Mazda Rotary - what are the issues to be overcome?

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rv7charlie

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Andrew makes some good points. Some of the other posts seem to be from people who've heard from other people who heard from their wife's brother-in-law's uncle who heard from someone that rotaries are {fill in your choice here}. :) I haven't flown my rotary, but the Renesis is on the motor mount and could fly within a few months, if I could just get 'life' out of the way. I've been around rotary powered a/c and their builders for a couple of decades now.

Conventional wisdom: Rotaries are almost impossibly tough on reduction drives.
Fact: Tracy Crook (founder of Real World Solutions) now has hundreds of hours on a Renesis in his RV-4 (wood prop) using his RWS 2.85-1 drive, with no torsional isolation whatsoever. Now, he will tell you that it's not possible to do this in every scenario, but it certainly blows that myth out of the water.

Cooling: It's an internal combustion engine. Its conversion efficiency of fuel to power is roughly the same as any other IC engine. It's got to be cooled, just like a Subaru, or a Continental, or a Lycoming. Cooling a Lyc should be a piece of cake, especially on a cookie cutter a/c like an RV, right? Search VAF for 'oil cooling' or 'cylinder head temp'. Even an RV/Lyc can be done wrong, and very often is. If someone starts out knowing nothing, and has no template to work from (as is available with an RV/Lyc), then odds of success are even poorer.

3 rotor all-aluminum engine: I'm aware of exactly zero examples that have actually flown. I'm aware of exactly one 2 rotor all-aluminum example that's actually flown (though there may be others). That one likely has less than 50 hrs on it (used to set a time to climb record), before being retired. So impossible to even speculate on odds of success. And honestly, if it *does* work, your market size could likely be estimated using just your own personal appendages, since the price of an aircraft-prepped turbo'd all-aluminum 3 rotor with reduction drive and cooling system will be very close to (possibly higher than) the price of a Lyc/Conti with the same HP.

Uncertainty on one or two turbos: What's the logic for twin turbos, on a rotary in an a/c? And FWIW, assuming that the custom aluminum end housings can survive continuous high output, a 3 rotor engine (if P-ported) will make ~300HP without turbos. Cooling it will be another matter entirely, turbo or not. (Aluminum end housings will not make a significant difference in the ability to cool the engine.)

Auto-PSRUs: I spoke with them at SNF several years ago. Tread very carefully. Find a real mechanical engineer with at least a couple of decades of power transmission experience, and pay him several thousand dollars to evaluate it, before investing in one. Here's one 'tell' on their design: their torsional decoupler (clutch) they use approaches the weight of an entire RWS reduction drive, *including* its decoupler mechanism. Based on my conversation with them several years ago, they knew literally less than nothing about rotaries. Several things their rep tried to tell me about them was just...wrong. At that time, all they had was a dummy engine with their drive bolted over the end of the engine; it had never even run on a rotary. Unless things have changed radically in the past few years, I wouldn't consider flying with one. While reduction drives are not *easy*, they aren't rocket surgery either, for a qualified engineer. The RWS design, probably the most-flown reduction system ever, is effectively open source now; anyone can get a copy of the drawings. Just something to consider....

Noise: With straight pipes, the 13Bs really are painfully loud, due primarily to the high frequency harmonics caused by the exhaust port snapping open (on the 13B), very much like a 2-stroke engine's sound. But I'm aware of at least three flying examples that are no louder than a Lyc (though the *noise spectrum* might make them sound louder to some ears). One example (13B), in an SQ2000 pusher, used the stock cast iron exhaust manifold (~12 lbs) coupled to a short stack pointed aft through the prop. It sounded eerily similar to a small block V-8 with straight pipes. In another example (Renesis, still flying), you can barely hear the engine at all; virtually all noise is from the prop, very similar to a turbine. The entire exhaust system on that plane, from engine block to tailpipe, is around 20 lbs. A properly engineered muffler system could be significantly lighter. Perhaps Billrsv4 will chime in, or you can search here for his posts to see an example.

Strongly advise signing up for the Flyrotary email list; that's where you will find guys that are actually flying the engine.

