Reduction drives

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Robert Young

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Nov 24, 2003
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Apple Valley, CA
redives

Yes, I agree with the point about shifting gears. That will not contribute to the performance. The variable pitch prop is our gear change method and it works very well and is quite simple. My "second gear" comment was a flippant remark made in jest.

I do think however the CVT technology without the Variable may be worth looking at. The belts used in those devices are mettalic assemblys that seem to be very compact and simple. A mettalic belt system (aka chain) that is very efficient and rugged is the Morse HyVo
drive and I know it is being used successfuly by someaircraft buliders. The mettalic belts in the CVT units are friction drives whereas the Morse is a toothed belt. The friction drive may be worth considering as a way to deal with torsional problems. The service seen by the CVT must be severe since it is working with positive and negative loading in the car.
Anyway it is food for thought.

Engine power is related to air consumption and enough fuel must be provided to use the available air (oxygen). We can either turn the engine faster or increase the air density to gain power for a given engine displacement (cylinder swept volume). Increasing air density is the more efficient method, in most cases, when we are concerned with specific fuel consumption. The most effective way to increased air density is the supercharger, either mechanical drive or turbo. Increasing engine speed reduces mechanical efficiency and stresses reciprocating parts and bearings in proportion to the square of the speed. It also increases wear. High density air can be dealt with more easily I think. During WWII engine performance gains were considerable and most if those gains were the result of air density increases by use of multiple superchargers, turbos, and intercoolers and aftercoolers.

If I were to design a new GA engine from the whole cloth I think I would look to direct drive running at modest speed-no more than 2100rpm-using high boost with a mechanical drive suprcharger. To take it to the highest level it could be two stroke cycle direct injected into the combustion chamber and then it may as well be compression ignition.
The Delta Hawk is the closest thing I have seen to this scheme.
 

Robert Young

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Nov 24, 2003
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redrive info

there is a periodical called "Contact!" that is devoted to alternative engines and back issues are available. The magazines are loaded with real experience information and all back issues are available. To date they are up to issue 73.

They can be contacted at: Editor@Contact Magazine.com

The web site is: www.contactmagazine.com

Contact has published two books containing many of the articles from the magazines.

It is a GOLD MINE
 

Captain_John

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Feb 3, 2003
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Orion, 2 things...

1) Oil - I understand the problem with gear backlash in the descent. If we can successfully eliminate the problem in truck gears I don't understand why a good set of gears in a redux unit couldn't be as good a quality, if not better. I mentioned synthetic because of the supposed superior "cushion" effect it has on the gears. According to sales propaganda (which cannot be trusted) and some savvy mechanical folks I am familiar with (whom can sometimes be trusted) I am on the understanding that synthetic would be the choice of lubricants. I am also curious as to why a bath would be less desirable than a pressurized system, when the main function of the lubricant is to carry off unwanted heat.

2) I think you misunderstood my reason for a 2 speed unit - I want to sacrifice horsepower for a lower rpm, quiter, more efficient and albeit slower mode of travel. I don't expect to get a turkey for the price of a chicken. Please re-read my previous post and tell me what you think about my hair brained scheme.

I appreciate your take on this subject, as you seem to be what could be considered fairly well versed in the subject.

Thank you.

:D CJ
 

Robert Young

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redrive controversy

At Oshkosh in 2002 NASA presented a forum on the status of their General Aviation Power (GAP) program that they studied in collaboration with Teledyne Continental Motors(TCM). The forum discussion became lively and heated because NASA's Mark Moore and Andrew Hahn concluded the auto engine was the best avenue to follow for bringing down the cost of GA aero engines. The NASA/TCM diesel engine path is a dead end. The NASA members suggest a Corvette engine driving a ducted fan/propeller with direct drive would be the best approach for a so-called personal air vehicle (PAV).

It looks as though the NASA guys had put their heads in the sand when they wrote their report. Ducted fans have merit but on most GA aircraft they do not fit and some of those that have tried them where they could fit had great performance after they took the duct off and reverted to an open prop. Pushey Galore comes to mind. The only ducted fans in regular use today are the turbine fanjets and the reason for that is obvious.

We all know that the very first successful powered aiecraft used a redrive, without which they would not have got off the ground. All the big piston iron uses a redrive. In my opinion NASA would have gotten more bang for our bucks if they had concentrated on designing a versatile redrive for the GAP program. I frankly think the NASA suggestion at that forum was a red herring to extricate themselves from the GAP program.

