Reduction drives

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TFF

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How much boost is a lot, 100psi? A rotary at 2500-2700 rpm does not make a lot of torque without astronomical boost. Not at useful propeller speeds.
It’s an open ended question. Your airframe requires X prop. You pick Y diameter and pitch will be in a certain range to get what performance you spec. Same with pitch. Pick a pitch and you will force Y to a matching diameter. All assuming you are in the realm or reasonable aerodynamics.
If you want to go direct drive, you are going to have to make say 180 hp at 2700 rpm to drive your reasonable sized prop. A Lycoming is spec’d for that. Props are designed to that. To do that with a rotary, you will have to make the same 180 hp. but drop it from it’s natural rpm of 5000 to 2700 for that horsepower so the prop will work. You just took the equalizer out which is the gearbox.
You really are inserting engine in between the airframe, which is a set quantity, and the prop to fly it the way you want it which a set requirement.
You go the way of high boost, is the crank strong enough to hold a prop? Can the engine case handle lots of boost all the time? Can you keep the engine out of detonation? Can you cowl this engine? Do you have the lifespan to develop this? It’s not the route of the rotary gurus. You will have to know more than they know to pull it off.
 

wsimpso1

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Okay, having read what Orion had to say, what is wrong with the idea of putting a lot of turbo boost on a direct-drive rotary, no PSRU, and run the engine slower? The fuel burn would be more complete at the lower rpm.
Technically, you could select a turbocharger for that job, boost the manifold pressure and intercool the charge air down at those rpm, and raise the engine torque to 390 ft-lb at 2700 rpm (that is 200 hp) or thereabout.

Being as the base NA engine makes somewhere around 125 ft-lb at that rpm, you would need to more than triple the manifold pressure to get to 390 ft-lb there. I do not know if you can even find a fuel delivery and spark timing combination that will run without detonation and/or preignition at that much manifold pressure, so it might be a tall order. Then the engine may not structurally be reliable or adequately long lived at these manifold pressures and the resulting combustion pressures and temperatures. In addition, you might need to adjust the port sizes and locations keep it breathing well at those lower rpms.

Basically, the smart people at Mazda did not design the engine to do what you want. If they did, it would have been heavier and bigger diameter, which was counter to their needs in cars. I suspect that even with the base engines being pretty robust, they may not have enough metal in places where you would need it to be capable of standing these manifold pressures and the resulting combustion pressures. This MAY be a huge development task to get to the torques and then to get a reasonable engine life and reliability.

Knowing what I know about isolation of engine firing pulses and other inherent engine vibration, I suspect that a good isolation scheme and gear set can be built for these engines running where they should be run. Tracey Crook showed us a good solution for 160 hp, so we know it is possible. The Powersport solution also seemed to work well. Neither scheme is commercially available right now. Bummer. I would like a turbine cowled airplane with a P-51 belly scoop, but I am most likely to put a used O-360 with SDS EFII on the front of the bird I am building.

Billski
 

BBerson

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Reading this thread again I found orions comment interesting. Could help Peter eliminate the pesky belts on the Raptor!
" yes, a direct drive engine with a fan unit could be a solution (I've been saying this for years). By this I mean a fan without a shroud or duct. Designed right it could be quiet and very efficient. However, as was pointed out, it would not be really applicable to the current line of aircraft since most are configured around a relatively conventional power-prop combination." (post 25)
 

cheapracer

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Okay, having read what Orion had to say, what is wrong with the idea of putting a lot of turbo boost on a direct-drive rotary, no PSRU, and run the engine slower? The fuel burn would be more complete at the lower rpm.
it's a sound idea, and a number of engines, not Rotarys, are there now, 1.0 to 2.0 engines that have full boost by 2500 rpm, and very strong from below 2000 rpm.

Problem with Rotarys is that many know how to get big HP at high revs, but no one does work for low rpms,
 

Winginitt

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Don't know much about Rotary engines except they produce a lot of heat. Turbos produce a lot of heat and intake air temps go up exacerbating the already high engine temps. Add an intercooler and you increase weight and drag. Seems like a lot of difficult problems to solve. I'd just look at building a direct drive LS3 stroker (408 cu in) that will easily make 250 HP (or more) at 2700-3200 RPMS. No muss and very little fuss. You may be able to develop the power you want but I just think heat will be the elephant in the room.
 

Erik Snyman

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Having been out of the PSRU quest for some time, (flying certified), I wondered if anyone has learned anything useful about the (usable) design of a modern PSRU unit. I used one or two on planes I built in the late 80`s to around 2000, but as was previously stated, a lot of manufacturers promised a lot of things, and somehow the produced unit always seemed to not quite work as well as promised.
If we are no closer to an answer re. a modern PSRU unit, it looks like I will have to go and drag the old IO-470 out of the shed and give it a bit of a once-over.