Charlie
 

Vigilant1

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3 rotor all-aluminum engine: I'm aware of exactly zero examples that have actually flown. I'm aware of exactly one 2 rotor all-aluminum example that's actually flown (though there may be others). That one likely has less than 50 hrs on it (used to set a time to climb record), before being retired. So impossible to even speculate on odds of success. And honestly, if it *does* work, your market size could likely be estimated using just your own personal appendages, since the price of an aircraft-prepped turbo'd all-aluminum 3 rotor with reduction drive and cooling system will be very close to (possibly higher than) the price of a Lyc/Conti with the same HP.
Charlie, what's your assessment on the difference between an all AL engine and the iron-rotor-with-AL-housing stock design in the case of coolant loss? Obviously, avoiding coolant loss is the best answer, but I've heard of cases where an engine with iron rotors ran a long time after loss of coolant, due to the higher expansion rate of AL vs steel the engine didn't seize up (obviously, needed a lot of attention afterward). In an airplane, this seems like a positive attribute.
 

Tornado Flyer

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I would like to see a single rotor conversion with 80 - 100 hp. Seems a two-rotor engine could be used. Remove one rotor and use a different crankshaft and intake and exhaust.
 

rv7charlie

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I would like to see a single rotor conversion with 80 - 100 hp. Seems a two-rotor engine could be used. Remove one rotor and use a different crankshaft and intake and exhaust.
Here you go:
http://www.rotarypragma.com/
Richard is one of the smartest guys I know, and he has over 2 decades of development in his engine. One problem with a single rotor rotary is weight. With 2 rotors, each supplies part of the counterbalance for the other. With a single rotor, you're forced to carry 'dead' weight equivalent to the rotor as balance, and you still need 2/3 of the end housings, instead of 1/2. And one of the few very real potential downsides to a rotary is having only 2 power producing elements (instead of 4, 6, etc). If you start with one, *any* problem stops output right now. Torsional issues go up a lot (more weight to compensate). Etc etc. In the end, a pair of smaller rotors is just...better... than trying to cut down a 12A or 13B.

Charlie
 

rv7charlie

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Charlie, what's your assessment on the difference between an all AL engine and the iron-rotor-with-AL-housing stock design in the case of coolant loss? Obviously, avoiding coolant loss is the best answer, but I've heard of cases where an engine with iron rotors ran a long time after loss of coolant, due to the higher expansion rate of AL vs steel the engine didn't seize up (obviously, needed a lot of attention afterward). In an airplane, this seems like a positive attribute.
If you're asking about using aluminum rotors, I've seen speculation about what it would be like to make them from some unobtainium aluminum alloy, but I'm not aware of it ever being done in a production engine. (Just so the non-rotary guys are on the same page, the stock engine has chrome plated aluminum rotor housings (the section the apex seals run on), while the 'end housings' (front, middle and rear plates) are iron. The rotors themselves are also iron.)

You're right about the 'stock' engine's ability to run without coolant; I've experienced it in an RX-7 car on the highway and I know someone who's lost coolant in an a/c. But I really don't have any idea about the effect of all-aluminum end housings; edit/correction: even Mazda didn't do it with the LeMans winner. (just looked up the engine to refresh my memory.) The 16X prototype engine is all-aluminum, but they haven't pulled the trigger on production. So, who knows? Mazda might, but I doubt they'll talk about it.

Charlie
 
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RSD

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Ok, I'm going to try and be tactful, But your comment shows you really need to do more homework or your just going to be giving away your kids inheritance. I don't want to curb enthusiasm but from your posts I feel your looking at the rotary for all the wrong reasons. have you even gone outside & done market research and talked to some rotary flyers? You don't have to start from scratch, Search for and purchase an existing RV with rotary, get some experience with it before putting in your own firewall forward package and testing that for at least 1000hrs before even considering selling another half arsed package that just ends up giving alternative engines a bad name.
I built & fly behind a 13b Renesis & have a 20b ready for next plane. So I have a little experience in these, I love the rotary, and reckon it is a great aircraft engine and better than ever it was as a car engine. I built mine because I could, not because I ever thought it would be better or cheaper.
Everytime I hear someone say they want to use automotive engines because they cant afford a similar power Lycoming, I laugh, just shows naivety towards their project. basically power costs money. these also cost time. end result is similar or more, very rarely less.
As to your quote above, The only way you can direct drive a rotary is to have a ducted fan, that's been done, but not persisted with.
Andrew
Looks like there has been lots of posts in the last 24 hours so I will reply to each in turn.

