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Reduction drives

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flat6

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i also believe auto conversions is the way forward. Building reliable engines is well understood nowadays. the missing components are reduction drives and vibration dampers/absorbers. the most common engine is the 4 cylinder inline engine and thats where the engineering needs to focus.
 

flat6

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monaco
Hey, you seem like you are very knowledgeable.
Could you make some sort of bracket by the side of your motor, on it weld and reinforce a horizontal rod/shaft that runs parallel to the drive shaft. Then on the drive shaft attach a gear, on the new shaft bolt a larger gear and connect them with a chain (not a belt, a chain like a motorcycle). The propeller attaches to the larger gear, so it spins slower. Think of it like a bike geared for torque, the pedals spin the wheel gear which spins the wheel. (in this case instead of a propeller)


Wouldn't this work I'm surprised no one does this it seems so simple to me. Could you give me your 2 cents?
chains are heavy. the lighter ones will fatigue quickly. unless you want to change them frequently. in that case i would rather use belts. they are a lot cheaper and lighter. assuming that vibrations are damped out elsewhere.
 

Dan Thomas

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Is there no one looking into designing a prop to eliminate the NEED for a redrive?
Oh, you can get a prop that will run direct drive at 6000 RPM. It'll just be really small, 36" or so, and very inefficient. If you use anything larger the tip speed generates huge drag, which means you lose a big percentage of your power to just heating the air and making noise instead of getting thrust.

Lots of guys have used higher-RPM direct-drive setups with smaller propellers. They tend to eat a lot of runway on takeoff and the climb isn't too good.

The prototype Helio Courier (the Helioplane) was a converted Piper Vagabond, in which an 85-hp Continental (which redlines around 2700 RPM and swings a 72" or so prop) was converted using a redrive. A multi-V pulley was installed on the crank and a larger one on the prop shaft, and a much larger and slower-turning propeller was installed. Thrust gain was significant. Production Helios used the GO-480 engine, a geared Continental that allowed high engine RPM (and therefore higher horsepower) but a large propeller to get big thrust.

Dan
 

flat6

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Is there no one looking into designing a prop to eliminate the NEED for a redrive?
im afraid redrives cant be avoided. all turboprops have one and all new radials at the end of ww2 have them. Engines would be too big otherwise. witht the current downsizing trend with turbo auto engines its going to be essential unless lycoming can match the economy of scale of auto engines which is unlikely.
 

Kristoffon

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This guy tried it and then put an ordinary propeller in place so the logical conclusion is that the result isn't so great:

Mick Duckt N7XR

Duckt is now ductless! Duckt was converted from ducted fan propulsion to PSRU/prop December 2004 (still powered by a rotary engine - of course!). The short story is: takeoff distance remains the same, climb rate improved, cruise speed much improved.


Redrives aren't a black art. Every car has mutiple redrives which work problem free in worse conditions than aircraft having to withstand the jerks bad drivers cause on the transmission. There's no technical reason why we can't have good reliable redrives.
 

autoreply

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Redrives aren't a black art. Every car has mutiple redrives which work problem free in worse conditions than aircraft having to withstand the jerks bad drivers cause on the transmission. There's no technical reason why we can't have good reliable redrives.
Apart from the 3 zeroes difference in the engineering budget you mean? ;)
 

alr

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Looking at the efficiency numbers quoted in this thread (a few percent to 15% loss, depending on the type of reduction drive) makes me wonder whether a torque converter might be a viable option. According to some graphs I have seen a single stage torque converter operating at its most efficient reduction ratio can have an efficiency of about 90%. This puts it in the same range as the redrives discussed here.

It would have the added advantage of an infinitely variable self adjusting reduction ratio, which would translate into higher torque at the propeller when heavily loaded, such as during take off, and the fluid coupling inherent in a torque converter would reduce vibrations being transmitted from the engine to the propeller.

I am thinking that weight might be the biggest drawback, and torque converter fluid cooling would have to be carefully designed.

Any thoughts?
 

Dan Thomas

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Looking at the efficiency numbers quoted in this thread (a few percent to 15% loss, depending on the type of reduction drive) makes me wonder whether a torque converter might be a viable option. According to some graphs I have seen a single stage torque converter operating at its most efficient reduction ratio can have an efficiency of about 90%. This puts it in the same range as the redrives discussed here.

