Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

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Old 10-28-2004, 10:59 PM
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1. Why are Q-40 racers lacking tuned pipe? Why do they have side exhaust? Why don't they have retractable gears? Seems 50MPH are lost because of these ommitions. Are these the Q-40 regulations?

2. Seems that these engines (such as Jett) are good at astronomical RPM and deliver very little torque. The huge power is because of the RPM, and no need for substantiaal torque. So why don't we see high-end car engines in this application (with smaller heat sink)? One of the Novarossi engines delivers max RPM of 44000RPM (it's a .21). Since there's much bigger market for cars than for Extreme speed planes, I would expect to have the state-of-the-art in high performance engines to come from the model car market.
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Old 10-29-2004, 12:10 AM
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Q40 was designed to get people back into racing who were burned out with the complexity and expense of FORMULA 1. The rule makers did a pretty good job of making Q40 the event it is today, the lap times are plenty fast and the racing is close. I'm sure there are some speed freaks who are looking into using car engines. The question you gotta ask is ,"how much prop can this .21 handle and still be happy"? I don't know what the .21 weighs, but the car engines I have looked at were pretty beefy. Too bad there isn't more info coming in from guys who have tried it already. Imagine a 6.5"-6 prop hitting 44,000, comes out to 250 mph.
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Old 10-29-2004, 06:29 AM
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IMO there's no way a .21 car engine is going to hit 44K with a 6.5x6... for CLUB 20 racing over here we use a 7x6 turning in the +20K rpm band... to get that same prop to turn even only 30K or 35K you would need astronomical power levels... power levels provided by .40 racing engines...

Also, how long would that engine survive 44K? I mean, sure maybe the car guys hit 44K occasionally, but they running around at much lower rpm (and cooler temps) most of the time.
I don't think it is going to last long if you run it at 44K for three minutes at a stretch, 15 times a day...

Maybe some of the tiny carbon props that the electric racers use are more suited for this purpose...? Just a wild guess though...
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Old 10-29-2004, 08:57 AM
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As you noted, all of the QM40 specs are by rule. Infact, at the inception of the event, the QM40 engine was intended to be identical to the 428 Q-500 engine, and the aircraft were intended to operate off of the same flying facility (often grass) and on the same 'short course' (at the time). Things evolved over the years, since the Q-500 class has a specific prop size rule, and the QM class has no prop size regulations. Therefore, today the engines still look very similar, but are internally different.

The QM40 engines do have tuned pipes. The "mufflers" used on both 428 Q-500 and 422 QM40 engines are tuned exhausts. They contribute a great deal to the performance of the engine.

The QM40 engine and muffler deliver peak power just over 26K rpm, and our prop selection is critical to 'allow' the engine to turn up to that mark in the air. These are 7.5x8 thin blade carbon props. Typically, we rarely see more than 23K rpm on the ground with a flight prop set up to fly at 26K rpm at 180 mph.

R/C Car engines and the engine technology has been around for a long time. K&B, OPS, Picco, CMB and Rossi all have made car engines since back in the 1970s. That car technology came mainly from marine and control line speed applications. 15 years ago, any sort of 'nitro' car was hard surface/speed track vehicle... not off road/truck applications you see today. The off road applications are geared down substantially more than the road racers were. What makes r/c car engine a bit different today is the materials, CNC machining, and in some cases the "Cheap labor" that makes the higher performance engines easier to produce, more consistant from engine to engine, and the price makse them more commonly available. The same applies to the cars and trucks. They are produced now with strength and durability to 'handle' more power than they had previously been capable of.

The performance requirements of an RC car .21 size engine is far different than the needs of an aircraft engine. The smaller engines are capable of higher RPM because of the lower mass moving internal to the engine. The car engines are essentially lightly loaded due to the gearing involved. In many cases they are very highly timed to achieve very high rpm to take advantage of the gearing, with the design constraint that they generally do not spend a great deal of time sustainted at that maximum RPM. Car engines spend a great deal of their time throttling and accellerating.

On an aircraft, the prop load and the power required to turn the prop is far greater than you might imagine. There is also a constraint to try to keep the prop tip speeds below 'sonic' speed, which means the engines are typically designed for power/torque/timing to develop their max power at RPMs in this range.

The r/c application that most closely approxmates r/c cars are r/c boats. Tyically lighty loaded, high rpm, with limited excursions well into insain rpm ranges. The closest fixed wing application would be ducted fan. With those, engines are allowed to turn much higher rpms beacuse the tip speed of a 5" fan (example) at 25000 rpm is well below sonic, while say that same "90" size engine with an 11" prop turing 17K rpm .... the tips are trans-sonic. One would typically not want to turn that engine/prop combination up a whole lot higher than that... its just tends to make noise, it becomes inefficient, and it better be a real good prop to handle the load and sonic tip speeds

Jett makes "20" size engines. These are the BSE .25-35 engines. All of these put out extraordinary power. But we rarely turn them over 19K rpm in an aircraft application. Some of the free flight versions turn up in the high 25K range. They can phyicially turn up much more, but its generally not practicle for the prop driven applications.

If you want to learn a bit more about .21 size aircraft speed engines, do some research on control line speed. Most of those engines are pushed about as far as one might want to go. [&:] You may even find some info on car/aircraft engine technology exchanges, per your orignal post.