Why do I say that. Because we now learn that TCM and Honda have been collaborating for a couple years on a GA aero engine project which they showed at Oshkosh in 2003. It is a four cylinder opposed layout that is liquid cooled and produces 225hp at 2700 rpm and is direct drive.
Connect the dots and it becomes clear what path this project is taking. A modern engine using modern technology built by two great engine companies that can fit into the cowl of all those old but good aircraft flying around with those post WWII engines that are good but not as good as they should be. The new TCM/Honda engine will run on most gasolines in world as well.

There may someday be more used but good four and six cylinder aircooled aero engines available than the the homebuilt market will know what to do with. Honda does not announce entry into a market until they have concluded there is a market so I think this is a real effort on their part.

There is still a need for a very good redrive during the interim whether it be gear, chain, friction belt, toothed belt, or something else.
 

orion

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There are a couple of points I'd like to address from the above posts. Earlier, Robert Young made an excellent point and one that hasn't received a lot of attention for some reason. The CVT transmission technology could potentially be an excellent approach to our reduction needs. Although it functions as a result of contact friction, the metal based belt design allows it to operate at very high efficiencies (I've seen SAE data indicate that the transmissions run at 98% or so) and very quitetly. The system does not have a catastrophic failure mode and as was mentioned, could be an excellent approach to dealing with torsional feedback issues. If I had time, this is an area I'd certainly like to do a bit of research within since this would result in the simplest redrive configurations out there.

The CVT drive also does not have the RPM limitations that some chain drives do so it would be applicable to some of the higher turning engines (such as the rotaries) that would serve well in our applications.

I've seen several drives that use the HyVo chains and given the extremem stength and durability of that particular chain design, I would be willing to bet that in an aircraft application, the HyVo configuration would make a bullet-proof drive. The small pitch variants of the chain are applicable to very high surface speeds so even in applications where the small member turns in excess of 6,000 RPM, the chain still functions within its design limits. Yes, there is a small weight penalty associated with these drives but in my opinion, this is minor given the durability of the configuration.

Regardng Captain John's comment above, I have nothing against synthetic oil and agree with your analysis of its application. However, in a conservative reduction drive design, the specifics of oil selection are a bit more flexible. Yes, I'd use the best that is available but others may have different preferences and the details of the selection is not all that critical.

The purpose of the oil is three-fold. First, as you say, it is to carry away heat. For this purpose most oils will serve just fine. Personally I like the idea of using the engine oil since it circulates through the integral system of oil cooler and filter, so as long both are sized for the additional effects of the gear drive, this seems to be the most convenient solution.

Second, the oil of course lubricates the rotating components such as shafts and bearings.

And third, the oil reduces the negative effects of the contact of the gear teeth. Here it acts in two ways. First, as was mentioned above, it reduces the "impact" loading of the teeth coming into mesh. As such, its purpose is to create a benign tooth-to-tooth interface so that locallized bearing stresses are minimized. Here the "bearing" stresses are those encountered as a result of the high locallized loading of one tooth face when it touches another under high torque.

The second purpose of the oil in the tooth contact situation is that unless the teeth are designed with perfect geometry and instalation tolerances, there will always be a small amount of relative motion between the teeth in the reduction drive's operation. Perfect tooth design results in a contact such that when the teeth come together, the faces rotate radially so that the contact motion resembles a cylinder rolling along a surface, with no slipping. In practical application though, this is not always possible so the result is that there will often be a chance for a small amount of slippage.

When oil is placed in a situation of extreme pressure (such as is seen between the contact faces of two gears), not all the oil can move out of the way When the surfaces come together. As such, a very thin "plasitc" film is created that forms a protective barrier between the teeth. This "cushions" the contact, and protects the metal surface in case any slippage occurs.

For this reason it is a good idea that the gears are lubricated with a pressure fed system that sprays directly onto the gears just prior to the mesh. At high speed, a bath system will most likly be insufficient to provide the necessary penetration of the oil into the teeth since the high viscosity of the oil will not allow it to flow fast enough to where it needs to go.

Submerging the gears is not the solution since that may just froth the oil, increasing unnecessarily the heat input on one hand, and reducing its capacity to remove heat on the other. Remember that in many of our applications the gears will turn much faster than what we convetionally see in automotive transmissions. It is therefore important to look at a few additional factors, ones that you would not necessarily need to account for in a more benign application.

Finally, regarding the engine development mentioned above (GAP and others), the problem that I've seen with many of those is that the results, although impressive from a technical standpoint, do nothing to aid the general aviation kit builder to be able to purchase an affordable engine. All the programs thus far have resulted in nice "gee-whiz" technology but the projected costs of the engines, even before certification, are outrageous.

Personally, I think the idea of building on automotive engines is the right solution but I think it will take a bit of development and testing before most of us are ready to really depend on the long term viability of the reduction drive. As a matter of fact, I have no problem with the engines at all. But I would like to see a more resposible effort made in the redrive development field.