Fortunately I don't have kids so not worried about the inheritance - might as well spend it as apparently you can't take it with you when you go...

I've talked to some rotary fliers but seeing as you are just down the road (in Australian terms) I will shoot you a message and see if we can arrange a time for me to give you a call.

In this case it is a firewall back case initially so different airflow/cooling challenges but not unsolvable.

Have read up about Mick/Duckt's ducted fan, but that seems more like a ducted prop than a fan you would choose for a ducted fan.

Will message you.
 

RSD

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Some of the other posts seem to be from people who've heard from other people who heard from their wife's brother-in-law's uncle who heard from someone that rotaries are {fill in your choice here}.
That's something that we have become very aware of.:(

I haven't flown my rotary, but the Renesis is on the motor mount and could fly within a few months, if I could just get 'life' out of the way. I've been around rotary powered a/c and their builders for a couple of decades now.
What HP are you aiming for and which PRSU are you using?

Conventional wisdom: Rotaries are almost impossibly tough on reduction drives.
Fact: Tracy Crook (founder of Real World Solutions) now has hundreds of hours on a Renesis in his RV-4 (wood prop) using his RWS 2.85-1 drive, with no torsional isolation whatsoever. Now, he will tell you that it's not possible to do this in every scenario, but it certainly blows that myth out of the water.
A lot has to do with the balancing of the engine, and that is something that my engine guy has a lot of experience in (Mistral engines used to fly their engines from their base in Europe out to him in Australia to get him to balance them for him which gives you some idea of his balancing knowledge)

Cooling: It's an internal combustion engine. Its conversion efficiency of fuel to power is roughly the same as any other IC engine. It's got to be cooled, just like a Subaru, or a Continental, or a Lycoming. Cooling a Lyc should be a piece of cake, especially on a cookie cutter a/c like an RV, right? Search VAF for 'oil cooling' or 'cylinder head temp'. Even an RV/Lyc can be done wrong, and very often is. If someone starts out knowing nothing, and has no template to work from (as is available with an RV/Lyc), then odds of success are even poorer.
Rotary cooling is somewhat similar to a liquid-cooled piston engine, but you have to pay a lot more attention to the oil cooling.

3 rotor all-aluminum engine: I'm aware of exactly zero examples that have actually flown. I'm aware of exactly one 2 rotor all-aluminum example that's actually flown (though there may be others). That one likely has less than 50 hrs on it (used to set a time to climb record), before being retired. So impossible to even speculate on odds of success. And honestly, if it *does* work, your market size could likely be estimated using just your own personal appendages, since the price of an aircraft-prepped turbo'd all-aluminum 3 rotor with reduction drive and cooling system will be very close to (possibly higher than) the price of a Lyc/Conti with the same HP.
I think that one of the engines that Tracy is currently using is an all-aluminium engine (I could be wrong but I thought that I did read that somewhere).
Price might be higher than a conventional Lyc/Conti, but certainly will be cheaper than a FADEC Lyc/Conti

Uncertainty on one or two turbos: What's the logic for twin turbos, on a rotary in an a/c? And FWIW, assuming that the custom aluminum end housings can survive continuous high output, a 3 rotor engine (if P-ported) will make ~300HP without turbos. Cooling it will be another matter entirely, turbo or not. (Aluminum end housings will not make a significant difference in the ability to cool the engine.)
My engine guy did suggest that two smaller turbos was possibly the better way to go - but I can't remember why. A PP 3 rotor will make 300hp at ground level, but without a turbo it will struggle past 12,000 feet

Auto-PSRUs: I spoke with them at SNF several years ago. Tread very carefully. Find a real mechanical engineer with at least a couple of decades of power transmission experience, and pay him several thousand dollars to evaluate it, before investing in one. Here's one 'tell' on their design: their torsional decoupler (clutch) they use approaches the weight of an entire RWS reduction drive, *including* its decoupler mechanism. Based on my conversation with them several years ago, they knew literally less than nothing about rotaries. Several things their rep tried to tell me about them was just...wrong. At that time, all they had was a dummy engine with their drive bolted over the end of the engine; it had never even run on a rotary. Unless things have changed radically in the past few years, I wouldn't consider flying with one.
I received an answer back today about how many hours of testing that had done with their PRSU with rotaries and wasn't satisfied with the numbers so will be looking elsewhere.