It would have the added advantage of an infinitely variable self adjusting reduction ratio, which would translate into higher torque at the propeller when heavily loaded, such as during take off, and the fluid coupling inherent in a torque converter would reduce vibrations being transmitted from the engine to the propeller.

I am thinking that weight might be the biggest drawback, and torque converter fluid cooling would have to be carefully designed.

Any thoughts?
Torque converters, as found in transmissions, are fluid clutches that can overdrive. Anytime the output RPM is below the input, efficiency is suffering and energy is lost in the form of heated oil. As a reduction, it would convert only a small part of the total hp to usable thrust at the prop.

Dan
 

orion

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+1 to Dan's post - many torque converters today have a locking feature so that when the car is up to speed, the power transfer is direct. As such, using the converter as reduction would only work for a limited time. However, if we're talking simple reduction of rpm out versus rpm in, the loss of rpm is simply a waste of energy, meaning the efficiency will be far lower than most of the transmissions mentioned. Then, couple that with the relatively high weight, and you really have an inefficient mess.
 

alr

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Actually, what I have in mind it a classical, i.e. non-locking torque converter.

I think there may be some misunderstanding about how a torque converter works. A torque converter is not a simple fluid coupling, but is a devices which functions like a variable ratio transmission. It is designed to work optimally at a particular ratio. Think of it as a gear ratio. A typical example would be when the output speed is ~60% of the input speed. At the optimal ratio the power efficiency can be something like 90%.

Whether the device is operating at its optimal speed is determined largely by the loading. If the loading is zero then the efficiency is zero and the rpm ratio is one. At moderate loading the efficiency is about 90% and the rpm ratio is about 60%. (This depends somewhat on the design of a particular unit.) At maximum loading the rpm ratio is zero, i.e. the engine is working hard but the output shaft is stalled. This corresponds to maximum torque multiplication (i.e. output torque divided by input torque), something like a factor of two, but the efficiency is zero.

One result of this type of operation is that when you apply maximum power the engine rpm goes up and the torque at the output shaft goes up as well. Because of this torque multiplication effect torque converters are popular with certain types of race cars.

An aircraft containing a torque converter could be designed so the maximum efficiency condition occurred at cruising speed. There would be a 10% power loss compared to a direct drive to the prop. However, because the prop would operate at lower speed than it would with direct drive you could use a larger and more efficient prop and would therefore regain at least some of the lost efficiency.

An advantage would occur under takeoff condition. In this situation the engine would speed up (more so than it would with a direct drive or conventional redrive) and the torque on the prop would increase (more so than it would with a direct drive or conventional redrive). This would allow more takeoff power. The efficiency would drop during this time, but the trade off is that it would get you off the ground quicker and your climb out would be faster. (This is without using a variable pitch prop.) The reason this is possible is that the motor would run closer to its maximum power setting, something which is not possible when using a fixed prop optimized for cruising being driven by a direct drive or conventional redrive. Drag racers refer to this sort of thing as "getting into the power band early." (Note: when I say "maximum power" I mean maximum power, not maximum throttle. The two are not the same.) All this occurs naturally without the pilot having to change any settings.

One more thing. If we were to compare a torque converter (90% efficient if run under optimal conditions) to a v-belt drive (85% efficient according to some posts in this thread) the torque converter wins in efficiency. It loses compared to a geared drive (what is it, 97% efficient), but the torque converter has the advantage of torque multiplication at take off or other highly loaded operating conditions.
 

Kristoffon

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This is just based on some irrational fear of plain gears. Gears are much simpler and lighter than a torque converter and frankly if you're afraid of engineering issues with a gear redrive you should be utterly terrified of a torque converter.

The point of the torque conerter is to get a heavy load (car or truck) moving. "Heavy load" here is defined as something the starter motor can't turn along. This is not a problem in aircraft as propellers are light (compared to a car chassis) and their load on the engine is not significant at the starter motor cranking speed.

Also your understanding of the fixed pitch propeller is flawed. Granted a torque converter could let the engine spin more and work like a variable ratio redrive. But a fixed pitch prop's speed will be a function of the torque applied. So in the proposed scenario of take off with a fixed pitch prop optimized for cruise, you can't make the propeller absorb more power/torque at its proper speed. An increase in applied torque will in turn make the prop turn faster and overspeed. The torque converter can't magically make the prop take more torque at a give rotation. The only solution for that is the variable pitch prop.