Bob
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Old 10-29-2004, 10:34 PM
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Wow, these were really educating responses!
Still, it is hard for me to believe that all has been done for extracting all from the engines: First, there is such a huge investment in fluid and combustion simulation, optimization, and imaging (e.g., PIV) that the progress really shows in some auto and aero manufacturers. It is hard to believe that Jett has a team of PhDs to do the same in his R/C engine applications. Just look at the huge progress in European Turbodiesel cars (yes, diesels!) in the last 5 years to imagine what could be done in the R/C industry with computer technology. Then look at generic engines: OS FSRs of the 70s and equivalent of the 90s, the FX to understand that we should have expected more. Or am I buls%$**ing?
Back to extreme speeds, maybe we are not at the limit of the engine potential - it's this market that cannot support high-end technology.
And BTW, are there extreme speed planes with variable pitch props? Also, are there ones where the air is tunneled directly into the carb from the front, to have the full stagnation pressure at its entrance? Works for motorbikes, should work here too (especially at 200MPH).
Just my thoughts.
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Old 10-30-2004, 03:10 PM
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[link=http://www.rcpics.net/media/EngineGoBoom.wmv]THIS[/link] is what happens when you run a car engine at 44K rpm continuously: thermal self destruction...

Of course it might last a bit longer if it had proper cooling...

At the end of the vid the guy notices that the engine doesn't have compression... which makes sense without a head on it and a hole in the piston... lol...
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Old 10-30-2004, 03:38 PM
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Default RE: Beginner's questions

The same thing happens in NHRA TOP FUEL DRAG RACING, but that's besides the point. I don't think anyone expects longevity in the 200+ club. It only takes a few seconds worth of max running to blast through the 660' [1/8th mile] timing area. I would love to get some time with a hot .21 to see how much of a 6 pitch prop it could handle, and still get into the 200 mph club. Maybe it's time to look at the RC car bulletin boards?
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Old 11-01-2004, 12:44 PM
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ORIGINAL: Gold

Wow, these were really educating responses!
Still, it is hard for me to believe that all has been done for extracting all from the engines: First, there is such a huge investment in fluid and combustion simulation, optimization, and imaging (e.g., PIV) that the progress really shows in some auto and aero manufacturers. It is hard to believe that Jett has a team of PhDs to do the same in his R/C engine applications.
ROFL....... I definately have to send this to Dub

Of note.... Jett Engineering is basically a one-man show. Dub. I believe has a Masters degree..... but I do not believe he has a doctorate. And at this point, he is having enough trouble at times using the computer no less a fluid dynamics simulator [&:] Then again, he seems to have the capability to do fluid flow visualization in his head. He also has a great deal of patience. He does seem to wear a lot of hats in the shop...machinist, engineer, project manager, salesman, accountant, secretary, product support, shipping clerk, bathroom attendant... so I guess he could be considered a team

All of the technology tools are nice. The problem with aircraft engines is there are SOOOOOOOO many variables. Everything from ambient conditions and pressures........ to operating altitude........ even that last .0005" of manufacturing tolerance... all play into performance.

One could, in theory, design an "optimum design" engine, built using top notch materials and processes, that will funtion its very best, for a limited amount of time, at a given altitude, temperature, prop, fuel, and airframe. The closest example to this in real life. would be a top-fuel dragster or funny car. They adjust absolutely everything every single run. They also tend to replace a lot of engine parts every single run. Look at the money involved. In the model example the minute the airplane climbs to 100 feet, all of those variables have varied. Something has to give. Ask some of the old F-1 pilots how many engines they melted ( I had a FEW, look at the money involved!).

I'm sure someone with time could build a "silver-bullet" engine. We know the ducted fan many of the ducted fan planes flew well over 200 mph and up into the 250mph range, and my guess its not impossible to design a prop-driven engine and suitable airframe to produce that sort of power. It would be a one of a kind engine (or more like 10 of a kind).

As far as a production engine, that is not all that likely. Recovering the R&D cost on limited product sales would result in "military" type pricing. I already get grief on $250 engines

Bob
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Old 11-01-2004, 02:20 PM
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Default RE: Beginner's questions

Here's a confirmation for Bob's trans-sonic claim:
Trans-sonic speed (~.9Mach, where at some point on the prop M=1): ~300 m/sec.
Air speed of 250MPH=114 m/sec
Tangential speed at the tip (a^2+b^2=c^2)= sqrt (300^2-114^2)= 277.5m/sec
44000rpm= 733rps= 4608 rad/sec
r=277.5/4608 = .06m=2.37" radius (~5" prop).

So,
max prop for 44000rpm is hardly 5" (250mph)
==============================
It's inversely proportional: for more prop you are limited to less RPM.
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Old 11-01-2004, 07:51 PM
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A WARNING TO GOLD:

when calculus is involved do not ingest alcohol



don't drink and derive
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Old 11-01-2004, 09:16 PM
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Default RE: Beginner's questions

Ask some of the old F-1 pilots how many engines they melted
I used to figure on at least one melt down per contest. Sometimes you got lucky, but sometimes you would hear a lean run and not know it was your engine until it was too late (3 or 4 seconds). The big difference however with sport flying a fast airplane and racing is that you can take-off richer and monitor the engine performance as it comes to peak. As soon as it starts to strain, shut it down. Racing is a bit more cruel if you are in the points chase.
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Old 11-01-2004, 09:16 PM
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I got a C in calculus, and this was in 1997. Now you can imagine how very old I am.
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Old 11-01-2004, 09:17 PM
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