.15 glow engine suggestion

Thunder Tiger manufacturers both a GP-15 and GP-18 engine that might fit your needs. The Magnum XLS .15A is another good possibility as it is a dual ball bearing design instead of a bushed design like LA or GP series.

As for which engines will turn a 8x7 prop at what rpms, I can't speak to that. You might check the Tachometer Readings forum on RC Universe and see if anybody has actually tach'd these engines with an 8x7 prop.

Good luck and good shopping!

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I doubt that even a full race .15 with a tuned pipe would swing an 8x7 that fast.

However, because a .15 sized model is so small, I can guarantee that a person with at least average vision is going to have one heck of a time flying a .15 powered model running a 7x4 and keeping up with it at wide open throttle. The little rascals can get too far away to see well in a hurry.

I flew a model named "The Peppermint Pattie" in the early eighties. This model resembled a Banshee pattern ship. I hand launched the model, so I did not use landing gear. I was able to run Peppermint Pattie right past the piped .60 sized Curare models of the day with an easily visible faster speed. The pattern ships back then were routinely flying at over 100 mph. Pattie left them in the dust.

My engine in those days was a baffle piston HB .15 with muffler and Top Flite 7x4 nylon prop and the engine was burning 10% nitro club fuel. Nothing exotic. It was all I could do to keep it in sight and my vision was pretty good back then.

Ed Cregger

What he said. Very quick plane it is. I have one myself powered with an OS .15FP which is the precursor of the .15LA. More than enough power and its plenty fast enough for me! Zing! It's a dot. Zing! It's in your face. As far as a screamer .15, this OS engine would fit the bill:

OS 15CV-A engine. Airplane ready, no conversion needed. $110. I have one that I was going to put on a Sig Wonder but never finished building the kit for some reason. I have thought briefly of putting it on the Hyperbee but that might make it supersonic. Also thought of putting an MVVS on it but if they run anything like the MVVS .26 I have once again, it might be too fast. Plus I don't like the pipe. Maybe back when I was 20 but now that 50 is staring me in the face I dunno. Either way, that Hyperbee is a flyin sumbitch.

Dan KE6ERB Fubar of The HillPeople

Those are some pretty ambitious figures for any .15. I generally run

8x6 props on .20 size engines. My OS 15 FP usually gets a 7x5, depending on what plane it's in.

3for3 wrote:

I seriously doubt that you are going to find ANY .15 engine that will turn that prop at that speed. You are looking at something in the neighborhood of 0,85 HP requirement.

even with the inflated claims of power and RPM published by most engine manufacturers, I doubt that you will find anybody claiming much above 0.50 HP

a 7X7 (if such an animal exists) would require approximately 0.49 HP

for old school Topflite 7x9. used to fly the 7x6W, it think on the old K&B sandcast, a real screamer on Missile Mist. Needed really long lines

** mike **

You might get close to 100mph with a Cox Special .15, I believe it was rated at close to 17000rpm on a 7x6 and 30% nitro. Obviously you need the pitch to get the desired MPH but the planes drag affects how large you can go with the diameter before you exceed the engines power. If the drag is low enough, a smaller prop will let you rev the Cox up even higher.

Indeed. It takes a racing 36 to get an 8x8 to 18K RPM.

TNP, how could he do this with electric? mk

Something like a MP Jet 28/10 motor, 7-8 LiPo cells, 100 amps, 7" or

8" prop .......... needs analyzed via Motocalc

With extreme difficulty and probably not at the same weight for any sensible duration.

Top class Hacker B50 and a shed load of cells can do it, but tends to weigh a lot more.

Hi Bob, you indicated hp vs rpm numbers. What is the math fromula behind this? My physics is a littly rusty.

Raymond

Horsepower = Torque x RPM / 5252

Unless you have a way of measuring torque, there is not a direct relationship between HP and RPM.

However, if you know a certain prop requires a certain HP to turn it at a certain HP, you know about how much torque that engine must be putting out to do those RPM's. You then can work backwards to estimate the HP of an engine, given a similar RPM range. Indeed, there are "club" props (a chunk of wood that has no pitch) around that people use to work out HP, lacking a device to measure torque, like a dynamometer would do.

the math is based on some work done many years ago (prior to 1998 for certain) by Bob Adkins and Brian ????? (I can't remember his last name at the moment) Their formulas were derived from many engine runs measuring various parameters. with props in the 7 to11 inch diameter range (I'm not trying to be vague, I'm working from memory here)

At one time, various versions of thrust / HP calculators were online at several different web sites, and they all seemed to be based on similar math and assumptions.

I have the formulas stuffed into a calculator, and in an Excel spreadsheet,

******** DISCLAIMER ******** For MY purposes, these formulas have served me very well over the years for predicting prop performance on various planes. the further away from the observed data you get, the greater the error in the predictions. ****************

These formulas have been posted in the newsgroup several times, and almost every time they were posted, the thread degenerated into a fiasco where the "pure math" types claimed that the formulas were worthless because they didn't take into account some specific parameter.

As far as I am concerned, they work well enough for me, BUT I subscribe to the " T L A R " school of thought and the "That'll get you close" theory of information management when precise information is not available.

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I'm with you, Bob. Close enough is close enough.

I can't remember Brian's last name either. Bummer. I wish that he and Bob Adkins would "come on down" and begin participating on RCU. Who knows? Maybe they are...8>0

Ed Cregger

I do know that Bob Adkins is still around, (at least when I checked a couple weeks ago ;-) ) It has been years since I last had any contact with Brian,

Not sure if either of them haunt any of the on-line forums,

I know that I don't,

3 reasons, SLOW internet connection, lack of time to go digging, AND at least some of the old hands still show up here to help keep us headed the right way ;-)

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There's not a single thing wrong with being right here. I had brain fade when I made the comment about RCU. I thought I was there when I made the comment. Here is just fine.

Ed Cregger

Hi Bob, Will you share your excel spreadsheet? do you subscribe to RCUniverse? would you PM me ?

3 for 3 Regards raymond

"brain fade",,, I get that a lot, It doesn't bother me, the part that bothers me is that mine never seems to fade back in ;-)

bob