The RWS design, probably the most-flown reduction system ever, is effectively open source now; anyone can get a copy of the drawings.
OK I certainly wasn't aware of that! Thanks!

Noise: With straight pipes, the 13Bs really are painfully loud, due primarily to the high frequency harmonics caused by the exhaust port snapping open (on the 13B), very much like a 2-stroke engine's sound. But I'm aware of at least three flying examples that are no louder than a Lyc (though the *noise spectrum* might make them sound louder to some ears). One example (13B), in an SQ2000 pusher, used the stock cast iron exhaust manifold (~12 lbs) coupled to a short stack pointed aft through the prop. It sounded eerily similar to a small block V-8 with straight pipes. In another example (Renesis, still flying), you can barely hear the engine at all; virtually all noise is from the prop, very similar to a turbine. The entire exhaust system on that plane, from engine block to tailpipe, is around 20 lbs. A properly engineered muffler system could be significantly lighter. Perhaps Billrsv4 will chime in, or you can search here for his posts to see an example.
I can certainly remember the noise of a 13B - I spent a few years in the navigators seat of several 13B powered forest rally cars.

Strongly advise signing up for the Flyrotary email list; that's where you will find guys that are actually flying the engine.
I've tried to join that list a few times but the email keeps bouncing back - it seems like the email address is no longer registered
 

wsimpso1

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I have some technical disagreement on some of the comments...

Some very smart folks have had trouble solving the vibe problem from Wankel rotaries, but it has been solved too. In cars, the dampers look conventional, but they are not the same as for piston engines of similar power and firing frequencies. For each builder, I would want to see evidence that their system is durable.

The RWS does have vibration management - there is a set of rubber bushings that do serve as a rising rate spring set between the engine and gear set. This will lower the resonant frequency - usually a good thing as it will drive resonance to a lower rpm.

The AutoPSRU system has two vibe control systems: First is the clutch that does not engage until rpm gets above some target level. that level is usually set below idle; Second is that it has a spring hub with coil springs imbedded in polyurethane, giving it a rising rate - usually a good thing. This too should drive resonance to low rpm...

Billski
 

rv7charlie

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Billski,

Just about all of the RWS drives that are in service have the rubber bushing dampers you describe. But shortly before he retired, Tracy redesigned the input shaft on the 2.85-1 drive to bolt directly to an aluminum racing flywheel. That's what has been flying on his Renesis powered RV-4 for at least 5 or 6 years. He has *not* converted the drive on his 20B (3 rotor) powered RV-8 to direct coupling, and doesn't recommend it for high MMI props. He did offer the updated version for sale, but retired shortly after it was developed, so I'm not sure how many (if any) actually made it out into the field. My info is from in-person conversations with him (I've flown with him in the -4), but IIRC, there should be some discussion on the Flyrotary list if you can find your way around its archives.

Charlie
 

rv6ejguy

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I believe Tracy discussed development of his drive with built in "excessive" backlash clearances to put the TV impulses developed at certain rpms out of phase with reaction from the prop inertia. He considered this vital in making it all work. I can no longer find that reference though.
 

rv7charlie

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That's something that we have become very aware of.:(


What HP are you aiming for and which PRSU are you using?


A lot has to do with the balancing of the engine, and that is something that my engine guy has a lot of experience in (Mistral engines used to fly their engines from their base in Europe out to him in Australia to get him to balance them for him which gives you some idea of his balancing knowledge)


Rotary cooling is somewhat similar to a liquid-cooled piston engine, but you have to pay a lot more attention to the oil cooling.


I think that one of the engines that Tracy is currently using is an all-aluminium engine (I could be wrong but I thought that I did read that somewhere).
Price might be higher than a conventional Lyc/Conti, but certainly will be cheaper than a FADEC Lyc/Conti


My engine guy did suggest that two smaller turbos was possibly the better way to go - but I can't remember why. A PP 3 rotor will make 300hp at ground level, but without a turbo it will struggle past 12,000 feet


I received an answer back today about how many hours of testing that had done with their PRSU with rotaries and wasn't satisfied with the numbers so will be looking elsewhere.


