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Thread: Propeller design for greater efficiency and much higher RPM's

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    Propeller design for greater efficiency and much higher RPM's

    This idea came to me for how to make an auto engine conversion work without the normal complications and expense. I'm not an engineer, I'm a new pilot who from the beginning has been fascinated with how antiquated and expensive the current options for engines are in the general aviation world. So, I started looking at auto engines and the biggest issue is that they produce their max HP and higher rpms thereby requiring a complicated/expensive PSRU. So, I started thinking about what if one made a propeller that could be fitted directly to a Honda Fit engine for example that would spin at say 5000 rpms. I realize the problem with doing this is that you would then have to make the prop length short enough that it avoided the speed of sound issue and if you made it short enough it's efficiency would be greatly reduced. And the reason for this is that the air is relatively thin and not enough of it would be going across the smaller area of prop to produce the same thrust as a bigger prop.

    But what if you added elongated fins protruding from the nose of the prop. Such fins when spin with the prop and would create a vacuum that would suck air from the area outside the prop to inside and push more air across the prop itself. I don't know if I explained that very well, I will try and draw a diagram and post it here. But has this idea been discussed before? What would the limitations be of this, and would it work?
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    Registered User Jay Kempf's Avatar
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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by tilopa View Post
    This idea came to me for how to make an auto engine conversion work without the normal complications and expense. I'm not an engineer, I'm a new pilot who from the beginning has been fascinated with how antiquated and expensive the current options for engines are in the general aviation world. So, I started looking at auto engines and the biggest issue is that they produce their max HP and higher rpms thereby requiring a complicated/expensive PSRU. So, I started thinking about what if one made a propeller that could be fitted directly to a Honda Fit engine for example that would spin at say 5000 rpms. I realize the problem with doing this is that you would then have to make the prop length short enough that it avoided the speed of sound issue and if you made it short enough it's efficiency would be greatly reduced. And the reason for this is that the air is relatively thin and not enough of it would be going across the smaller area of prop to produce the same thrust as a bigger prop.

    But what if you added elongated fins protruding from the nose of the prop. Such fins when spin with the prop and would create a vacuum that would suck air from the area outside the prop to inside and push more air across the prop itself. I don't know if I explained that very well, I will try and draw a diagram and post it here. But has this idea been discussed before? What would the limitations be of this, and would it work?
    Many of us have thought along the same lines. There are threads on small block Chevy conversions, small 2 cycle, motorcycle engines, etc... If you use the search function you should find a ton of info. In general the supersonic tip problem is a show stopper limiting prop RPM.
    Jay K.

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    Registered User Autodidact's Avatar
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    Re: Propeller design for greater efficiency and much higher RPM's

    I would think that just about everything has been tried and tested when it comes to props. One thing that controls prop design is materials. When metal props became available, they could be made with thinner blades so that the critical Mach number was higher and they could be turned faster for the same diameter. There are some new ideas for racing props with highly tapered blades from about 1/3rd span to the tip. They look like elongated spades from a playing card. There are some propeller specialists on the forum here who would know more about it.
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    Registered User Dan Thomas's Avatar
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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by Autodidact View Post
    There are some new ideas for racing props with highly tapered blades from about 1/3rd span to the tip. They look like elongated spades from a playing card.
    The Ellipse propeller:

    Propeller design for greater efficiency and much higher RPM's-rearcover.jpg

    In this link EAA - EAA Experimenter - ELIPPSE Propeller they talk about 3150 RPM, far short of the 5000 RPM (or much more) that an auto engine is rated at these days. They reduce the tip drag by making it so narrow but the mach limits still apply.

    Propellers are a thorny problem. At low RPM a large prop can generate large thrust but without variable pitch it won't pull an airplane very fast. At low RPM a normal prop (72" or so) won't pull much at all at any pitch. Large props can't be spun at high RPM since the tip drag mounts rapidly above mach 0.8 or thereabouts and power is lost to drag rather than thrust. Small props at high RPM have really rotten performance in takeoff and climb.

    Much has been tried, but there are few really revolutionary ideas. I think we're stuck with the 2000-3000 RPM range until someone comes up with a propeller that somehow fools Mr. Mach.

    There was one supersonic propeller I know of, on a prop/jet combination that failed partly because it made so much noise. The "Thunderscreech."