I realise this is a redrive forum, but another avenue that could be pursued is to just get the humangous aluminium GM or Ford engine that tickles your fancy, and invert it, and accept the fact that you will "only" get 1 hp out of every 2 ci of your engine capacity, for a normal size prop.
I am investigating this option quite seriously for an install on a BD-4 I am resurrecting, and I have a book written by a person that did just that with a ZZ4. Minimal mods, with most needed to the oiling system, but way less hassle than a redrive, imho. Your individual requirements may differ from mine.

Erik in Oz.
 

henryk

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I realise this is a redrive forum, but another avenue that could be pursued is to just get the humangous aluminium GM or Ford engine that tickles your fancy, and invert it, and accept the fact that you will "only" get 1 hp out of every 2 ci of your engine capacity, for a normal size prop.
I am investigating this option quite seriously for an

install on a BD-4

I am resurrecting, and I have a book written by a person that did just that with a ZZ4. Minimal mods, with most needed to the oiling system, but way less hassle than a redrive, imho. Your individual requirements may differ from mine.

Erik in Oz.
-iff CONTERROTATING gear/propellers ?

(zero giroeffects, much lower reactional momentum...)
 

BBerson

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I realise this is a redrive forum, but another avenue that could be pursued is to just get the humangous aluminium GM or Ford engine that tickles your fancy, and invert it, and accept the fact that you will "only" get 1 hp out of every 2 ci of your engine capacity, for a normal size prop.
Might get somewhat more power running at say 3200 rpm. The O-470 can do 2900 rpm with a 88" prop, a GM engine could turn a bit smaller prop at more rpm. A bit supersonic, but so is a C-185. Just throttle back after takeoff.
 

cheapracer

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Might get somewhat more power running at say 3200 rpm. The O-470 can do 2900 rpm with a 88" prop, a GM engine could turn a bit smaller prop at more rpm. A bit supersonic, but so is a C-185. Just throttle back after takeoff.
My prop knowledge is pretty limited, what is the highest practical rpm/propellor combination for reasonable performance for light aircraft in general .. Yes I understand the loss of efficiency as they get smaller,, but what's real world application work out like, has there/is there any examples?

There's a number of suitable engines around now making serious hp 3600 ~ 4000 rpm.
 

Vigilant1

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what is the highest practical rpm/propellor combination for reasonable performance for light aircraft in general .. Yes I understand the loss of efficiency as they get smaller,, but what's real world application work out like, has there/is there any examples?

There's a number of suitable engines around now making serious hp 3600 ~ 4000 rpm.
Here are some sample numbers from Jan Carlsson's program (below). I didn't have time to fiddle around with more numbers now, but the small prop looks like it might have trouble at speeds below 80 MPH. It might take some monkeying around with blade numbers, camber, and aspect ratios to get it to work. If the small prop is unworkable, it will be due to performance issues at low speed (as we'd all have guessed):

All: 160HP, both props are either variable pitch OR the numbers apply for a fixed-pitch prop optimized for that airspeed

Prop..........................................................................................................80MPH (climb) .............................................150 MPH cruise
2 Blade wood, 72" dia, 2700 RPM, tip speed approx876 fpm).....32.16" pitch, 63.8% eff,478 lbs thrust.......59" P, 81% eff, 324 lbs thrust
4 blade wood, 50" dia, 3800 RPM(tip speed approx 840 FPM).....23.39"pitch, 63.3% eff, 475 lbs thrust.......41" P, 82% eff, 328 lbs thrust
 

TFF

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Also type of plane. A fast airplane can use a small diameter; it can help with speed. An STOL plane would just sit wondering if it was going to move fast enough to fly. It needs a bigger prop.
The column of air size, the trust is made, is important. A fast plane wants a fast column so it will put up with a smaller area if the air is accelerated faster. STOL wants the column slower with more area. It needs immediate usable thrust. The compromise is why we have so many engines that are close we can use. The airframe dictates the prop needed. We decided where to bias the compromises. Less takeoff thrust for more speed or the other way around. Then you get into how safe the plane is because of the choices. Fast plane on a short runway might mean you could be taking off at stall before you run out of real estate. Longer runway could build more speed to get going. Varying pitch starts becoming a necessary part if all around performance is required.
 

BBerson

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My prop knowledge is pretty limited, what is the highest practical rpm/propellor combination for reasonable performance for light aircraft in general .. Yes I understand the loss of efficiency as they get smaller,, but what's real world application work out like, has there/is there any examples?

