Propellor thoughts

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Dust, Apr 12, 2004.

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  1. Apr 12, 2004 #1

    Dust

    Dust

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    ok, in general terms, prop tip speeds are limited to 80 or 90% of the speed of sound.

    I know the plane will indicate 210 MPH at 6000 feet, been there, done that. I know what in theory an indicated 200 will true out to at 25000, all great numbers.

    I am planning at cruising at full cruise power at 25000 feet. No actually, i am planning to turbo to full power at 25000 and will probably have full cruise power to 30000.

    So here is my question, as the air thins the speed of sound goes down, so at 25 to 30 thousand feet the speed of sound is less, does this mean i will no longer be able to run 2800 RPM full climb power or 2400 RPM, full cruise power at those altitudes???????

    Can you compensate for this by adding pitch to the prop and slowing engine RPM?

    Or am i all wet and should start building a snow mobile, for those of you who don't know, a vehical most think we need in SE michigan 10 months a year and which i have never been on.

    I could make a cozy snow mobile that really flies.

    enjoy the build

    dust
     
  2. Apr 12, 2004 #2

    orion

    orion

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    Turbo to full cruise at 25,000 ft? Are you sure? I know that in theory this can be done but usually it requires a pretty good quality and/or complex turbo setup. Personally, I'd be a bit more pessimistic before depending on those numbers.

    Side note - for those of us with acrophobia, I would prefer to leave out one zero out of those altitudes. Personally, I tend to prefer "fly low and fast" rather than high. ;)

    However, on to the subject at hand. Your reasoning is correct. The drop in sound speed and the cruise speed is one reason that most of your high altitude performers use large diameter props turning very slow. The output speed of most turboprops is between 1,800 and about 2,100 rpm. Larger aircraft with larger props of course turn even slower.

    Your power is a function of two things - manifold pressure and rpm. Yes, you can add pitch and slow the rpm down but at the lower rpm you will not make as much power so if this is the route you wish to go, then you'll have to trade off a bit between the two. If you have it available, you can add more boost to offset the drop in rpm in order to maintain power however then you may not have a normalizing system and you'll have to be a bit more careful in applying throttle and boost at lower altitudes.
     
  3. Apr 12, 2004 #3

    Dust

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    Orion said

    "Turbo to full cruise at 25,000 ft? Are you sure? I know that in theory this can be done but usually it requires a pretty good quality and/or complex turbo setup."

    Well it really isn't that much of a stretch, this 1969 tsio360a is currently turboed to full power to 18000, I'm just adding 7000 capacity.

    The reason i was looking at the exhaust material is for the good quality.

    The controll area is next, i am planning to pull off all 1969 technology and try to replace it with 1999 technology.

    I think that i will be working closer with MT propellor than i thought.

    Well, the engine is rated to 31.5 manifold preasure, does this mean something like

    2100 rpm
    28 manifold preasure

    instead of 24 mp and 2400 rpm

    for full cruise at 25000 or 30000?

    enjoy the build, just trying to wrap my head around lots of new concepts

    dust
     
  4. Apr 12, 2004 #4

    orion

    orion

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    I'm not sure how the relationship forms, nor even if it is linear (although I doubt it).

    I have however had some experience with the newer technology turbos. Several years back we turbo'd an IO540 for a Glasair III. This was not a certifed engine so we decided to see if we could do better than the rather sizeable Rayjay that was recommended. At the time we were dealing with a company out of San Jose, CA., called Turbo Power. The owner configured a nice, small (about half the size of the Rayjay)turbo with a built-in wastegate, which actually performed at a higher efficiency than the aircraft product. The turbo incorporated not only an oil jaket for the bearing but also an outer jacket that acted to cool the bearing oil gallery. The cooling could be done either through water (for a water cooled engine) or oil.

    In addition to providng us with the turbo design and hardware, he also provided us with all the performance maps matched specifically to our engine/turbo combination.

    Unfortunately I wasn't available to see the project through completion but from all accounts, the performance was up to spec.

    In short, the newer technology products can deliver some pretty impressive performance so I hope all this works out for you.

    As far as the relationships of boost, rpm and performance are concerned, hopefully someone here can chime in with a more specific and accurte answer than I can provide you with.
     
