Yes. I am apoplectic, but it goes good with my morning coffee. :o) There's a lot more going on here than just the throttle position and pitch of the prop. Things like drag and torque curve are really important in this discussion.
What you say is correct in many regards, but some assumptions likely are not. In your test example, going to more coarse prop on your plane to further load down the engine is likely to further reduce your fuel burn, but your assumption that you are going to be making the same speed is likely incorrect. Your engine output is going to be lower on the torque curve while trying to pull the same amount of drag through the air. That equals lower speed. I don't know what the VW torque curve looks like, and that varies significantly from engine to engine depending on displacement, heads, valve size, camshaft and exhaust design. Once you reach the peak of the torque curve, more RPMS is just what you describe, flailing the prop generating noise and heat, but you are in the range of diminishing returns, not because the engine doesn't have more output, but because the additional output is lost to additional drag being generated as the speed goes up. As for running full throttle all the time, that works well with some types of induction, and not so well with others. The VWs have a long enough induction system that I would expect the mixture to the cylinders to be reasonably even with the carb at full throttle. Of course this depends on the induction system and carb as well. This is not necessarily true for all engines. The O-200 Continental is a good example. The Marvel carb tends to have an uneven mixture depending on the direction of the air flow from the spider mounted to the top of the carb. That makes for an uneven fuel distribution to the cylinders. Additionally, the MS carbs are designed to dump more fuel through the carb at full throttle to augment cylinder cooling and to prevent detonation. Pulling back the throttle enough to drop just 1/4" of Manifold pressure will make for a much smoother running engine with more even CHTs while yielding a significant drop in fuel burn with very little loss of power and speed. Getting the throttle plate in the way of the airflow tends to cause a bit of turbulence in the carb and makes it mix better. MS also sells a different main discharge nozzle (pepperbox nozzle) that is supposed to help the carb to do a better job of mixing the fuel/air better at full throttle. I didn't have mine modified with this nozzle, although it likely would have helped. While you can pull the mixture back to reduce fuel burn at full throttle, I found that pulling the throttle back a bit in conjunction with proper leaning produced a much smoother, leaner running and happier engine. I find the carbureted Lycomings to have much more even fuel distribution at full throttle, so they will run smoothly at full throttle setting, but will burn a lot of fuel unless you pull back the throttle enough to close off the economizer/enrichment valve. I would guess they would be happy running lean at altitude at full throttle. I haven't operated mine that way as it's just a waste of fuel to thrash like that in the SuperCub. I may try a trip or two like that in the RV to see what it will do for economy. Past experience tells me the injected Lycomings with constant speed props like to run as Mike describes; full throttle at altitude with a lean mixture and the prop loading them down to a good cruise RPM. As a general statement a longer prop does perform better for take off and climb. It doesn't necessarily work better for cruise. In the SuperCub world, the guys love the long props for getting off the ground as short as possible. And they work great for that application. But generally speaking, they usually give up 10+ mph of cruise speed to get that take off performance. There is always a trade off. On my KR, I sent one prop back to the manufacturer to have some pitch taken out of the prop. The manufacturer cut 2" off the diameter instead. While cutting down the diameter of the prop did produce the additional RPMs I wanted for cruise, it killed the take off performance of the plane. I could tell the difference just taxiing the plane on the ground before I ever flew it. The prop was effectively ruined for my application. As Mike correctly points out, when props are making a huge amount of noise like a T-6 on take off, the tips are either approaching or are supersonic. The drag on those tips is tremendous and generally a waste of power. Even before you get trans-sonic with the tips, you get into an area of compressibility where the prop tips are generating so much drag just to compress the air that they aren't making a lot of thrust. Additionally, the forward speed of the aircraft has to be added into the equation of the prop