Mohawk Aero

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About Mohawk Aero

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  • Birthday 06/09/1954

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  • Gender Male
  • Location Savannah, GA
  • Interests Yamaha powered aircraft

Mohawk Aero's Activity

  1. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    But then, that motor was oil cooled, and racing temps hit 300°.  No Yamaha aircraft conversion should ever see those kind of grueling conditions, unless something is wrong.  I asked Steve Henry about his operating temps, water side, and he said he's never seen anything over 170-180°.  I use the idiot lights on the stock multi-function display, the only time it ever gets hot is sitting still doing tune ups.  So I started putting an electric fan over the radiators when I tune the motors.  All of the new cars run 20 weight synth.  I don't think running 50 weight synthetic is such a great idea for any Yamaha engine, in any temperature, so long as it has less than 1000 flight time hours on it.  Unless you plan on running it flat out all the time.  I guess then it might make sense.  I know of a couple of guys who run YG3s with shorty exhausts 3-into-1 into a can, no mid-range power to speak of, so they run WOT all the time.  Seems strange to me, but that's what they do.  Some people won't listen to reason when told the exhaust is a poor design and needs to be lengthened.  
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  2. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    Hmm.  Ya know, NASCAR crews run 20w synth for time trial laps, then replace with 50w synth for the race.  I always ran 50w Mobil 1 in my GT1 turbo porsche when I was racing.  In the Yamaha motors I have never run anything but 5w-20 Mobil 1, changing oil every 25-30 hrs, and changing K&N oil filters every other oil change.  I have noticed that as the oil ages it does tend to disappear a bit more toward the end of that oil change cycle I use, perhaps using heavier oil would slow that down?  It isn't but an ounce here and there though, maybe 1/2 cup at most after 20 hrs.  
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  3. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    OR you can just ask me for the tool.  I sell it to you, you use it, and then you send it back to me and I refund your money.  
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  4. Mohawk Aero added a post in a topic RX1 Motor Mount   

    I don't understand the big diameter tubes, since they are heavier (and clunky-looking).  Instead of making the tabs, or ears, like this, you make them large enough so you drill a hole through them, slip them over the rail tube, and weld a complete circle around the tube.  This is how it is has been done on gyroplanes since 2011. 
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  5. Mohawk Aero added a post in a topic YAMAHA APEX (EFI) 150HP 118# Engines   

    2010-2011 Ernie Boyette of Rotor Flight Dynamics installed an APEX on a new Dominator gyrocopter with an Auto Flight gearbox and custom aluminum cast adapter which fit over the crankcase cover.  A clutch was later fitted, I think it was a sprag type.  The gyrocopter was sold and I've lost track of it since then, last I heard it had 50+ hours and that was many years ago.  (After, Ernie and I both thought the RX1 was the way to go for a while, since it was simpler electronically so I built an RX1 conversion on a 1980's  Air Commander gyroplane the following year, 2012, using an Arrow gearbox with internal sprag clutch, on an aluminum adapter cut on a water jet table with a universal adapter plate for use with any other gearbox combination.)   
    Steve Henry has an acquaintance who is flying an APEX with a cog belt PSRU.  The conversion reportedly has 300+ hours on it, trouble-free.  You might want to contact Steve for more information, he sent me a copy of his wiring notes he got from this fellow. 
    Steve's APEX engine was running on a test stand in early 2017 with a prototype adapter I made for him for a Rotax C gearbox, and one of my prototype clutches I gave him for free to test.  We discovered that the clutch had to be redesigned to accommodate a second set of idler bearings due to gearbox vibration.  The clutch was removed and I think he then continued running the engine with either an RK400 clutch or maybe even a rubber donut.  He then started experiencing intermittent shut-downs on the stand, and traced the problem back to a faulty ECU harness plug - apparently someone had stuck a multi-meter probe into the plug and bent some of the tiny brass connector strips inside, causing the thing to disconnect while running and vibrating.  
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  6. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    GOOD NEWS!  I finally found the part number for the Rotax C gearbox bearings:  25mm, 6306.  Using this info I updated and found that 90% of the bearings will last to 1000 hrs TBO.  Whew, that's a huge relief.  

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  7. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    Attached here is the bearing data that indicates Rotax C bearing life when used on an RX1 engine, as well as info for all other PSRU's in use today by every other shop selling RX1, Apex and Nytro  conversions today. The bearings in the  Rotax C box  when mounted to a Yamaha RX1 engine turning 7200-9000 ERPM / 6000 - 7500 SRPM @ 90 lbs torque at the output shaft have a life of 310 hours with 90% certainty.  That data is arrived at by using the exhaustive formulas published by every major bearing manufacturer in the world, and the information how to get to that answer is all over the web.  I have arrived at this conclusion using the NTN engineering handbook, and it is available in PDF for free by contacting
    You are invited and more than welcome to do the research and math in order to check my work.
    The calculations include a torque adder of 6% for the additional radial load that the bearings see due to the gear tooth angle of contact (not to be confused with a 19.8 degree helical cut, and the resultant axial loading); and a prop harmonics fudge factor of 10%.  Even if there are no harmonics present, then the best one can hope for is roughly 340 hrs TBO.  While we're at it, let's take out the 6% adder, just in case:  still just about 360 hrs, hm? 
    It is probably a good thing that the Rotax C must be disassembled for removal to inspect a RK400 clutch every 75 hours, that way you will be made aware of the condition of the bearings at the same time.  
    As to the alternative Rotax Hardy disk, we are aware of catastrophic failure of those at  just 12-15 hrs under normal operating conditions and therefore you need to inspect your rubber Rotax donut by removing it and looking for cracks - every ten hours.  Once again, you can check your gearbox bearings at the same time.
    When you put the Rotax C box back together every 10, or 75 hours (depending on what kind of coupling you are using), after inspections be sure to put the shims back in proper order and install a new gearbox gasket as necessary.  The datum indicate, and real-life experience bears out, that loading a single Rotax C bearing in the axial direction, and not getting both bearings loaded equally, results in a failure almost immediately.  
    PS:  I deleted the earlier post, as it did not go far enough in presenting hard-core engineering evidence, while at the same time going too far in the irritation direction.   

