Can we have an open and honest discussion about RPMs?

Discussion in 'Hangar Talk' started by SixPapaCharlie, Sep 29, 2018.

  1. tspear

    tspear Pattern Altitude PoA Supporter

    Joined:
    Dec 10, 2010
    Messages:
    1,672
    Display Name:

    Display name:
    Timothy
    Oops. Autocorrect

    Sent from my SM-J737T using Tapatalk
     
  2. FormerHangie

    FormerHangie Pattern Altitude

    Joined:
    Oct 28, 2013
    Messages:
    2,339
    Location:
    Roswell, GA
    Display Name:

    Display name:
    FormerHangie
    I used to work for a Mazda shop back in the early 80's. At the time there were still a decent number of RX-3s and RX-4s on the road, and some would occasionally come in with a can of starting fluid sitting in the front seat. As soon as you saw that, you could pretty much diagnose the problem as low compression, and the fix was a rebuilt engine from Mazda Technical Center, which was not something the customer liked hearing. What we would offer them is a compression test for a half hour's labor to confirm this. For a rotary engine, the compression tester was a recording device, way back then it wrote the results on a piece of paper, since we needed to see what the compresson was on each of the three lobes of the rotor. It never failed to confirm the initial diagnosis. One of the guys who was very experienced with rotaries told me it was usually one of the large O rings on the sides of the rotor that failed rather than the apex seals. There are some corner seals that were troublesome as well.

    The engines in the RX-7 never gave us much trouble, in the seven years I was there we replaced one, and that was for high oil consumption. One thing that we did see that would disqualify the rotary for aviation or marine use was something we called "body shop syndrome". For those cars that were street driven and then in an accident, if they sat for a few weeks without being run, the engine would not turn over. If someone did something to try to force them to start, then one or more of the apex seals would break as it had stuck to the rotor housing, probably from carbon buildup. The way to get the rotor unstuck was to turn it in the reverse direction from its normal rotation. I could see this happening to a infrequently flown airplane or pleasure boat.
     
  3. Dan Thomas

    Dan Thomas En-Route

    Joined:
    Jun 16, 2008
    Messages:
    4,983
    Display Name:

    Display name:
    Dan Thomas
    The Mazda rotary has flown in a number of homebuilts. One of the issues was the redrive, and another was cooling. Generating a lot of power in a small package means a lot of waste heat as well in that small package, and getting the heat out before it warped the engine's major castings was a problem.
     
  4. Brad Smith

    Brad Smith Pre-takeoff checklist

    Joined:
    Nov 26, 2017
    Messages:
    433
    Location:
    Klamth Fall, OR
    Display Name:

    Display name:
    Brad Smith
    Airplanes are designed as a total package. Keeping things simple when designing airplanes keeps costs down and improves reliability. Adding gearing to an engine creates a bucketload of extra parts, increases complexity, cost, weight and does nothing for reliability. In most piston engine aircraft you can dispense with the gearbox and keep your goal of keeping things simple which results in many of the benefits that makes both the manufacturer and buyer happy. Some larger piston engines do incorporate gearboxes but the balance is tipped when efficiency vs. initial cost is considered. Turboprops and helicopters wouldn't even be around if it wasn't for gear reduction as they turn in the neiborhood 30,000 RPM, well out of the range of props and rotors. For them a gear box is a necessary evil (ask any helo pilot how he feels when he sees a "chip light".)

    In the days when Lycoming, Continental and Franklin where designed (70 yrs. ago?) the theory was to keep things simple and reliable and gearing an engine was usually not chosen when all things were considered. These days, high spinning ROTAX engines with reduction drives are available but suited more toward the LSA market. To keep cost in line, the LSA builders universally opt for the ROTAX. Your still going to need something a little stronger to cart your Cherokee, Bonanza, Baron etc. around. The mainstay of the GA market will continue to be large cube, non-geared, slow turning engines that are matched with large, efficient props that get the job done with a minimum of fuss. The market is the one that ultimately decides what works and so far simplicity has been the answer.
     
  5. Dan Thomas

    Dan Thomas En-Route

    Joined:
    Jun 16, 2008
    Messages:
    4,983
    Display Name:

    Display name:
    Dan Thomas
    Yup. The Cessna 421 had geared engines. Ask any owner about issues with the operation, and about overhaul costs. A 421 with runout engines is worth very little.
     
  6. mwagg737

    mwagg737 Pre-Flight

    Joined:
    Aug 13, 2016
    Messages:
    81
    Display Name:

    Display name:
    Mooney man
    This^^^^^^^

    Think tuner car vs muscle car.

