -- what shape exactly is the prop? Master airscrew classic, most likely. > -- what is behind/around the prop? > (the cowling/engine/etc will affect this, as will your test bench) Exactly the same as it would be in the air. The test bench won't make any discernable difference > -- how about the cooling of your engine? Your engine will cool a lot > better in flight than it will on the ground or hovering, and this will > affect how it performs. Well not significantly. It is a strimmer engine which is normally cooled by the flywheel fins and used to running hot. The difference of extra cooling from a prop running stationary or in the air is barely going to affect it. I could even trim the fins and loose some weight. > -- at what altitude above sea level? 500m max. > -- where did this 30 mph figure come from? That's the approximate cruising speed I intend my plane to fly at and optimum tune my engine to. As you might have gathered, *any* prop a *bit* less powerful than the one actualy used for flight, would better simulate flight conditions in stationary testing. The only question is how much less. > | OK different props have their own slightly different drags and > | efficiencies but I'm only looking for an approximate equivalent so I [quoted text clipped - 3 lines] > > Maybe if you can pick a specific RPM speed target ... About 7000rpm. > -- > Doug McLaren, dougmc@frenzied.us Game over, man! Game over! > > -- where did this 30 mph figure come from? > [quoted text clipped - 16 lines] > > -- > > Doug McLaren, dougmc@frenzied.us Game over, man! Game over! If I did the math correctly and you want to fly at 30mph air speed at 7000 rpm engine speed you need a 4.5 inch pitch. So lets say ground testing gives you the 7000 rpm you want with a 15X5 prop. In the air you will need a 16 or 17 X 5 prop to get the same engine loading. A lot depends on airframe drag which can only be determined by testing in the air. Such a combo does give an acceptable prop tip speed. This seems to me like an awful lot of low gear and I think you would be a lot happier with the way the plane handled with a bit more pitch and a bit less diameter. For instance if you ever try to fly in any wind it is going to be tough to get the plane up or down safely with such a low pitch. Even if wind is only 10 mph gusts of 20 are common. At 30 mph airspeed you are not going to be much above stall speed even with a floater. Set up to land in a gust and when the gust passes your airspeed could be below stall speed and no altitude to recover. You do have a throttle control for a reason. But throttle only works if you have enough pitch to use it. Your specs are going to limit flying to calm, overcast days I think. Back in the 70's a friend of mine used a simple AF generator and ear phone. He had the dial calibrated and just read the rpm's directly off the dial. Phil > Strikes me one can measure the RPM of a flying plane by measuring the > frequency of the noise it makes (beware of doppler effect) with a > frequency counter. If one isn't handy, one could record the sound and > look at it with any number of programs on a PC. Plenty of DIY kits and > circuits on the net too. > Back in the 70's a friend of mine used a simple AF generator and ear > phone. He had the dial calibrated and just read the rpm's directly > off the dial. > > Phil It certainly seems the easiest method, and the cheapest for ground use too. Nice one. I'll check if I've got something lying around. I've got loads of NE555 ic's to put one together. > > Strikes me one can measure the RPM of a flying plane by measuring the > > frequency of the noise it makes (beware of doppler effect) with a > > frequency counter. If one isn't handy, one could record the sound and > > look at it with any number of programs on a PC. Plenty of DIY kits and > > circuits on the net too. |
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