  5. Apr 13, 2004 #5

    StRaNgEdAyS

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    The realm of turbocharging can be a tricky one. :ermm:
    With the right engine management computer you can however configure it to switch programs according to your altitude with the push of a button. Some of the better ones will give you four (or maybe more now, it's been a while :ponder: ) different programmable map configurations, which should be ample for you needs. The only thing you will need is to have a tech (unless you are familiar with the software and the mapping procedure :ermm: ) with a laptop to come up with you to help fine tune it at the different altitude ranges for you. (a hint, before you do it, negotiate with him/her about it, some places here in Oz will cut off the interface plug once they have configured it, because they retain I.P. rights over the mapping) :angry:
    Orion is correct when he says you could do better than the RayJay turbo. (what turbo are you running?) I initially had a RayJay F-40 on my 1100 fuel injected Suzuki, but I switched to a T-04, retaining the external wastegate (as the T-04 has no internal one) with much better results. The Rayjay uses a oils suspension bearing, to provide both cooling and friction reduction, and for my money, it just spooled down too quickly, which won't be a problem in your application, but I also feel it has a tendancy to wear too quickly. The ball bearing turbo's have much less friction in the bearing and thus tend to have a longer life span.
    I also had some trouble with the RayJay's with the spring clamp holding the compressor together coming undone at odd intervals, not such a drama on the bike, as it just sat there and stopped the turbo running, but on a plane I'd really not be wanting it to come adfrift! :eek:
     
    Last edited: Apr 13, 2004
  6. Apr 13, 2004 #6

    Dust

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    Welllllll, this aviation stuff is hard to do, as all of it seems to be for street/industrial applications.

    I am slowly wrapping my head around it.

    I fortunatly do have engine specialists available, you know the detroit thing, and this is really interesting stuff.

    Like on designing an intake manifold, on a car you have to get it good over a whole range of rpm's, on a plane you are looking at a very small range, splimplyfying things to a great degre.

    enjoy the build

    dust
     
  7. Apr 14, 2004 #7

    Dust

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    67 diameter inches
    105.2433539 circum inches
    8.7702794913 circum feet
    0.00166103778 circum miles

    2800 RPM
    168000 RPH

    propellor tip speed
    279.05434745 mph

    Do I have to add the speed of the plane going through the air?

    Looks to me as though the prop size should be bigger for better effeciency

    enjoy the numbers

    dust
     
  8. Apr 14, 2004 #8

    StRaNgEdAyS

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    hmmmm, this could get tricky.
    Your plane is a pusher no?, the problem with prop size is that if you go too big, you will run the risk of the prop hitting the ground during rotation on take off or flare out on landing.
     
  9. Apr 15, 2004 #9

    orion

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    To answer the above question, yes, you need to also include the tip speed of the prop into the equation. Basically, take the square root of the quantity: airplane speed squared plus the rotational tip speed squared. The resultant number should be no more than about 850 feet per second.

    As a general statement, yes, a larger diameter prop turning slower tends to be more efficient than a smaller diameter prop turning faster. However, this is not a rule set in stone. If you design for it, the smaller prop can be more efficient than a bigger one, however this depends on the performance range you are proposing to operate in or are proposing to optimize the prop for.
     
  10. Apr 15, 2004 #10

    Largeprime

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    from here
    http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
    mach varies from 761mph at sea level to 678.2 at 30KFt. Which is 90% of sea level.

    to the point
    30K mach is 994.7 ft/sec
    85% of 994.7 = 845 ft/sec (i guess we know where orion's number came from)

    to answer dust
    the root of 279^2+300??^2 = 600Ft/sec

    Somebody check my math but looks like a non event

    orion
    is it your assertion that a fully optimal small prop is more efficient than a fully optimal large prop? I have only read the oppisite
    I understood that diamater gave and efficency gain as a cube? Is this not true?
     
  11. Apr 15, 2004 #11

    orion

    orion

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    As a general rule, you are right. If for instance you fly with an 80" prop and for some reason want to go to a 40" prop, there is really no way to equate efficiency.

    However, if the standard prop available is around 80", and you need something around say 72", then it is possible to design a prop of the smaller diameter that is for all practical purposes as good as the larger one. Also, since you would be designing a prop better matched to your airframe and performance envelope (as opposed to an off the shelf one), there is a good chance that it could be made better than the larger unit.

    Several years ago we designed a prop for a customer who was flying a very modified SuperCub. The airplane was used for all types of flying normally but he also used it for short field competitions. One of his modifications was an IO-360 that had Nitrous injection and thus was capable of putting out over 300 hp. His competition prop was a 90" monster that at full throttle static had the outer 6" of the blades running sonic. Despite that though, it gave him plenty of static thrust (almost 1,100 pounds). The prop however was too big and way too heavy for what he was trying to achieve so he asked us to design a new, lighter prop with the same or better low speed characteristics.

    The new prop we designed was about 75" in diameter, with about a 6% improvement in the static performance. The new blades were fit into a Hartzell constant speed hub, reworked to be fixed pitch but ground adjustable (getting rid of the hydraulic stuff on the inside reduced the weight significantly). By being ground adjustable, he could now use the prop for competitions and for his every day flying.

    For those curious, he was able to take-off in less than 10 feet. But because the SuperCub did not have sufficient tail authority for flare (the overall flight surface planforms had to be kept stock to meet the rules) due to the addition of a large set of flaps, it took him just over 60 feet to land. That was one of the reasons he wanted a lighter prop - to move the CG a bit aft to aid the flare.