    Sent from my Pixel 2 XL using Tapatalk
     
  7. jesse

    jesse Administrator Management Council Member

    Joined:
    Oct 2, 2005
    Messages:
    15,651
    Location:
    Lincoln, NE
    Display Name:

    Display name:
    Jesse
    Any twin with runout engines is worth very little. Geared or otherwise.
     
  8. SixPapaCharlie

    SixPapaCharlie May the force be with you

    Joined:
    Aug 8, 2013
    Messages:
    13,551
    Location:
    North Texas
    Display Name:

    Display name:
    6PC
    This has been a highly informative thread.
    I learned quite alot.

    Thank you POA.
     
  9. eman1200

    eman1200 Touchdown! Greaser! PoA Supporter

    Joined:
    Mar 10, 2013
    Messages:
    10,611
    Display Name:

    Display name:
    eman1200
    I’m here for you, bud.
     
  10. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

    Joined:
    Jun 7, 2008
    Messages:
    10,804
    Location:
    DXO124009
    Display Name:

    Display name:
    Light and Sporty Guy
    Can we get back to bacon now?

    [​IMG]
     
    Stingray Don and Checkout_my_Six like this.
  11. Omalley1537

    Omalley1537 Line Up and Wait

    Joined:
    Mar 13, 2015
    Messages:
    663
    Display Name:

    Display name:
    Sean
    Haha, that line was the first thing that popped in my head when I read this title...
     
  12. Omalley1537

    Omalley1537 Line Up and Wait

    Joined:
    Mar 13, 2015
    Messages:
    663
    Display Name:

    Display name:
    Sean
    The Merlins in modified P-51’s like Strega, Voodoo, and Dago Red (RIP...sniffle) are good examples. Prop RPM is still limited by tip speed but the engines are making 3400 HP or more, compared to mid-teens for a stock Merlin. 120-plus inches MP vs 70. More pitch, etc make use of the additional power to generate more forward motion.
     
    Cooter likes this.
  13. FormerHangie

    FormerHangie Pattern Altitude

    Joined:
    Oct 28, 2013
    Messages:
    2,339
    Location:
    Roswell, GA
    Display Name:

    Display name:
    FormerHangie
    About these engine things...

    The engines in our airplanes, and most of our cars are four stroke spark ignition engines. They pull in an air and fuel mixture on the first (intake) stroke, then compress that mixture on the second (compression) stroke. Just before the engine reaches the top of the compression stroke, the ignition system provides a spark, and the mixture starts to burn, which causes it to expand, for what we call the power stroke. This is what we are after here, this is what provides the useful work that the engine is going to do for us. Finally, the combustion gases exit the cylinder during the final (exhaust) stroke. There's an animated diagram here, if you're not familiar with how an engine works. Back when I was in middle school they taught this sort of stuff in science class, I don't think they still do. Anyway, that's the basics.

    Probably the best way to think about what an internal combustion is doing is to think of it as a pump. If you want to fly faster or haul a bigger load, you need a bigger pump. The more air you can get into the engine, the more fuel you can burn, which provides more power, which will drive a bigger propeller, which produces more thrust. For those of us who want to fly, thrust is what's going to get us off the ground. I'm going to start with the Continental O-200, which has four cylinders, arranged in a horizontally opposed order, and has a total displacement of 200 cubic inches. When we say displacement, that is the amount of air that the engine would move in one complete cycle of two crankshaft revolutions, assuming it is at full throttle and the process is 100 percent efficient. (Remember, a four stroke engine needs two revolutions to complete its entire cycle.) It has a cylinder bore of 4.06 inches and a crankshaft stroke of 3.88 inches. At 2750 RPM, it produces 100 hp, and has a dry weight of 224 lbs. There is a lighter weight version that is closer to 200 lbs but I don't know of any uses for it.

    The Rotax 912 S and ULS are also have four cylinders in an horizontally opposed arrangement, but only has 82.6 cubic inches of displacement from its 3.3 inch bore and 2.4 inch stroke. it's also considerably lighter at 140 lbs. Now you'd look at these two and think that the 0-200 would produce way more power than would the 912, and it would if they both ran at the same engine speed. The Rotax turns 5800 rpm. Remember the pump analogy? We can do a quick calculation to see how much air each engine would consume at maximum power. Since it takes two revolutions for a four stroke engine to fill all of its cylinders one time, the formula is RPM/2 * displacement. So,for the O-200: (2750/2) * 200 = 275,000 cubic inches, and for the 912: (5800/2) * 82.6 = 239,400 cubic inches. This is the theoretical amount of air that each engine could consume in one minute at full throttle and maximum power RPM.

