panther3001

I know not all of this fits into "aerodynamics" exactly, but here is a detailed document I wrote/am writing (I know, it's incomplete still, sorry) covering many general RC topics, with a large portion of it covering a crude flight testing technique which could help you get an estimate for lift coefficient, CL, drag coefficient, CD, and glide ratio, L/D. To make the thrust calculation more accurate I'm also working on identifying a couple "scaling parameters" to account for lower actual thrust values, as theory doesn't really match up perfectly to reality.

For the Pitch Speed and Tip Speed equations see the section titled, "Propeller or EDF (Electric Ducted Fan) Unit."
For a crude flight testing technique to get CL, CD, L/D, etc, see the section titled, "Some Advanced Calculations and Flight Testing."
For a detailed step-by-step tutorial on how to use the Doppler Effect, for FREE, to calculate the aircraft's speed, see the section titled, "How to Measure Your Aircraft’s Speed (for FREE—no Radar Speed Gun Required!!!) Using the Doppler Effect."

Refer to other sections as you see fit, and feel free to use this document as a reference for general info on batteries and motors, ESC's, etc. "Ctrl+F" can be a helpful resource to search for things that may interest you.

Here it is: https://sites.google.com/site/electr...raftguru/doc-2

Lastly, be sure to check out the other page links in the upper-left-hand column of the website, and if this information can help someone then I've succeeded!!!! Thanks for reading!

BMatthews

A couple of small corrections which you may want to consider.

First off the history of model aviation actually predates the Wright Bros successful Kittyhawk flight by quite a few years. Also there was a fairly healthy model building and flying community in most countries even prior to WW1. It really blossomed following the end of WW1 and was massive by the outbreak of WW2.

I used to think that the pitch speed was something that could not ever be reached. But numerous folks showed me otherwise a couple of years back here on RCU. It seems that the pitch stated on the prop is based on the chord line or airfoil center line. But cambered airfoils such as used on most props actually still make lift even down to a slight negative angle. The angle for each airfoil where no lift is made is the "zero lift angle". But since this can often be a -2 to 4 degree angle it would seem that the effective pitch of most props is actually somewhat higher than the geometric pitch.

Then there's the simple fact that the stated pitch is often not the true pitch. The fellow that started the old Y&O propeller line before it went to the current owners painstakingly checked a lot of modern props. He found that often the stated pitch value wasn't much better than a guess. So all in all there's a lot of "free form" thinking when it comes to props and how accurate the pitch speed will turn out to be.

panther3001

BMatthews, thanks for the corrections! I'll incorporate them when I get a chance. As for the history of model aviation thing, do you have any links for sources I could add as well, or where did you get that info? If I can put a source into the paper for better credibility I will (Wikipedia is ok by me by the way in case that's it). Also, the pitch speed thing really makes a lot of sense! Looking at lift coefficient vs. angle of attack plots in my undergrad, I've known for many years that a cambered airfoil is still capable of producing lift at a slightly negative angle of attack, but it never occurred to me that's what was happening with props. I did have an inkling it was occurring recently though as I was taking data just for fun....one of my planes has a top speed slightly higher than its pitch speed. However, that could also be due to rpms increasing as airspeed increases, since effective angle of attack decreases as airspeed increases, and hence drag and resistance on the prop decreases, so perhaps rpms could increase (I'm not sure on this, I have no live, in-flight RPM sensor)...in either case though, what you said makes a lot of sense so I appreciate your comments!

PS. It sounds like you are actually reading the document too. That's pretty cool I think. Hopefully you find it interesting, accurate, and useful....even if you already knew what I've written. If not though, at least you are giving me good suggestions, so thanks again!

BMatthews

You've got a lot of text on your page. I have to admit that I mostly jumped in where key words made me stop to scan through specific areas. Most of what I quickly scanned seems pretty well OK. So all in all I'd say you're on the right track.

Links to the history of model avaiation are mostly found in those old things we call "books". But if you check You Tube there's some slick stuff from the old british days and a few movies posted from some early US Nationals by a guy named something like "bomberguy". If you feel it's worthy you might include links to those videos in your website page. But the material posted by that guy shows that even by the early to mid 1930's that model aviation was well entrenched. And in actual fact it was FAR more highly regarded than it is today or even back in the early post WW2 era. Back then our hobby was seen as a stepping stone to a career in aviation. In fact there was a movement back in the mid 30's called the Junior Birdment of America that was some 200,000 members strong! Doing a quick search for "junior birdmen" led me to a Wikipedia entry that indicated that the theme song was used in a 1955 movie. For that to mean anything to the audience of the time suggests just how strong this whole model airplane movement was back then. Model aviation was a HUGE attraction back then. And all manner of local and state officials used to come out to award prizes to the modelers. It wasn't at all uncommon to see pictures of such presentations and write ups of the contests in fairly prominent newspapers.

We actually MEANT something back then! Now we're simply seen as grownups playing with toys ! ! ! ! !