OK I certainly wasn't aware of that! Thanks!


I can certainly remember the noise of a 13B - I spent a few years in the navigators seat of several 13B powered forest rally cars.


I've tried to join that list a few times but the email keeps bouncing back - it seems like the email address is no longer registered
Tracy's Renesis powered RV-4 performs at power levels roughly equivalent to a 200HP Lyc, but comparisons are difficult due to radical differences in cooling methods, etc. The engine is rated at around 210 HP in the car; I'll be happy with 180-190HP from the engine, hoping that I can keep cooling drag to a minimum. As a general principle, I would never expect more from an automotive-derived powerplant than the mfgr expected from it. Thinking that we can 'hot rod' an auto engine and then expect it to survive at the sustained high output levels required in a/c has never struck me as a reasonable idea. I'm using a RWS RD1C 2.85-1, (6 pinion planetary gearset); the same model is running on Tracy (RWS) Crook's normally aspirated 20B in his RV-8, and has run for hundreds of hours on a P-ported 20B/MT electric prop in a Lancair ES (though that drive endured a *lot* of abuse).

Balancing: While it's always a good thing to reduce vibration, I think you'll find vibration has only a tangential relationship to torsional resonance. Billski just posted here; perhaps he'll weigh in.

I know Tracy pretty well; if he's running an all-aluminum engine on anything, his black-ops operation has better secrecy than any military operation. :) Both his Renesis and his 20B are very-nearly stock automotive 'short blocks'.

Turbo: All depends on your goals. If you *need* 300 HP at altitude, then yes, you'd need a turbo. But how much fuel are you carrying in your design? 300 HP of turbo'd power will burn between 150 & 180 lbs of fuel (25-30 gallons) per hour. A NA 300 HP engine will still be making close to 200 HP at 12K feet. How much airframe are we talking about?

If you want, I'll try to contact the Flyrotary list admin to get you signed up. You can PM me the email address you want to use.

Charlie
 

rv7charlie

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I believe Tracy discussed development of his drive with built in "excessive" backlash clearances to put the TV impulses developed at certain rpms out of phase with reaction from the prop inertia. He considered this vital in making it all work. I can no longer find that reference though.
That's essentially correct, though the term 'excessive' might be a bit misleading. The inherent lash in the planetary gearsets, combined with the very low impulse strength around idle rpms of the rotary, allowed relatively small dampers, got resonance freq below desired a/c engine idle rpm,and eventually led him to try the direct-coupled idea.

Powersport decided to get resonance *above* any likely operating rpm, while Tracy decided to get resonance *below* any likely operating rpm. My understanding is that he chose that route because it could be lighter, much less expensive to make, and didn't have the risk of a 'tight' (Powersport) geartrain getting 'loose' (lowering resonance freq into operating range) with wear and/or heat.
 

RSD

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Tracy's Renesis powered RV-4 performs at power levels roughly equivalent to a 200HP Lyc, but comparisons are difficult due to radical differences in cooling methods, etc. The engine is rated at around 210 HP in the car; I'll be happy with 180-190HP from the engine, hoping that I can keep cooling drag to a minimum. As a general principle, I would never expect more from an automotive-derived powerplant than the mfgr expected from it. Thinking that we can 'hot rod' an auto engine and then expect it to survive at the sustained high output levels required in a/c has never struck me as a reasonable idea.
Certainly agreed - whether it is a LS3 or a rotary. The way to significantly more power in a rotary is via adding more rotors and small turbos rather than developing the hell out of it.

Turbo: All depends on your goals. If you *need* 300 HP at altitude, then yes, you'd need a turbo. But how much fuel are you carrying in your design? 300 HP of turbo'd power will burn between 150 & 180 lbs of fuel (25-30 gallons) per hour. A NA 300 HP engine will still be making close to 200 HP at 12K feet.
It's about having the power available when you want or need it, and then cruising when it suits for lower fuel burn.