    The Rotax is still about 12 percent behind the Continental, according to our theoretical air flow calculation. It turns out that with careful design, it is possible to get more air into an engine than the theoretical number would suggest. For an engine without a supercharger or turbocharger, it is possible to get as much as 130% of that theoretical value into the engine. For more on that topic there's a brief introduction to the concepts in Wikipedia's article on volumetric efficiency. The Rotax, being a newer design, is apparently a more efficient engine. It's also much more compact and about 90 pounds lighter than is the 0-200, and has a couple of complications. Propellers don't do well at 5800 rpm, so the Rotax has a propeller speed reduction unit, a gearbox that reduces the propeller speed so something more reasonable. Also, since so many more combustion events happen per minute in a Rotax, the cylinder heads are liquid cooled, which is an added complication. (The cylinders are air cooled.) So now you've added a water pump and coolant. Even after adding these things, the Rotax installation is still somewhere around 80 pounds lighter than is a standard O-200. That's why you see the Rotax in so many LSA's, and why the Skycatcher has such a low useful load.

    Back to the pump analogy for a minute. I suggested earlier that if you wanted more power, you needed a bigger pump, which would mean more displacement (bigger engine). We just saw that running the pump at a faster speed works, too. I'm going to add a third option, which is to force more air into the engine. You can add an air pump to your engine. During World War II, designers added superchargers, which are engine driven air pumps, to push more air into their engines. The supercharger did consume some power, but that was more than offset by the added power from the additional available air. Nowadays there are lots of cars, and a few airplanes, that use turbochargers to boost their maximum power. There are a number of Lycoming TIO-540A engines that use turbochargers to boost the manifold pressure to 40 or 42 inches for takeoff. There are also airplanes that use a turbocharger to offset the loss of power that comes with altitude, which is not what we're talking about here.

    Power comes from burning fuel, which requires air. You can make the engine bigger, turn it faster, or add forced induction to get more power. Each has its pluses and minuses. I got to fly in front of a Rotax recently, and I was very impressed. It's also nice that it can run on 91 octane E10 mogas. I have a reasonable number of hours flying behind an 0-235 and an O-320, and I preferred the Rotax. At cruise the propeller is turning fairly slowly and it's quiet. YMMV, but if you get a chance to fly in something Rotax powered, by all means take it.
     
    AuntPeggy, FastEddieB and nauga like this.
  14. vman

    vman Pre-takeoff checklist

    Joined:
    Sep 16, 2018
    Messages:
    306
    Display Name:

    Display name:
    vman
  15. bflynn

    bflynn En-Route

    Joined:
    Apr 24, 2012
    Messages:
    4,487
    Location:
    Fuquay Varina, NC
    Display Name:

    Display name:
    Brian Flynn
    It would. Or you can expand RPMs again and say "Revolutions per Minutes".
     
    eman1200 likes this.
  16. SixPapaCharlie

    SixPapaCharlie May the force be with you

    Joined:
    Aug 8, 2013
    Messages:
    13,551
    Location:
    North Texas
    Display Name:

    Display name:
    6PC
    Yes because I rarely fly for just 1 minute.
     
    bflynn likes this.
  17. SixPapaCharlie

    SixPapaCharlie May the force be with you

    Joined:
    Aug 8, 2013
    Messages:
    13,551
    Location:
    North Texas
    Display Name:

    Display name:
    6PC
    I also motion that we change "I LOL'd" to "I L'dOL"
     
    bflynn likes this.
  18. jsstevens

    jsstevens En-Route PoA Supporter

    Joined:
    May 18, 2007
    Messages:
    4,388
    Display Name:

    Display name:
    jsstevens
    Second!
     
    Checkout_my_Six likes this.
  19. eman1200

    eman1200 Touchdown! Greaser! PoA Supporter

    Joined:
    Mar 10, 2013
    Messages:
    10,611
    Display Name:

    Display name:
    eman1200
    now that's just silly.
     
  20. bnt83

    bnt83 Final Approach

    Joined:
    Dec 31, 2012
    Messages:
    8,128
    Location:
    Lincoln NE
    Display Name:

    Display name:
    Brian
    Gotta be like a Harley owner, never talk about HP, always talk displacement. So if your talking to a tuner with a 600 HP 350 chevy be sure and tell him you've got a 320 cubic inch engine in your airplane and your Dad's is 550 cubes. Ignore the fact that the engines in our airplanes turn so damn slow they don't make big power. They'll just assume the 550 makes around 900 HP and not 300.
     
    mwagg737 likes this.
  21. Dan Thomas

    Dan Thomas En-Route

    Joined:
    Jun 16, 2008
    Messages:
    4,983
    Display Name:

    Display name:
    Dan Thomas
    They don't, and it sure shows. Just try finding people under 40 or so that know anything about what goes on under the hood.