Apart from the 3 zeroes difference in the engineering budget you mean? ;)
I said technical differences.
 
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alr

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Irrational fear of engineering? I can assure you that nothing is further from the truth. How about if you quit that sort of silly comment and let's have a rational discussion?

There are several errors in what you have said. Let us start with the comment about a starter motor. Basically, other than starting the engine, the starter motor has nothing to do with accelerating either a car or a propeller. These acceleration forces are applied by the engine, not the starter motor.

Next, let's discuss the propeller load. It is composed of two components, drag and inertia. (For this discussion I am lumping lift and drag of the propeller into a single term that I am calling “drag”.) I am oversimplifying the language a little here because strictly speaking words "drag" and "inertia" refer to a linear load (which relates to linear force), not a rotational load (which relates to torque). However, the inertia and drag acting on the propeller blades become transformed into torque on the propeller shaft as a result of the lever arm of the propeller. In a more correct terminology I would use "moment of inertia" rather than inertia when discussing the inertia of a rotational load, and analogous terms for drag. However, just to keep the language in common terms I will use the terms "drag" and "inertia" to refer to torques arising from drag and inertia of the propeller.

Both drag and inertia oppose the torque applied to the propeller. If the torque applied to the shaft exceeds the drag then the angular velocity of the shaft will increase, i.e. the shaft will be accelerated. The rate of acceleration is governed by the inertia of the propeller and the difference between the torque and the drag.

When an airplane is taking off the propeller is being accelerated. Both drag and inertia are in play. At the very beginning, when the propeller is turning slowly, drag is relatively low and inertia is the main force that is opposing the torque applied to the propeller shaft. As the shaft turns faster drag forces become more and more important. In any case, the more torque applied to the propeller the faster it will accelerate, and that is the most important concept.

The lift of the propeller is what causes the aircraft to accelerate. (Here I am temporarily talking about lift of the propeller and separating it from drag.) In general, the faster the propeller turns the greater will be the lift. Of course, if the angle of attack of the propeller blade is too high then the propeller will be in a state of stall, but nevertheless for a given angle of attack the lift will be greater as the propeller spins faster. Thus, for a given propeller geometry the aircraft will accelerate faster if the propeller is turned faster.

Now let’s look at torque, and let us assume there is a device between the engine and the propeller shaft that transforms the torque. This device could be a gearbox, a torque converter, a belt drive, or anything else that causes the output speed (i.e. rotational velocity of the propeller shaft) to be different from the input speed (i.e. engine rpm). If the device is lossless, i.e. 100% efficient, then the torque ratio is the same as the rpm ratio. For example, if the engine is turning at twice the speed of the output shaft then the output torque will be twice the input torque. If the device is less than 100% efficient then the output torque will be less than the ideal case. For example, if the efficiency is 90% and the speed ratio is 2 then the torque ratio will be 1.8, and the “missing” torque is wasted, i.e. not applied to the load.

Let’s go back for a second and look at the propeller. The propeller (if it could think) knows nothing about the presence of the torque transforming device. All it knows about are the drag forces, the inertial forces, and the torque being applied to the propeller shaft. For example, a torque of 1 newton meter coming out of a torque transforming device and applied to the propeller has exactly the same effect on the propeller as 1 newton meter applied to the propeller shaft from a direct drive.

Now let’s think about a torque converter for a minute. A torque converter multiplies torque, i.e. the output torque is greater than the input torque. It also reduces the rpm of the output shaft relative to the input shaft, i.e. in the context of driving a propeller it is a reduction drive. However, it is not a fixed-ratio reduction drive but a variable ratio reduction drive, and it has the interesting property that the reduction ratio is load-responsive. For example, if the system is driven by a certain amount of power input (let us say 100 kw) the load is such that the system is in steady state then nothing is changing, i.e. there is no acceleration.

