combatpigg

My Feedback: (3)

http://youtu.be/J-KnwuBBH8E This Webra .50 has the exhaust port raised to 180 degrees, the cranshaft port closing moved back to 60 degrees and the crankshaft tunnel enlarged as much as I dared.
I made a nylon venturi...then kept opening it up until it was obviously too big, then made another one, probably 11mm IIRC.
I lightened the piston internally, but do not remember having to rebalance the crank or not.
I also made my own head buttons with different chamber volumes and shapes to try.
My "target" prop was a 8x8 APC and after many trials arrived at 7.5 x 8 and the Mac's header and pipe cut very short. The header ran out of donor material, and I remember cutting the pipe until no further gains were made with the 7.5 x 8 prop.
I think the model [a Screamin Demon Delta] runs170 mph or so. Even on the most sophisticated airframe with the most sophisticated carbon prop, there just isn't enough air handling ability to make the engine turn much faster since the Webra .50 uses a standard .40 size crankshaft design that you would find in any .40 size sport engine. This is the limiting factor.

Pond Skipper

Sounded like you was at the race track. Apprent best pass 29,160 rpm / 221 mph if you had no losses. Sounded like my Z on the way to work this morning or maybe it was the coffee talking

MJD

My Feedback: (1)

Gotta be something talking - no way it's spinning or flying that fast. The shifted pitch on the approach sounds like 28-29k. Play it at the same time as this video to hear the sound of 28.5 k for reference.

At 0:02 you hear the same pitch - from a static run of the .061 that is running 28.5k, and the very Doppler shifted pitch of the Webra. At 170 mph, 22.3k will sound like 28.5k.

https://www.youtube.com/watch?v=53v5Z--nkig

Pond Skipper

Yes if no slip loss or drag loss 22,440rpm with 8p would give you 170mph. I was just using my audio smartphone app. - (Apparent best pass 29,160 rpm / 221 mph if you had no losses) was the reading it gave. Old rule of thumb I always went with is 20% loss in slip / drag of a clean frame puts it back at 176.8 if that was the case. 8p prop puts out its own sound mixed with the engine.
I agree the vid had a higher pitch sound to the ear and the photocell tach was talking

Pond Skipper

My app reads the vid at .06 seconds in at 28,746 and 28,989 almost at the start (looking at two things at once here). It followed the engine gradual reduction in rpm within 100 rpm in relation to the tach and 27,777 at 1.02 min. as an additional sample it has always road tachs shown on vids very well for a free app.
So fight video's are worth checking with it. Given the 20% rule in conjunction its reasonable in lieu of radar.

aspeed

I estimate about 18,500 on the slow part of the doppler, and yes around 28,500 on the high end. That is an average of 23,500 rpm. Max of 188 mph minus slippage and plane drag etc. I watched an onboard video of a pylon racer going around 200 mph, and the revs went up around 3,000 immediately after release, and just stayed there for the whole flight. It sounds much different from the ground. With a pipe, the 3,000 rpm could even double, although the ground rpm would be less. One octave is double the rpm and a pipe is only happy at one powerband rpm which is half or double increments. The pipe powerband and motor powerbands have to match of course for true happiness;-) Control line is much simpler. You know exactly the speed because you are timing one kilometer or 1/2 mile and you have the exact speed. An audio app form inside the circle can give you the air rpm and ground rpm too. You can even differentiate each lap as it is quieter and louder for each lap if the microphone is not in the exact centre of the circle. One of the newer radios could give the rpm and gps speed too if you really need that type of info.

combatpigg

My Feedback: (3)

There is another way to look at this.
Raw fuel consumption in pounds per hour or ounces per minute.
The difference between a truly 200 mph capable .40 sized engine and the max fuel demand of a modified Webra .50 is like night and day. When you look down into the intake of a Nelson .40 Pylon engine it looks like Niagra Falls, or a toilet flushing...it's just incredible.
I seriously doubt that Weston can take their modded Webra .50s much further than what I've done with mine. About the only tricks I haven't done are "turboing" the crank's counterweight and making a "stuffer" backplate to increase pumping efficiency a little.
I don't want to "rain on the parade" that the Webra .50 is capable of 200 mph +, in fact I'd love to be proven wrong..!
What's nice about the level of power with my Webra .50 is it can do it all day, every day, it's very easy to run, it uses $4.00 APC props and it's still fast enough to be scary. I think you could get about the same level of power out of many brands of .45 to .52 sized sport engines that are out there with some work, but I'm not sure about longevity. With the Webra ABC .50, I can vouch for it's durability. I've owned it since 1989 and rebuilt it just once.