How much airframe are we talking about?
A Berkut on steroids

If you want, I'll try to contact the Flyrotary list admin to get you signed up. You can PM me the email address you want to use.
I'm finally sorted now - was actually trying to join the wrong list.
 

wsimpso1

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That's essentially correct, though the term 'excessive' might be a bit misleading. The inherent lash in the planetary gearsets, combined with the very low impulse strength around idle rpms of the rotary, allowed relatively small dampers, got resonance freq below desired a/c engine idle rpm,and eventually led him to try the direct-coupled idea.

Powersport decided to get resonance *above* any likely operating rpm, while Tracy decided to get resonance *below* any likely operating rpm. My understanding is that he chose that route because it could be lighter, much less expensive to make, and didn't have the risk of a 'tight' (Powersport) geartrain getting 'loose' (lowering resonance freq into operating range) with wear and/or heat.
This powertrain vibe geek finds this sort of stuff "uphill"

First, balancing of the rotating components is useful for keeping the vibe you can feel out of the airplane, but it does NOTHING for torsional vibe. Torsional vibe is kept from destroying things by keeping resonant mode frequencies away from forcing frequencies. Putting a deliberately "soft" spring between the flywheel and the rest of the system produces a low first order, and usually lets you use stuff with lash down stream. Millions of cars and trucks made this way every year. The other way is a "stiff" system, and everything has to to vibrate together... Props and flywheels solidly bolted to beefy crankshafts are an example.

Build a system with a low Inertia propeller and no soft springs sound like a "stiff" system, yet the Tracy Crook scheme has and acknowledges lash in the gearset and splines. This is where it goes "uphill" - the whole system accelerates and decelerates with the firing pulses, which will try to open and close the lash, with predictable impact loading on every firing pulse. Small lash, and maybe the impact is low enough, but has the hazard that the splines and gears will crash on both coast and drive sides on every firing pulse. Large lash will prevent crashing on the coast side, but will have more impact on the drive side... then if you are trying to time the impacts away from each other, well, you might get away with that at a very narrow range of speeds. What it sounds like to me is that the no bushing approach is in a system that is beefy enough to stand the lash opening and closing on firing pulses. That six pinion planetary is quite the sturdy gadget. If the rest of the box is as sturdy as the planetary and if prop Inertia and flywheel Inertia are kept down, maybe it can be durable at reasonable weight. Since I believe in demonstrated durability, particularly on unusual designs, I shall await a couple of them running a few hundred hours each on flight duty cycles...

Billski
 

rv6ejguy

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I think Tracy said the high backlash road wasn't his first choice and it doesn't sound like a nice way to solve the issue but his drives have accumulated thousands of hours collectively on Wankels with good reliability.

The 3 rotor will have different TV characteristics from 2 rotor engines, could well function fine without the damper plate installed and so far seems to be according to Charlie. After a couple hundred hours of flight we usually know whether it's a big fail or not. Long term life won't be known until it flies long term- like 1000-2000 hours.
 

rv7charlie

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The non damper drive is on the 2 rotor Renesis. The 3 rotor still has the dampers; higher mass large 3 blade prop to handle the higher hp
 

wsimpso1

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I think Tracy said the high backlash road wasn't his first choice and it doesn't sound like a nice way to solve the issue but his drives have accumulated thousands of hours collectively on Wankels with good reliability.
Oh, understand that I have no quarrel about Tracy's damperplate equipped 2 rotor designs- they are well demonstrated. Their design makes much sense to me.

I am only concerned over durability of the damperless version.

The 3 rotor will have different TV characteristics from 2 rotor engines, could well function fine without the damper plate installed and so far seems to be according to Charlie. After a couple hundred hours of flight we usually know whether it's a big fail or not. Long term life won't be known until it flies long term- like 1000-2000 hours.
Agreed

Billski
 

rv7charlie

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Again, the 3-rotor RV-8 still has the dampers (same reduction drive assembly).

If memory serves, Tracy has accumulated a couple hundred hours since converting to the non-damper setup on his Renesis powered RV4. It's still being flown. He had enough confidence in it to start marketing it (he's one of the very few vendors who wouldn't sell anything he hadn't flown & come to trust), but retired shortly after.
 
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