    Or that know that if we all buy electric cars we're going to need a LOT more powerplants. Big powerplants. WInd and solar will never do it.
     
    Omalley1537, bnt83 and FormerHangie like this.
  22. PaulS

    PaulS Final Approach

    Joined:
    May 29, 2007
    Messages:
    7,571
    Display Name:

    Display name:
    PaulS
    I Laugh Out Louded….. good point.
     
  23. SoCal RV Flyer

    SoCal RV Flyer Pattern Altitude

    Joined:
    Apr 27, 2016
    Messages:
    2,147
    Display Name:

    Display name:
    SoCal RV Flyer
    Tell 'em your engine will happily run at 75 percent power for 2000 hours! At, just for example, 150 KTAS, which works out to nearly 350,000 statute miles, assuming roughly equal head- and tailwinds.

    Horses for courses...
     
    Checkout_my_Six likes this.
  24. Omalley1537

    Omalley1537 Line Up and Wait

    Joined:
    Mar 13, 2015
    Messages:
    663
    Display Name:

    Display name:
    Sean
    This goes for all hands-on trades. I have talked to several folks in a trade who have said that there are very few younger folks interested in taking over when their generation retires. This isn’t helped by the fact that we’ve become a throwaway society where things aren’t designed to last or be repaired.
     
  25. Checkout_my_Six

    Checkout_my_Six Final Approach

    Joined:
    Mar 4, 2007
    Messages:
    8,596
    Location:
    Maryland
    Display Name:

    Display name:
    Check_my_Six
    But....you could if you wanted to.o_O
     
  26. Cricket1

    Cricket1 Pre-Flight

    Joined:
    May 19, 2017
    Messages:
    44
    Location:
    Ohio
    Display Name:

    Display name:
    Brian
    I had always wondered about the same thing, so awhile back looked it up, and there is a simple calculation to figure out torque.

    hp x 5,252 / RPM = torque

    So, Lycoming 360 makes 180 hp at 2,700 rpm

    180 x 5,252 / 2,700 = 350 ft lbs of torque.

    or my favorite: IO520

    300 x 5,252 / 2,700 = 583 ft lbs! That's a lot of twist :)
     
    SoCal RV Flyer likes this.
  27. Dan Thomas

    Dan Thomas En-Route

    Joined:
    Jun 16, 2008
    Messages:
    4,983
    Display Name:

    Display name:
    Dan Thomas
    Yup, and all of it goes into that propeller. Further, it's not smooth when the prop gets it. Every time a cylinder fires there's a big push, momentarily and considerably more than 583 foot-pounds.
     
  28. Crashnburn

    Crashnburn Pre-takeoff checklist

    Joined:
    Aug 8, 2018
    Messages:
    177
    Display Name:

    Display name:
    Crashnburn
    Actually, VW engines were originally aircraft engines.
     
  29. Kenny Phillips

    Kenny Phillips Cleared for Takeoff

    Joined:
    Jul 29, 2018
    Messages:
    1,029
    Display Name:

    Display name:
    Kenny Phillips
    We sold those "back in the day" for motorcycles. Are you familiar with the eccentric who invented the SuperTrapp? For decades he's been chasing a dream, that now seems to have been eclipsed by giant drones: https://www.moller.com/
     
  30. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

    Joined:
    Jun 7, 2008
    Messages:
    10,804
    Location:
    DXO124009
    Display Name:

    Display name:
    Light and Sporty Guy
    I'd say he found the dream - On average he rolled in $2.5 million per year for the last 40 years and never had to deliver an actual product.
     
  31. pigpenracing

    pigpenracing Pattern Altitude

    Joined:
    Feb 8, 2012
    Messages:
    1,991
    Location:
    Brenham Tx
    Display Name:

    Display name:
    pigpen
    Why does a F1 engine turn 20,000 RPM... Because it wouldn't make 10 horsepower at 2700.
    My 447 2 stroke Rotax turns 6800 because it needs rpm to make power.
     
  32. dell30rb

    dell30rb Final Approach

    Joined:
    May 18, 2011
    Messages:
    7,034
    Location:
    Raleigh NC
    Display Name:

    Display name:
    Ren
    My single cylinder YZ250F had a redline of 13,500 which I hit regularly. I have no idea how many hours I put on that thing but I ran it almost every weekend for 3-4 years with no maintenance except for regular oil/clutch changes and it never showed any signs of internal engine wear.