    From Wiki: Republic XF-84H - Wikipedia, the free encyclopedia

    The XF-84H was quite possibly the loudest aircraft ever built (rivalled only by the Russian Tupolev Tu-95 bomber [13]), earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger".[14] On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away.[15] Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run.[15] Coupled with the already considerable noise from the subsonic aspect of the propeller and the dual turbines, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[10] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[16]
    The pervasive noise also severely disrupted operations in the Edwards AFB control tower by risking vibration damage to sensitive components and forcing air traffic personnel to communicate with the XF-84H's crew on the flight line by light signals. After numerous complaints, the Air Force Flight Test Center directed Republic to tow the aircraft out on Rogers Dry Lake, far from the flight line, before running up its engine

    Dan
    Last edited by Dan Thomas; July 25th, 2012 at 07:50 PM.

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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by Dan Thomas View Post
    ...I think we're stuck with the 2000-3000 RPM range until someone comes up with a propeller that somehow fools Mr. Mach.
    Something derived from Unducted Fan technology is probably where that's at, but developing such props takes aerospace-prime levels of technology. Even Rolls Royce and GE have only, just recently, begun claiming to have built test fans that are both efficient and relatively quiet. This is not something the homebuilder can hope to accomplish, unless you happen to already be an expert in the technology and have the necessary software tools at-hand.
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    Re: Propeller design for greater efficiency and much higher RPM's

    The RPM depends on the engine size. A model airplane engine might turn 15,000 rpm for example.
    A VW has shown it can turn 3500 rpm (with up to 64" prop) because it is a much smaller engine than a Lycoming.

    The Honda Fit would be similar to the VW, so 3500 rpm is probably the most efficient limit for direct drive.

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    Re: Propeller design for greater efficiency and much higher RPM's

    Thanks for all the input. I apologize, when I wrote this thread I was running late for work and just wanted to get something off, but I realize now I was not very clear and vague about what I was asking.

    Let's take the Honda Fit engine just for an example. It's max hp of 118 is obtained at 6400 rpm. If we ran it at 80% of max (which would be around 5000 rpm) attached to a propeller directly, we would need to use a prop that was no longer than 46 inches in diameter, which at 5000 rpm would make it well under Mach speed at the tips and therefore avoid the noise vibration issues at or near mach speed. Now a 46 inch prop (even with 3 or 4 blades) would be much less efficient, and produce less static thrust than a 72 inch prop spinning at 2700 rpm. And the reason for this (please anyone correct me if I'm wrong or missing the big picture) is because given the relative density of air (meaning it is thin) there is not enough air flowing over the area of prop to create adequate thrust. In other words, the prop does not have enough air to 'bite' into to move it along at a powerful speed. If you look at a boat motor which moves water which is much denser than air the props are smaller and spin at much higher rpm's at best efficient thrust.

    So, my question is really about what can be done to make a 46 inch prop at 5000 rpm be more efficient? And here is where my lack of engineering knowledge comes into play and I am really just throwing out very general ideas for the solution. But what I was thinking was perhaps if you could attach fins (or long thin blades of a type) from the nose cone base, perpendicular to the propeller blades, that would then attach at the forward end, kind of like (to give a visual example) a greatly elongated cone but constructed of five long blades instead of a solid piece. And the blades or fins would be shaped in such a way that when they spun along with the prop at 5000 rpm would create a vacuum or suction of air around it that would in effect push more (and therefore more dense) air through the propeller. Conceptually like creating a turbo charger for your propeller. A jet engine works on a similar principle, it has fins in the front of the engine that spin and create a vacuum of air that gets compressed and then mixed with jet fuel before being ignited. So, I'm talking about some type of prop design that pulls more, and therefore more dense, air into the propeller area.

    I realize I'm talking conceptually here and to actually create something like this would take some real engineering calculations and tests for the right material and shape etc. But conceptually speaking why could this not be done? And it seems to me that it would take less engineering and be less expensive than to create a really well designed and reliable PSRU. If one could create such a prop it could really open up the use of auto engine conversions in experimental aviation. You could have props that could be fitted to not a specific engine but rather to an rpm setting.

    Any input would be greatly appreciated, even if you just want to call me an idiot.