As for the pitch speed? Yeah, there's a lot at work there. And most of us underestimate the amount of RPM pickup on models with high pitch props once airborne.

I might add that the tales of those which ended up meeting and beating the pitch speed limit were flying very clean "racing" models. Our average sport model with landing gear and dumpy looking fuselages isn't going to meet the pitch speed limit other than in a screaming death dive. For those designs achieving a speed of even 80% of the theoretical pitch speed would be considered excellent.

I'll try to look over your material in more depth when time permits and post back here or PM you. But I've got a lot on my plate at present. If you don't hear from me in about a month send me a PM and jog my memory.

alasdair

As BMathews says, model aeroplanes go way back
Alexandre Gustave Eiffel, the Frenchman who built the Eiffel Tower in 1889 and the engineering structure of the Statue of Liberty in 1886 also built flying model aeroplanes. I saw an exhibit of his models (in the Eiffel Tower I think).

Achieving pitch speed depends on the choice of prop. A 61 2-stroke will turn a wide range of props on a wide range of models.
Fit a prop that is too fine-pitch for the model (an aerobat with 13x4) and it will easily exceed its pitch speed in level flight
Fit a prop that is too coarse for the model (a scale biplane with a 12x12) and it will struggle to fly at all, and its level speed will fall well short of pitch speed.

Indeed, I maintain that youcan tell thatprop choice is optimised if the model achieves a level flight speed of 90-100% of pitch speed.
That is the envelope wherein most props reach their peak efficiency.

UStik

panther, that is a good idea that you share your knowledge collected with hard work and over a long time. I think it's like "what would I have liked to read when I was a beginner". But it was the thread title what sparked my interest.

As to measuring of model airplane speed, there's now even a fourth way: GPS as part of the telemetry system. Admitted, it would make you go beyond the 250$ limit but you already mentioned telemetry so this is for completeness.

By the way, radar guns are based on the Doppler effect as well as the sound method you mentioned. However, I don't trust them both, not for some unavoidable inaccuracy but because you don't really know if the airplane flies level. Even a shallow dive may have a very noticeable effect on speed. That's why speed records once had to be flown at very low altitude, just to force level flight. In the 1930s, Heinkel complained that he had to fly speed records at sea level while Messerschmitt was favored having his racetrack at 1700 ft above MSL.

Anyway, another time I'd like to gainsay BMatthews. I think most of us overestimate the amount of RPM pickup on models with high pitch props once airborne. That may be true only for electrics since I/C engines lose torque when RPM decreases while an electric drive's torque is reverse proportional to RPM. But in both cases torque doesn't increase when airspeed is so low that the prop blades are (at least partially) stalled, instead it may even decrease. That's because prop torque comes not only from drag but even more from lift, which is perpendicular to the blade surface and not in flight direction. This effect is bigger on high-pitch props.

You may notice this on low-pitch props as well though, for instance in a full-scale with fixed-pitch prop, say a C172. There are only a few hundred rpm difference between runup, climb, and level off, not that much compared to the runup rpm. Of course leaving full throttle when levelling off would overrev the engine, but still you'd never come close to pitch speed. That's because (1) that would require no prop thrust and hence (2) no airframe drag, which increases roughly with second power of speed, though (an oxymoron).

So I would say in any case someone ended up meeting and beating the pitch speed limit it was due to some dive, not necessarily a death dive. It's sufficient to get a bit beyond that limit to have the prop windmilling and overrevving the engine. I'll add two graphs showing the behavior of an electric F3A drive. You'll notice that thrust decreases rapidly when flight speed is near pitch speed and that windmilling is required to reach it. On the other side, below 20 m/s some blade stall begins so thrust is no longer reverse proportional to speed and torque no longer an inverted parabola.

I have to admit that I only skimmed your document, but may I add another unit conversion calculator: just Google for Convert.exe!

If you'd like to add more info about electric drives, [link=http://www.rcgroups.com/forums/showthread.php?t=185271]here[/link] and [link=http://www.rcgroups.com/forums/showthread.php?t=182903]here[/link] are nice explanations of the basic equations. If you want to know how I came up with the diagrams, [link=http://time.fh-augsburg.de/cgi-bin/dl.pl?id=ElectricDrive.pdf]here[/link] is the derivation of the basic equation.

As to history of R/C model flying, also look [link=http://www.modelaircraft.org/mag/Jan2011/onthefly2.htm]here[/link] and google for Walt and Bill Good and their Big Guff. There are videos.

panther3001

wow. Boy do I still have a lot to learn! This could take a while. Meanwhile, I've got so many mini projects going on I'm not sure how to organize my time well either. I think I'll have to just try to finish the missing sections in the document and leave the rest basically as it stands now, then make subsequent versions with new sections and more clarification in the years to come. I'll do my best. A quick sentence or two addition for the history I can do in the document soon, but it looks like I have a ton more to learn about motors still. Always learning...it never stops! The more I learn the more I realize I know so little!