Now let us assume that the input torque is suddenly doubled, such as if the throttle setting of the engine were suddenly increased. In this case a fixed ratio reduction drive and a variable reduction drive (torque converter) will behave differently. Both will cause the output shaft to accelerate, but the torque converter will accelerate the output shaft faster than the fixed ratio drive. The reason is that in the case of the fixed ratio drive the input and output shaft are locked in a fixed ratio and the speed ratio and torque ratio is unchanged. However, a torque converter behaves differently. When the input torque is doubled the engine speeds up faster than the output shaft, the speed ratio therefore changes (the input speed is greater than the output speed), and the torque ratio increases. This multiplies the torque on the output shaft, causing it to accelerate faster. This is exactly the same thing that happens in a drag racer fitted with a torque converter, which is why this technology is favored by some drag racers, i.e. it gives them greater acceleration.

Of course, you don’t get something for nothing. During this enhanced acceleration period the fuel consumption increases. It increases for two reasons, the first being that there is more net power being applied to the output shaft, and the second being that the torque converter is now not operating at its optimum efficiency point (let us say 90%) but rather at a lower efficiency point (let us say 70% for sake of discussion).

Now, let’s compare the system to a direct drive with an efficiency of 90%, such as you might find with some belt drives. At cruise both the direct drive and the torque converter would have the same fuel consumption. Over the course of the whole flight however the torque converter will consume more fuel because, assuming the engine is set to maximum acceleration during take off and climg in either case, the aircraft equipped with the torque converter will use more fuel during take off and climb out. However the tradeoff is that the take off distance and climb rate will be improved for the aircraft fitted with the torque converter. If the flight is a long one and most fuel is consumed during the cruise phase then the percentage difference in fuel consumption between the two cases will be very small. If the flight is short then the percentage difference in fuel consumption will be larger.

Now, compare the hypothetical 100% efficient redrive compared to a torque converter. In this case the torque converter will probably have improved take off and climb performance, but it will also have greater fuel consumption during all phases of the flight and/or a slightly lower top speed.

Of course, a variable pitch prop is also a possibility, and it has different engineering trade offs.
 

Dan Thomas

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The torque converter still does nothing for the fact that at or near 1:1 the prop will be rotating at engine RPM. Propellers have to be small to spin at auto-engine speeds, and small propellers are much less efficient than large props. Small props have really poor takeoff thrust regardless of the speeds they spin. Large props must spin more slowly to avoid the huge drag losses, and forward speed increases tip speeds, too.

Jim Bede tried, in his first iteration of the BD-5, a belt-type torque converter as found in many snowmobiles and even a few European cars. The purpose was to let the engine spin up to full RPM on takeoff, and then the drive pulley would expand, pulling the belt lower in the driven pulley, and accelerate the propeller for cruise. Two problems: the small prop necessary didn't have good low-speed performance anyway, and at higher RPM it lost efficiency. Bede abandoned it soon in favor of a fixed-ratio redrive and larger prop, which worked much better. Propellers are not snowmobile tracks or auto wheels , and need something different.

Dan
 

alr

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Dan,

I think you are missing part of the point. The point is to set the design point so that under cruise conditions the torque converter is not approaching 1:1 rotational speed ratio but instead is operating at the peak efficiency point, which would be something like a 60% speed ratio for a typical torque converter.

Also, as I mentioned before the intent would not be to use a small prop but rather a large diameter prop, which of course is the whole idea of using any kind of reduction drive.

I am not necessarily saying that a torque converter is ideal or even the best way to do things, but it does have certain desirable features. However, in order to discuss it we do need to all be on the same page, e.g. with respect to operating points and so forth, and so far I don't think the discussion is quite meshing that way.

I was not aware that Bede had tried a (non-hydraulic) torque converter, which I think would more properly be called a continuously variable transmission (CVT) in this case. If, as you imply, Bede used a small prop then one of the two reasons for using a torque converter (rpm reduction to enable the use of a larger propeller) was defeated from the beginning and could not have produced a favorable result. There can be no doubt Bede knew the significance of using a larger propeller, so I suspect the reason he abandoned the approach was not for the reason you suggest.

Regarding the use of a CVT, I could be wrong, but I don't think that in general they have the self-adjusting feature that a torque converter has. In other words, as far as function is concerned I think a CVT is like a geabox, i.e. the operator still has to change the ratio, but instead of having a few fixed ratios available he has an infinite number of ratios available. By contrast, a torque converter automatically adjusts its operation without any user intervention.