There's a number of suitable engines around now making serious hp 3600 ~ 4000 rpm.
Both Wittman and Beachner flew direct drive. I don't know the size or rpm.
But today you would probably go with a very thin custom composite prop, using high tip speed. A variable pitch like Raptor would be needed for highest performance. Fixed pitch for less utility. It depends on mission.
 

BJC

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My prop knowledge is pretty limited, what is the highest practical rpm/propellor combination for reasonable performance for light aircraft in general .. Yes I understand the loss of efficiency as they get smaller,, but what's real world application work out like, has there/is there any examples? .
Some examples that might help define the outer limits:

Pitts S-1 full throttle, level flight, 3250 RPM with a 76” diameter. It gets really noise, on the outside, at about 3,300 RPM and 220 MPH.

Cassutt full throttle, level flight, 4,200 RPM with a really short prop, don’t recall the exact diameter, but VB may.

Neither example is intended to last very long.


BJC
 

wsimpso1

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Prop fundamentals:

Props are rotating airfoils, making thrust through lift while using up crank flange torque through drag.

Props convert power at the prop flange into power in the airstream. The rate of work made good in the airplane is less than the rate of work available at the prop flange. Efficiencies generally run in the 82 to 90% range. Input power in foot-pounds/second is torque (ft-lbs) times flange speed (rpm) times 2*Pi/60. Output power is thrust in lbs times speed in ft/s. This usually comes into play at flight speeds. The thrust is specifically air mass flow rate times acceleration of that air in the flight direction. The rotating portion of the airflow is the 10% to 18% lost energy...

Maximum thrust that any prop (or helicopter rotor) can make is related to its diameter and the power available. The bigger it is, the more thrust it can make. In conventional airplanes, this usually is the upper bound on thrust available from the prop, usually limits thrust through the takeoff run.

Props have two limitations on what they can do. Prop flange horsepower times prop efficiency sets thrust times airspeed possible while prop flange horsepower, efficiency, and prop diameter squared sets the static thrust possible. The lower of the two is what is actually expressed in accelerated airflow.

Prop efficiency drops dramatically when any part of the airflow around the blades approaches sonic velocities, so we really need to keep tip velocities below about 0.7 Mach. Good prop design can get to about 90% efficiency, but bad prop design can make the expressed power much lower. Supersonic tips is one way to get much lower efficiency. Some airplanes take the air velocities to and above Mach at max power settings. These airplanes put out a crack as each tip passes and are loud on takeoff. The rest of us tend to keep prop tip speeds lower.

Prop tip speed in flight is the vector addition of: rotation speed times prop radius, and; Forward speed of the airplane. So, for slow and fast airplanes with the same engine, the slower airplane can run a somewhat bigger prop than the faster airplane and get better acceleration on the runway than the faster airplane.

If you want to run a really big prop, for a fixed horsepower, you have to gear it to run it slower.

In our standard airplane engines, we usually have redline for normal operation somewhere around 3000 to 2700 rpm. And that puts props in the 72" to 88" range. In geared engines, we run the props in similar ranges. The 100 hp Rotax in my wife's RANS runs a min prop rpm at takeoff power of 2420. Merlins and Allisons and big radials are all geared, run bigger props and run them slower. Get to turbo props on the C-130 and they turn even bigger props even slower. All in the name of big thrust at takeoff speeds and keeping the flow around the tips below sonic at cruise speeds.
 

GESchwarz

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Very good responses! Thank you very much.

So I have been building my scratch design airplane for 10.5 years based on the Mazda rotary engine which I purchased 10.5 years ago. So now it's time to start working firewall fwd and I can't seem to get my hands on a PSRU.

Where can I find a Tracy Crook PSRU?

I believe Neil Unger took over that product line, but when I asked him for one, all he seemed to have is a completed system, engine and all, and it's sitting on a pallet somewhere on the other side of the Pacific Ocean.
 

BBerson

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The supersonic tips lack of efficiency is only for a few seconds at takeoff to get off the water or clear the trees. Then the rpm is reduced for climb and cruise. It's a compromise, most personal airplanes are direct drive for simplicity. If an auto engine weighed the same per cubic inch as a Lycoming it would be simple to fly with direct drive auto engines.
 

pictsidhe

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Very good responses! Thank you very much.

So I have been building my scratch design airplane for 10.5 years based on the Mazda rotary engine which I purchased 10.5 years ago. So now it's time to start working firewall fwd and I can't seem to get my hands on a PSRU.

Where can I find a Tracy Crook PSRU?

I believe Neil Unger took over that product line, but when I asked him for one, all he seemed to have is a completed system, engine and all, and it's sitting on a pallet somewhere on the other side of the Pacific Ocean.
I believe that the Tracy Crook redrives used Ford gears. That means that it shouldn't be too expensive to have a single one made if you can get enough details. Or borrow one to copy.
 
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