    When designing a new prop, the important question to ask is "What part of the fight envelope are you looking for more efficiency in?" As in most engineering disciplines, the design of a prop is a compromise. If you want high speed efficiency, you're going to give up some performance on the low speed end of the envelope, and of course the opposite is true also. The situation is aided by constant speed units but even then the design of the sections and the twist distribution is really optimized for only a narrow range of performance.
     
  12. Apr 16, 2004 #12

    Dust

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    So when you do this computation, do you use IAS or TAS.

    My guess is that you use indicated air speed, but totally a guess as the logic i just backspaced through didn't read as though it made sense.

    OK, OK Calibrated airspeed. heh heh heh

    enjoy the build

    dust
     
  13. Apr 16, 2004 #13

    orion

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    All calculations for airplane performance and behavior are based on indicated air speed, since that is what the airpalne actually sees. Airplane flight is based on "q", or dynamic pressure. This is defined as:

    q = (.5)(ro)[(V)squared]

    where ro is the air mass density and V is the airspeed. The airplane itself does not know how to calculate things like calibrated or true air speed, all it cares about is the "wind in its face", or the indicated speed.

    This is why it is important at times to really pay attention to the airspeed indicator (uncorrected), especially at times of high density altitude, since the airspeed indicator is the only true gauge that tells you how the airplane will behave.
     
  14. Apr 16, 2004 #14

    Dust

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    Gotch yah, gotch yah, so then for these calculations, calibrated would be the correct one as it just changes indicated by the amount that your indicator is computing the speed wrong by.

    But that is a 1 or two percent error, I made a 100 percent error above. I used PiR instead of PiD or 2PiR, heh heh heh

    enjoy the build

    dust
     
  15. Apr 16, 2004 #15

    orion

    orion

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    Technically, calibrated would be correct however, before you know the effects of the installation (such as during the design process) I generally use the term "indicated" since that is the wind in the airplane's face. Also, during flight test you tend to install a very sensitive and accurate pitot, which has the capability to change attitude apart from the motion of the airplane. In that case, the IAS and the CAS are, for all practical purposes, the same.
     
  16. Apr 17, 2004 #16

    Largeprime

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    So given any fully optimal prop, the bigger prop wins, Right? All other thing being equal and with the blessing of the tooth farie and santa.

    But given a typical lycon engine (with a rpm of , what, 2400-2900?) is there a size that is a good foundation for an optimal prop? or is it a case of essentialy no difference (+- very small) between a range of diamaters (like +-2"'s) no matter how optimized they are.

    Or can a fully optimal 72" kick a fully optimal 66" butt?

    Or am I off in left field?
     
  17. Apr 17, 2004 #17

    orion

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    As a general rule, applied to basic general aviation speed envelopes, yes, the larger optimized prop will be better than a smaller one. There are always exceptions but if we limit our discussion to airplanes of moderate performance, the rule of thumb can apply.

    Given that, selecting a prop for your particular airplane will be a balancing point between the optimum performance and geometric requirements. The latter include things such as ground clearance, weight and balance, etc.

    However, given the range of standard props (all within say about 4" dia.), the differences in propulsive efficiency will be so small that you most likely will be hard pressed to detect a significant difference. This is especially true if you are looking at selecting a constant speed unit.

    For fixed pitch props though, I have seen some measurable difereces in performance, even for props that were theoretically designed for the same application and envelope. Here the differences will be in the design details and to the approach the designer/fabricator uses in the construction of the prop. As a general rule of thumb though, fixed pitch metal props will perform better than wood ones.

    As far as "kicking butt" is concerned, we're generally talking relatively small differences. They may be measurable but nothing that I would classify as overly dramatic.

    Another point, although somewhat a secondary one, is that with the smaller prop you can turn it faster at cruise than the bigger one, and still be below critical tip speed. The higher rpm will generate more horsepower and so your end performance may be in line with the larger prop spinning slower, although you might be consuming a hair more gas.

    In short, it's probably not so critical a choice for you to lose sleep over.
     
  18. Apr 19, 2004 #18

    Dust

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    it is really fun being a newby. I get to ask stupid questions and get great answers.

    When doing a project, thoughts creep in. Ohhh Ohhh the speed of sound goes down as you go up so maybe i have a problem.

    Well, i assumed it went down, like the diferencial between TAS and IAS as you go up. I mean the same percentage change.

    Well, it doesn't and that is what i should have looked up first.

    Thanks for all of the info, just sitting trying to think up more stupid newby questions, just can't help it , it is part of the build process

    enjoiy the build

    dust
     
  19. Dec 21, 2004 #19

    dustind

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    Last edited: Dec 21, 2004
  20. Dec 21, 2004 #20

    Craig

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    Dust -
    Tip speed is distance and time - the distance is the circumference of the circle, or pi x diameter.
    pi x radius squared gives area.
     

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