As an interesting note, the wikipedia page on CVTs states that they were banned from formula 1 auto racing in 1994 because they were making the cars to fast. (Continuously variable transmission - Wikipedia, the free encyclopedia)
 

Dana

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Back around 1980 a neighbor gave me an old snowmobile engine. Not sure which one it was but it was single cylinder Rotax. It had a belt torque converter. Both pulleys were adjustable; the motor pulley was its widest (smallest diameter) at low speed, and flyweights pressed it in, increasing the running diameter. The driven pulley was simply spring loaded in. At idle, the engine pulley would open far enough so the belt would ride on a bearing at the center, providing a clutch.

Not knowing much back then (I was only 21), I intended to use the drive as is on an ultralight I was building. The project got interrupted and the state of the art passed it by, so I don't know how (or if) it would have worked.

-Dana

A truly wise man never plays leapfrog with a unicorn.
 

Richard Schubert

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The point is to set the design point so that under cruise conditions the torque converter is not approaching 1:1 rotational speed ratio but instead is operating at the peak efficiency point, which would be something like a 60% speed ratio for a typical torque converter.
Are you saying that for every 100 rpm the engine turns in cruise, the propeller will turn at 60 rpm because its more efficient for the torque converter?? :think:
 

Dan Thomas

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Dan,

I think you are missing part of the point. The point is to set the design point so that under cruise conditions the torque converter is not approaching 1:1 rotational speed ratio but instead is operating at the peak efficiency point, which would be something like a 60% speed ratio for a typical torque converter.

Also, as I mentioned before the intent would not be to use a small prop but rather a large diameter prop, which of course is the whole idea of using any kind of reduction drive.

I am not necessarily saying that a torque converter is ideal or even the best way to do things, but it does have certain desirable features. However, in order to discuss it we do need to all be on the same page, e.g. with respect to operating points and so forth, and so far I don't think the discussion is quite meshing that way.

I was not aware that Bede had tried a (non-hydraulic) torque converter, which I think would more properly be called a continuously variable transmission (CVT) in this case. If, as you imply, Bede used a small prop then one of the two reasons for using a torque converter (rpm reduction to enable the use of a larger propeller) was defeated from the beginning and could not have produced a favorable result. There can be no doubt Bede knew the significance of using a larger propeller, so I suspect the reason he abandoned the approach was not for the reason you suggest.

Regarding the use of a CVT, I could be wrong, but I don't think that in general they have the self-adjusting feature that a torque converter has. In other words, as far as function is concerned I think a CVT is like a geabox, i.e. the operator still has to change the ratio, but instead of having a few fixed ratios available he has an infinite number of ratios available. By contrast, a torque converter automatically adjusts its operation without any user intervention.

As an interesting note, the wikipedia page on CVTs states that they were banned from formula 1 auto racing in 1994 because they were making the cars to fast. (Continuously variable transmission - Wikipedia, the free encyclopedia)
The CVT is automatic. Flyweights in the drive pulley, plus loading on the belt, control the positions of the two pulleys. If you've ever run a snowmobile you'll know how awesome it works. The acceleration can be scary for the uninitiated.

Dan
 

RJW

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Feb 9, 2011
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Orion,

What happened to your Mazda PSRU? Was it a success? If not, why? I’m drawing up an idea for a smallish PSRU for a V8—about 300hp and hopefully weighing less than 65 pounds (probably dreaming on the last point). I’ve been gathering “internet knowledge” and information from books for the last few years. I have corresponded with a very knowledgeable man who designed and started building a gearbox some time ago who has been very helpful. But it would be nice to get some details from someone who went through the testing phase. Any help would be greatly appreciated. I also talked with Bud Warren two years ago at Oshkosh. He was very nice but was unwilling to share much about his design, which is understandable.

Attached is a screen of the idea. At this point “analysis” is little more than the “looks about right” kind. I understand that thorough analysis and testing must be performed before the thing is put into an airplane. What is attached is only a concept or general arrangement. Criticism is welcome.

The arrangement is similar to the old Blackhawk PSRU but for a lighter installation. I have only been able to find scant information on this gearbox. If anybody has and is willing to share any information on the Blackhawk box it would be greatly appreciated.

Thanks,
Rob
PSRU RW1.jpg
 
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