tip speed. But, without actually doing the math, I don't think there is much danger of getting into that range with the short props used on the KRs. Our planes would need much taller gear legs to run a prop long enough to generate any compressibility issues. Aircraft drag is another important part of the discussion regarding propellers. As a crude example, I'm now flying a SuperCub and a RV-6, both with the same aircraft weight and the same engine HP. Obviously, the performance of those two planes is vastly different and one wouldn't even consider using the same prop on the two of them. The difference is in the drag of the aircraft. Of course that is an extreme example, but the point is that comparing KR to KR is nearly impossible as unlike Cessnas, Pipers, or Mooneys (or even RVs), no two are alike. That's one of the really unique things I love about the KRs. During the 23 years I owned my KR, I spent a lot of energy working on HP enhancements, and more importantly, drag reduction. Over the years as I made little tweaks here and there, the plane continued to get faster and faster. When I sold the plane this summer, it was more than 25 kts faster than it was when it first flew back in 1997. The plane didn't really look a lot different but the prop that was on it when it was new would not have been the correct prop for it now. In fact, the prop I put on it in 2007 was quickly becoming too little prop. If the current owner ever sends that prop back to Prince for repair, he might want to have Prince add a bit of pitch into the prop. The apoplectic episode is over and the coffee cup is empty. Time to go fly somewhere. -Jeff Scott Arkansas Ozarks > Sent: Saturday, October 24, 2020 at 2:34 AM > From: "Mike Stirewalt via KRnet" <[email protected]> > To: [email protected] > Cc: [email protected] > Subject: KR> Propellers > > Sam said re his Sterba 56 x 56: > "(I could only get 3000 RPM with wide open throttle) > which saves a lot of wear on the engine internals . . ." > There ya' go gentlemen. The fastest prop of the three he tried was the 56 > x 56, a prop most KR people would almost unanimously presume would be too > much for our VW engines. With my 52 x 56 Sterba I get 3100+ WOT so this > experience of Sam's matches perfectly my own observations. Lower RPM's > mean lower engine temperatures which means less stress and wear and > liklihood of failure over time. Plus, less fuel burned means more range. > Sam's surprise at how little difference the coarse prop made on his > takeoff and climb also agrees with my experience. The coarse pitch really > comes into its own when you get up into the thinner air at altitude. Down > low, there's very little penalty. I've been claiming this for years so > its good to have Sam's confirmation. The prop may only turn 2800 static > but those coarse blades are chewing more air with each revolution than a > flatter-pitched prop and up high, in the thinner air, they get a better > bite that translates into speed and efficiency. > Jeff or any engineer will surely have apoplexy over what I'm about to try > and explain, but anyway . . . . no one has ever confused me with Stephen > Hawking. There's two ways of extracting power from a given engine. RPM or > torque (twisting force). Of course, any power setting is going to use the > two together but one can choose to emphasize one or the other through > one's choice of propeller. The coarse prop increases manifold pressure, > perhaps better thought of as cylinder pressure or, better yet, BMEP. > Through a series of gears and levers and explosions, this power is > channeled into a twisting force that bites into the air and shoves it > rearward. A flat pitched prop does the same thing without any particular > emphasis on BMEP. Flat props shove air rearward by flailing wildly, > generating heat and noise that certain types of pilots consider "sporty". > Aviation is for everybody so I'm not being judgmental here . . . it's > simply that there is something very _wrong_ with these people. Smiley > face. > Engines. They run better when they have no obstructions (like butterfly > valves) obstructing the passage of air through the engine. Running WOT, > the ideal configuration for any engine, a flat-pitched prop lets the > engine spin without restriction, resulting in lots of heat from all the > friction being generated inside the engine, and noise from those same > sources in addition to the prop tips exceeding the speed of sound > creating the screech of tortured air. The sound of a T-6 taking off is > the textbook example. Paul Lipps, the brilliant engineer behind Phantom's > many wins at Reno, used to say it hurt his ears since that sound is the > very essence of wasted power. To run a VW at such speeds for even > moderate periods will soon cause something to melt, bend