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    Registered User Dan Thomas's Avatar
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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by tilopa View Post
    If you look at a boat motor which moves water which is much denser than air the props are smaller and spin at much higher rpm's at best efficient thrust.
    A jet engine works on a similar principle, it has fins in the front of the engine that spin and create a vacuum of air that gets compressed and then mixed with jet fuel before being ignited. So, I'm talking about some type of prop design that pulls more, and therefore more dense, air into the propeller area.
    Two things:

    First, a boat's propeller is of much smaller diameter than an airplane's, and operates in a medium that's 800 times denser. A large diameter is not necessary. The speed of sound in water is also 4.3 times faster in water, so mach limits don't enter into the problem at all.

    Second, those fins in a jet's engine are guide vanes. Jet engines have rotors that drive air back into the guide vanes, which stop the rotational component of the air (imparted by the rotor) and slow it, increasing its pressure, and guide it into the next rotor stage. This happens as many as 13 or 14 times in an axial-flow compressor. The object here is pressure, not velocity; the flame front in a jet engine moves at less than 100 feet per second and the airflow into the combustors is considerably less than that but at 350 psi or so.

    The whole jet engine thing is vastly different from propeller dynamics, where we want to accelerate as much air as possible to as high a speed as possible. We're not concerned with pressure. A 36" prop has only one-fourth the area of a 72" prop, so it has to move that tiny column of air to a huge velocity to get a thrust equal to the larger prop; the problem there is the large propeller blade pitch angle required to do that, resulting in a prop that's is stalled at anything much less than cruise speeds and giving us our rotten takeoff and climb performance.

    Dan

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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by Dan Thomas View Post
    Two things:

    First, a boat's propeller is of much smaller diameter than an airplane's, and operates in a medium that's 800 times denser. A large diameter is not necessary. The speed of sound in water is also 4.3 times faster in water, so mach limits don't enter into the problem at all.

    Second, those fins in a jet's engine are guide vanes. Jet engines have rotors that drive air back into the guide vanes, which stop the rotational component of the air (imparted by the rotor) and slow it, increasing its pressure, and guide it into the next rotor stage. This happens as many as 13 or 14 times in an axial-flow compressor. The object here is pressure, not velocity; the flame front in a jet engine moves at less than 100 feet per second and the airflow into the combustors is considerably less than that but at 350 psi or so.

    The whole jet engine thing is vastly different from propeller dynamics, where we want to accelerate as much air as possible to as high a speed as possible. We're not concerned with pressure. A 36" prop has only one-fourth the area of a 72" prop, so it has to move that tiny column of air to a huge velocity to get a thrust equal to the larger prop; the problem there is the large propeller blade pitch angle required to do that, resulting in a prop that's is stalled at anything much less than cruise speeds and giving us our rotten takeoff and climb performance.

    Dan
    Thanks Dan,

    Perhaps I should not have used a jet engine as a comparison. The point I was trying to make is that for a jet engine to make it's greater thrust it uses denser (higher pressure) air combined with fuel before being ignited. It is the denser air that allows for greater energy and thrust because more air is being moved out of the back of the engine.
    Similarly, with the boat example, it is the fact that water is more dense than air that allows a boat prop to be much smaller and spin faster to create higher thrust.

    Relative to an airplane prop, you say that we want to accelerate as much air as possible as fast as possible, this is true. But then you say, we're not concerned with pressure. The reason that a 72" prop creates greater static thrust than a 36" prop given the limitations dictated by mach is because the 72" prop moves more air (greater area of air, or volume) over it's blades than a 36" prop. If we did not have the limitations of mach we could then spin the 36" in prop fast enough (whatever that speed would be) to move the same volume of air past the prop blades as the 72" prop by virtue of it's greater speed. If we took that same 36" prop and made the air passing over it's blades more dense (however we did that) then we would need a lesser speed of air (lesser prop rpm) to create the same volume of air and therefore the same static thrust as the 72" prop. So, the density of the air is an important factor in thrust, whether or not it is physically possible to increase the density of the air moving past the prop is another matter.

    In my example of the 46" prop, since we cannot spin the prop fast enough to produce the thrust of the 72" prop because of the limitation of mach, I was wondering if there was a way to make the air going over the blades more dense. And I was merely wondering if creating a structure protruding from the center of the prop similar to prop blades but thinner, in some sort of a spiraling configuration could possibly create enough of a vacuum to create a flow of denser air over the prop blades themselves and thereby increase the static thrust. I realize that this is a very theoretical or conceptual question, but what might be the physical limitations of such a prop design?

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    Registered User Jan Carlsson's Avatar
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    Re: Propeller design for greater efficiency and much higher RPM's

    HI

    Just some quick notes before work, I guess you are all sleeping anyway, at this time.