I'd like to do this myself but unfortunately I don't have the funding necessary to do so.

Finally, regarding the comment about the fans - 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.

In order for the benefits of the multibladed high speed fan to be realized, the propulsion configuration would have to be attached to an airplane specifically configured to accept such a combination. Placing a multi-bladed fan that will be maybe forty some odd inches in diameter on the nose of a Cessna 172 would not only not work well, it would also be extrememly wierd and look pretty ugly.

But if an attractive configuration could be developed around such a power package, I think it would be the long term solution. The problem though is that in doing so, one would have several development excercises including the airframe, the engine, and of course the prop. Could be somewhat pricey.
 

Robert Young

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Nov 24, 2003
Messages
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Apple Valley, CA
redives

Yesterday I visited with the mechanic who is restoring my Citabria. He also performs maintenance on some police department Enstrom helicopters. The Enstrom uses a poly V belt drive for the primary gear reduction and clutch system. The belt is about seven inches wide and forty eight inches in overall length. This is on the 225hp Enstrom.

I asked about the belt life and he said it is conditional but he replaces the belt at the second engine TBO which means the belt is replaced at 3000hrs. Engine TBO is 1500hrs. He said the belts still looked good at 3000hrs but he replaces them anyway.

This seems to be evidence that a belt drive though seemingly crude is a quite reliable system. Though the complete drive is bulky the mass is no greater than a good gear drive. The belt offers the ability to deal with torsionals as well as provide a speed reduction. I do not know if helicopter rotor service is more or less severe than propeller service but my guess is they are about the same.

The Enstrom is an old design but still in production and a very good helicopter according to those flying them.
 

HeliDev

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Jul 7, 2003
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108
Hey guys just thought Id add my 2c on the belt issue.
Im doing my CPL(h) in a R22. This uses 2 belts with a sprag clutch.
While I know a pilot who had a belt break and tangle up with the other one, causing an emergency landing, on the whole it seems a very light, reliable system.
One difference with a helo is that the belts will drive a transmition which reduces the RPM to the main rotor, while increasing it to the tail rotor.
One more note, with the exception of the Bell 47G and J I dont know of one piston helicopter that doesnt use a belt drive. Enstrom, Robinson, Hughes/Schwitser, all use the belt drive.
 

michal

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Mar 25, 2004
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a breakthrough ?

Very interesting thread.
I recently engaged in a similar conversation on the Express-builders forum. A fellow names Jason Day claims he solved the reduction gear issue and he has a complete solution for auto-engine conversions. He will be in Oskosh in booth 132 and he had already equipped 4 aircraft.
 

StRaNgEdAyS

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Oct 20, 2003
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Northern NSW Australia
Looking at the areas of reduction drives, I am faced with the options of purchasing one from one of the many manufacturers who all say their product out-performs all others, who doyou believe??? Or going the whole homebuilt hog and doing up one of my own.:ponder:
I know Tracy Crooks' unit has recieved great praise from those who use it and seems to be more than happy to provide information to the inquisitive, and Eggenwhosits' belt redrive from his Soob units also seems to have merit (but I hear he is very reluctant to release any engineering data, to the point of being downright rude about it), but I'm looking at putting this on the front of an engine neither unit was designed for.
If I was to do one of my own, since I'm looking at power figures of less than 200HP I'd be entertaining either a wide toothed belt a-la blower redrive or something along the lines of a double row timing chain.
The other thing I'd have to consider, is once I've got the testing and the 40 hours flown off, I'll probably be flying the wings off this poor little plane (hopefully not literally:eek:) so it is gonna have to be up to the stresses it's going to have thrust upon it. I am NOT kind to my machinery, I have a long line of broken car and bike engines to prove it.
From what I've been reading on Orions' site, one of his units would be worth consideration once they are ready for public consumption. :D
 

Stu

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Oct 4, 2004
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Central California
planetary gears

Interesting that planetaries are made of sintered steel. I never heard this before. I spent a year rebuilding some rather large automatic transmissions. The only planetary failures I've seen were the bearings inside the planet gears. Virtually never the gears themselves even when ingesting metal. Evidently Allison did something right with these transmissions. They were coupled to very high torque but fairly low rpm diesel engines, and were used (and abused) very hard. I've seen snapped main shafts and turbine shafts, and a whole bunch of burned clutch packs. But no planetary failures unless something else fragged and got metal into the works first. There are larger planetary assemblies available which could have a redrive designed around. Wouldn't even have to be near as big as the ones I dealt with- those make a ford c-6 look like it came out of a tonka toy in comparison. I'd look at one from an Allison AT 540 transmission- essentially a GM Turbo-Hydro 400 on steroids. I wonder how they would like the sustained additional input rpm's it would see in an airplane? Might need to at least be balanced first? Let the engineers do their calculations and studies by all means, but I know those are stronger than what is currently being used.

