I don't recall ever seeing this actually discussed.  Other than the 4-2-4 break, why not use a .45-.51 R/C engine running at their optimum high RPM to power a .60 size plane.  I want to build a Lark 60 and I have many R/C engines in the .45 size.  With RPM around 11,500-12,000, it would seem the thrust would be as strong as a .60 C/L engine at 10,000 RPM.  What am I missing here?

Thanks guys,
Paul

I don't recall ever seeing this actually discussed.  Other than the 4-2-4 break, why not use a .45-.51 R/C engine running at their optimum high RPM to power a .60 size plane.  I want to build a Lark 60 and I have many R/C engines in the .45 size.  With RPM around 11,500-12,000, it would seem the thrust would be as strong as a .60 C/L engine at 10,000 RPM.  What am I missing here?

  

    Not to be a wise guy, but, *the last 30 years?. That's pretty much exactly what we are doing, with variations, with piped engines. 4-2 break at low revs is largely a thing of the past - it hasn't been a mainstream approach since around 1990. VIRTUALLY NO ONE competitive in the US is running a 4-2 break in the traditional sense. The only people doing it on a regular basis are the Discovery/Retro users, and it works well on this highly specialized engine, but it's not something many people are doing.

    The problem with doing this with a 51 is that a 51 running at max HP RPM has *far too much* power for any available prop. If you solve that by running the pitch lower with your $50 Australian prop, then you run out of responsiveness and the engine is too "flat". We ran that way a lot in the early 90's, in fact Ted Fancher and I had a discussion about trying that again within the last week.

    The excess horsepower problem is why people running stock 40FPs and the like in Fox 35-sized airplanes usually come to grief. No matter how you cut it, you only need and cannot use more than about .35-.4 hp in level flight, because that's how much power it takes to pull the airplane at the speeds necessary. That changes in maneuvers but run a RC sport 51 at full power and it will generate something like 1.2-1.3 HP. If you hook up even 75% of that with the prop, that's nearly 1 hp going in to the airframe. That will make the airplane go 80 mph instead of 55 - which is exactly what happened when someone first tried the 40FSR in a stunt plane. This is the dreaded "runaway", which is not a defect in the engine, it's a defect in the thinking. That's also why I suggest a 20FP, 25LA, etc, because the power available is a far closer match to the power required.

   You might actually need all that power in a corner. The problem is getting it to go away when you go back to level flight.

    Pipes mitigate this by regulating the HP to a usable response, essentially preventing the engine from being able to produce excess power, and still respond when it needs to.

    Note also that blocking boost ports, grinding ports for less blowdown, etc, is all an attempt to do something similar. For instance, blocking the boost port limits the breathing of the engine so that gaining power up to 16,000 rpm is impossible. It also had the beneficial effect of raising the gas velocity for a given power level. It's far easier and more effective to use a tuned exhaust as a regulator, but people still consider them "complicated" for some reason despite 25 years of experience and widely available information on how to make it work in ever modeling forum in the world.

     Brett

Man you lost me!  I was not talking about putting a strong .51 in a .40 size airplane.  I understand the issues there and also the tuned pipe mechanics.  I meant putting the .51 in a .60-.75 size airplane.  I was actually thinking of thrust required versus thrust produced.  Since the .51 can turn a higher pitch prop (or a lower pitch at higher RPM's), I would think the level flight speed of the larger airplane could be reasonably controlled and still pull well vertically.

    The original comment was about 4-2 breaks, and if there was some other way to do it by using R/C engines and low pitch props.  We have been doing it that way almost exclusively since 1990 and conventional 4-2 break engines (aside from the Retro) haven't been used a lot in competition for literally decades.

   The answer is "yes" you can do that, get a pipe to regulate the output and a prop around 4" of pitch, and it will be good enough to win 18 of the last 22 US Nationals.


  My description was "an example" o what you will likely run into if you don't use a pipe to regulate it. Using a 51 in a larger airplane is merely scaling up the issue. A 60-75 sized airplane (i.e. around 650 square inches) still takes around .4 hp or so, if you generate 1.3 hp, you are going to be going way too fast. If you somehow manage to blow off the power with a very inefficient prop (extremely low pitch, like maybe 3"), then it has no where to go in the maneuvers.

   You can't get more thrust without increasing the speed - the speed will increase until the thrust equals the drag. You are running maybe 2 lb of thrust in level flight, which amounts to about .3 hp. At the shaft you need something like .45 hp. Any more and it will go faster, or you will have to figure out how to dump the excess by choosing less-efficient props. That HP had to be either regulated, or it has to be sucked up some other way.

    I can tell you the answer, you can't be very successful with it peaked out without any regulation. If it's peaked out in level flight, it will die in the maneuvers. If it is not peaked out in level flight, it will very likely run away at some point.

    This has all been hashed out at extraordinary length for literally decades, people have tried just about every variation. We started out in the late 70's with this idea, and while it was some better than what we had at the time, it wasn't practical until we had pipe regulation at moderate RPM (11,000 or so in level flight).

    So, to skip to the end, go get one of the recognized good stunt engines like the 40VF, set it up the way it was in the 1991 "Impact" article, and go from there.

     A search of the SSW archives will find a lot of the relevant discussions, but understand that this was all a hot topic in the *EARLY 90's* before there was an SSW, Stunthangar, even the old RCO, so some of the basic information may have fallen off the radar since it is so old.

     Brett

Brett,

I get it now.  Thanks for your patience.  My original post was misleading regarding the 4-2-4 break.  (One of the problems trying to communicate via the written medium.)  What I was trying to do was preclude someone answering immediately by saying that you could not get the 4-2-4 break.  I know the 4-2-4 break has not been in vogue for 30 years, but my reference to it was in error.  I too have been flying constant speed settings for years.  I was thinking more along the lines of increased gyro issues, yaw problems due to prop wash, increased vibration, etc. as well as thrust available vs. thrust required.  I think the better question should have been "Can you power a .60 size airplane with a high RPM .45 engine?".  Sorry for the confusion.  I'll try to do better, but this writing instead of talking is just not my strong point.

   Sorry, I did get a little frustrated and maybe that came across. I did a search on SSW for "high rev low pitch" and found something like 1400 hits so you can see we have been talking about it for years. You don't see much of it now only because we have been doing this for 25 years and there is not much left to say about it.

    If you have a specific engine you want to use I am pretty sure we can figure out some good starting points for the pipe length, etc. If you are unsure the most bulletproof setup is the 40VF the way Paul Walker does it (and as documented in the SN article David Fitzgerald and I did).

       Brett