    The 5000 rpm will have a efficiency about 80% or less, depends on airplane speed and power.

    we want to accelerate the air as little as possible compered to the incoming air to get efficiency, to do that we need a bigger disk area.
    compering it to a jet isn't all that bad, a smaller prop will give a more rapid "slip stream" then a big propeller.

    even if we get a accepteble efficiency on the paper with a small diameter the air have to pass the engine cowling and fuselage. the smaller prop the more of its air hit the fuselage.

    I just came up with the idea of having a belt or gear reduction on a high speed engine to get the prop rpm down, Oh i might not be first on that thought.

    Dan's answer is very good.

    Tillopa Don't call your self a idiot, no one els does, we have all been at the steep slope of the learning curve. and we still all are at the curve. still learning, it is fun.

    Jan
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    Re: Propeller design for greater efficiency and much higher RPM's

    Try this:
    a 2 foot prop has an area of 3.14 sq. ft. Double the diameter and....
    a 4 foot prop has an area of 12.57 sq. ft. Doubling the diameter increses the area by 4 times. increase the diameter bt 50% and
    a 6 foot prop has an area of 28.27 sq. ft. or twice the smaller prop and nine times the original prop - tripple the diameter get 9 times the area.

    A bigger prop moves a lot more air that is why it is more efficient - since the tips are always near the speed of sound you simply can't duplicate the thrust of a big prop with a smaller one - when all other things remain equal.

    Paul

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    Re: Propeller design for greater efficiency and much higher RPM's

    Quote Originally Posted by Jan Carlsson View Post
    HI

    we want to accelerate the air as little as possible compered to the incoming air to get efficiency, to do that we need a bigger disk area.
    compering it to a jet isn't all that bad, a smaller prop will give a more rapid "slip stream" then a big propeller.
    Hi Jan,

    I found this equation for thrust:



    T is thrust, D is prop diameter, v is velocity of incoming air, v2 is velocity acceleration by prop, and p is density.

    I understand, people involved in prop design never really consider density because it is a constant. But according to this equation Thrust increases if density increases.

    Quote Originally Posted by Jan Carlsson View Post

    even if we get a accepteble efficiency on the paper with a small diameter the air have to pass the engine cowling and fuselage. the smaller prop the more of its air hit the fuselage.
    Damn, this is a direct answer to my question and it is something I had not considered, that is a real problem.

    Quote Originally Posted by Jan Carlsson View Post
    I just came up with the idea of having a belt or gear reduction on a high speed engine to get the prop rpm down, Oh i might not be first on that thought.
    If you can build me a PSRU that is inexpensive, very reliable, and will fit on a number of auto engines I'll buy it tomorrow.

    Quote Originally Posted by Jan Carlsson View Post
    Dan's answer is very good.
    I appreciate Dan's response but he was not addressing my question or acknowledging density in the equation.


    Quote Originally Posted by Jan Carlsson View Post
    Tillopa Don't call your self a idiot, no one els does, we have all been at the steep slope of the learning curve. and we still all are at the curve. still learning, it is fun.
    Thanks for that. I did not think anyone would call me an idiot I was just trying to be humorous.

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    Re: Propeller design for greater efficiency at much higher RPM's

    Quote Originally Posted by PaulS View Post
    you simply can't duplicate the thrust of a big prop with a smaller one - when all other things remain equal.
    That is my whole point! I'm talking about a scenery where all other things are NOT equal.

    You have just regurgitated the already stated fact that a larger diameter prop moves more air.

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    Re: Propeller design for greater efficiency at much higher RPM's

    Turbo Fan Engine | The Web's Where You Study In!

    -I think=you are on the right way...ejectors are very promissing solutions.

    =sorraunding air have huge energy=circa 150 kJ/m^3\average particles speed is 500m/sec!\

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    -another example...
    Iff I unerstand you=radial compressor+axial propeller=integrated?

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    Re: Propeller design for greater efficiency and much higher RPM's

    It sounds like you want to compress the air coming into the prop disk. This takes power to do and leaves less power to make thrust. Ducted fans have been thought of as a way to increase prop efficiency at high rpm, but, among other problems, there are losses from the drag created by the duct. When you make the air do something that it is not already doing, there is always a cost.

    I think, though, that you will get much more satisfying answers if you would make the best sketch/drawing of this device that you can and post it so that everyone can see precisely what you are describing.
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