Sintered steel is used in firearms developing a huge amount of pressure, so we know it can have some excellent tensile properties at least. Some folks are skittish about using sintered here as well, but I've never heard of failures there, either. From seeing these big bad Allisons and a number of auto trannies, I don't see this as a concern so long as the unit is sized appropriately. I wonder if the parts are x-rayed as part of the QA/QC process?

There can be defects in billet material also; it still would not be completely immune from trouble. And the heat treating required would certainly take extra effort if you did machine from billet. Gear faces need to be hard, yet not brittle. Doable, but not generally something someone can do in a home garage.

I must agree that the redrive remains a critical piece which must be incredibly reliable before I'll want to put my precious rear-end behind it and go play in the sky. But there may be more than one way to acheive that extra margin of safety required. My personal "eyeball" method of engineering (which I would not rely solely upon) would tend to avoid do it yourself machining of planetay gears in favor of properly sized commercially available ones in the first place. These at least already have that part of the engineering done and have a well proven track record in other applications. Not that any change of application can be taken quite so casually.
 

orion

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Thanks for the input Stu - the industrial drives are really a specialized area that I have not investigated to any level of detail - however I think I probably should given your promising report. That certainley could make one rugged system.

One addendum though - about two weeks ago I was informed that many of the current transmissions being produced no longer use sintered gears. The individual did not know which year this changed but was pretty sure that all of today's C6 gears are cut from billet. (Boy I wish now somone would pipe in with a confirmation of this.)

Finally, regarding the above promising post, the one thing that would have to be checked out on the industrial planetaries is the rpm applicability we need versus that of a transmission that is designed for a slower turning diesel engine. High rpm's do interesting things to gears and as such, the teeth geometry has to be specifically cut to make sure that the contact and mesh can handle the high rotation rate.
 

jrdeyoung

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Oct 25, 2005
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here is something to think about(from a car guy). if you are concerned about the sintered steel in automotive planetaries, look to the aftermrket/racing indistry. there are countless upgrades availble to automatic transmissions like ford 4R100 and C-6's. i would be willing to bet alot of these are billet peices, as they have to endure unreal amounts of input tourque. they can also be had in reduction ranges of something like 2.4:1 to 2.8:1. TCI comes to mind but there are many others. just something to consider, some of the racers come off the line with 700+hp @6500rpm in a 3200lb car, thats some serious abuse.
 

orion

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The aftermarket racing products are very impressive and of the highest quality. Many would be applicable to our airborne arena. The only consideration (and it's a major one) is that there is a world of difference when rating a product for street or racing use and determining its suitability for aircraft application. Automobile uses, even racing, do not require life ratings at continuous rpms and power (torque) levels - aircraft do. As such, any such application needs to be very carefully investigated before the selection is made.
 

jrdeyoung

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diffinately true, its a completely different application. i was just pointing out that there are higher strength components available than you would find at any dealerships parts counter. as far as the race peices are concerned, some are drop forged or billet 9xxx series induction hardened. pretty tough, but then that is what they are engineered for. they have to have great impact resistance to resist deformation of the tooth faces, without too much hardness wich wold result in cracks at the root. as far as longevety i think proper lubrication would go a long way here. maintaining a constant oil film and controlling heat is probably 90% of the battle. if i was intent on using a planetary in a homebuilt redrive, i would go staight to performance aftermarket to source parts before using oem.
 

Rhino

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Yours isn't the worst we've seen, so don't feel bad. You can always use the Edit button to go back and correct it.
 

SIIaCanuck

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Does anyone know what type of torsional damping system was used on Gred Geschwender’s Hy-Vo chain drive PSRU? From what I've read, these drives have worked very well in several applications and while quoted as heavy, for those of us in the replica fighter crowd, this is usually not a problem (ever weighed a Merlin 61?). I'd love to see that drive back in production for both Chevy and Ford engines. Make my super-normalised 302 SAL Mustang a much better prospect!

I read about someone suggesting a multi toothed belt drive for redundancy. This is a bad idea primarily as when one belt lets go, it tends to go through the others and cause a large BANG. If anyone wondered why Jan Eggenfellner gave up on his and went back to gears, it's because it did just this. Waaaaay to dangerous! One belt or none for PSRUs.
 

Jeremy Clarkson

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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?
 

flat6

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monaco
in my opinion nothing beats spur gears from automotive gearboxes running on plain bearings lubricated by engine supplied oil. the merlin does this.

thats the easy part. the key is reducing vibrations. the merlin used quill shafts between crankshaft and reduction gears. we need something like that.
 
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