or break. How > much better it is to make the engine work hard by forcing it to turn > coarsely-pitched blades. Flat pitched props allow the engine to fight > itself internally, a battle which has no winner. With the throttle fully > open - no artificial resistance impeding the passage of air through the > engine - the air resistance against the blades provides the natural > limitation to the engine's desire to fly apart. Without this constraint, > this limiting factor of the engine's ability to turn . . . there is no > end to the troublesome possibilities. In Sam's case, 3000 RPM is as fast > as it will go. In mine, it's 3100+. Assuming we both have the same > engine, his prop turning at 3000 WOT is producing the same thrust as my > engine turning 3100-3200 WOT. The difference is, I'm generating more > internal friction which must be overcome with fuel consumption. In both > cases, the engines are working at their maximum WOT capacity and > producing the same amount of thrust - but one is more efficient than the > other. In both cases the same mass of air is being moved. In an engine > running WOT with a prop insufficient to limit the RPM to appropriate > levels, a large percentage of power being generated is being wasted due > to engine parts furiously fighting each other and by the noise of a prop > creating drag with the tips in persistent stall. The resultant heat is > not only damaging various components, it's using the engine's own power > to overcome itself. Instead of generating thrust, it's generating heat. . > . a total waste. Wasted as well is the energy lost as the stalled prop > tips generate noisy turbulance instead of thrust. The slower turning > engine, its prop limited by the natural resistance of the medium within > which it moves, producing thrust and wasting little in the form of heat > and noise, is in harmony with the natural world. Jonathan Livingston > Seagull. > The emphasis with the slower turning engine, in my case, is inclined > toward torque. Cylinder pressures will be higher with this engine, but it > will run cooler. It is easy to imagine how much more sense it makes to > use a coarsely pitched prop, one using torque instead of RPM to move air > rearward. This engine runs cooler, lasts longer, uses less fuel for the > same amount of work and is utilzing an all-around better method of > extracting power. > I've read that prop efficiency, for a given power source, can be better > gained by going to a greater diameter prop than by increasing pitch or > adding blades. If I were to go to a more efficient prop than my 52 x 56, > I would choose a 54 x 56 rather than going to a 52 x 58. I'm estimating a > 54 x 56 would give me a WOT RPM of 3000+, which would be nice. Steve > thought the ideal RPM for the VW engine in aircraft application is 2900. > I suppose if I went to a 56 x 56 I would get full WOT power turning only > 2900+ once leveled at cruise up where I normally fly. Theoretically, if > the throttle is fully open and the engine isn't being bogged down by > having too much prop - that is, if it isn't "lugging", I should be > getting the same thrust and thus the same speed as I do with my current > Sterba, but using less fuel. Currently my average cross-country speed at > altitude is, averaging numerous trips in my log, is 149 MPH when at my > typical cruising altitudes of 11K to 14K. At 13.5 I'm using 2.5 - 3 GPH > and pulling 45 to 50% power. If I could get the same speed while turning > 2900+ instead of 3100+, I would gain another hour (or more) of cruise and > a minimum of 150 additional miles of range. Unless I decide to throw some > money at this theoretical efficiency boost, I'm not likely to try it . . > . but it would be an interesting experiment. More than just an experiment > though, doing this might give me the margin I need to make it from > Bellingham to Ketchikan non-stop, thus avoiding a stop in Canada. A trip > to Merrill Field in Anchorage has been on my bucket list for years and > with my current range that segment from Bellingham to Ketchikan is doable > only under ideal conditions and in that stretch of territory conditions > are very seldom ideal or predictable. I could really use another 150 > miles of range for that trip. > The point I'm taking from Sam's prop reports is one I've been making all > along. The rabbit might be sporty, but it's the turtle that gets there. > Mike > KSEE > _______________________________________________ > Search the KRnet Archives at > https://www.mail-archive.com/[email protected]/. > Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options. > To UNsubscribe from KRnet, send a message to [email protected] > _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/[email protected]/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to [email protected]