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  8. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

  9. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    How ironic is this:  before I brought to his attention the fact that the RK400 guys require a tear-down inspection of their clutch every 75 hours, Teal didn’t know anything about it.
    This is a fundamental issue and concern for anyone making comparisons when considering available choices to hot-rod their aircraft by installing a Yamaha beastie. 
    Teal has designed and fabricated (2014) one thing, and one thing only, to date: an RX1 adapter which fits a Rotax C gearbox; which requires either a Rotax Hardy Disk (flex shaft coupling) or an RK400 aftermarket clutch designed specifically for use with this gearbox. 
    I designed and fabricated the first RX1 adapter (2012) two years earlier, which I used extensively with first an Arrow gearbox with internal sprag clutch, then an Air Trikes SPG4, more recently the Mohawk AK7 (based on the SPG4), and a single, lonely, solitary Rotax C in all of my 6 years of production of Yamaha adapters.
    Then in 2015 I designed, built and flew the pants off my Hy-Vo chain Silent Drive, with a prop-strike protection clutch no less.  The chain and gears have been field tested to over 300 HP.  It was perfect right off the bench, and remains so to this day.  It even suffered a prop strike, and the engine was saved, as was the PSRU!!!  That is not just a victory, it is HUGE, and I have bragging rights that no other PSRU builder known to man to this day can claim as a result of this major, accomplished work let alone a guy who has built and sold but a single design anything in the same period, to date. 
    There have been other, successful, PSRUs using Hy-Vo chain.  There have been other, successful PSRUs built with prop strike protection break-away clutches (Rotax 914 option $1300.00) but no one has ever put the two together before, let alone successfully. 
    That PSRU is attached to the Yamaha RX1 engine in the same, exact manner that my adapters are.  Not only that, it (because it is a prototype) was put together in sectional layers – FIVE OF THEM!
    Six years, unknown hundreds of hours, 100% perfect track record with ZERO evidence whatsoever to support the imaginary crap that Harvey the invisible rabbit dreamed up regarding adapter alignment and slip.  Someone is blowing smoke, and it is not me.  The ONLY advantage of a Skytrax one-piece adapter that can be proven by anyone in this room is that a one-piece adapter coming hot off a CNC table is ready to go.  Teal doesn’t have to do one more thing to it when it arrives in his hands from the sourced shop.  My collar-and-plate adapter requires several days of water-jet, milling, and aligning and pinning everything together.  Once that is done, anyone at all can install it, aligned correctly and reliably, just like a Skytrax adapter. 
    The only discernable advantage in a Skytrax adapter is to the seller himself, not the customer.  It does look pretty, so two points to Slytherin for aesthetics.  However, any advantage at all is lost to Gryffindore because the adapter is strictly limited to use of a Rotax C box and it’s associated shaft couplings which are proven beyond any doubt, by any yardstick, to be inferior to what can be bolted to a Yamaha using a Mohawk adapter.  It's all about choices, guys, and not being forced into a corner by your vendor.
    I don’t know how many Skytrax adapters have been sold/installed, and could care less.  Neither do I know or care if Teal has ever purchased for resale any RK400’s to go along with the sale of one of his adapters, and a gearbox.  No one can argue that one way or another RK400 clutches have been a topic of detailed discussion that Teal has been aware of and engaged in, often and long enough such that by now, at this late date, he has had more than ample opportunity to have done his homework and thoroughly familiarized himself with this unit which is so integral and crucial to his product sales - a component which is a primary, basic device required by nearly all of his customers from day one.
    With only one thing to do and sell for the last 2-3 years, how could this not be part of his knowledge base, how could this escape his attention?  The man’s got his agenda, that’s moot:  promote the pants off one gearbox, and one gearbox only, to the exclusion of all else.  He’s stickin to it, head in sand, that’s obvious. 
    It has become crystal clear that Teal doesn’t know the first thing about the clutched coupling required for his product – not at least until I told him so last month, and there’s the irony.
    Which begs the question: what does Teal know about the only other alternative to the RK400 clutch, the Rotax Hardy disk? 
    More importantly, what is he willing to tell you about it? 
    Last APRIL 2017 Teal Jenkins and I were both told by the same source, separately, this account:
    This prominent STOL builder/pilot tried to use a Hardy Disk with a 74” Prince prop, powered by a YG4 (RX1) using a Skytrax adapter.  After just 12 hours or so, he noticed something wrong and took everything apart.  The brand new Hardy disk had ripped through and through – catastrophic failure.
    There, maybe now it will return on this forum in a search for HARDY DISK FAILURE connected to a YAMAHA engine using a ROTAX C gearbox. 
    You see, before I came here in response to someone else already ripping me, I searched “Hardy disk failure” and then simply “Hardy Disk” on here and no news of that failure appeared anywhere.
    How many posts has Teal put up here since last April?  How much news has he generated in that time to educate you folks on the known dangers of using a Hardy disk in conjunction with his adapter? 
    I should have posted it here, but I really expected Teal to do and I didn’t want to come off as a prickly, nay-saying competitor bashing inferior equipment.  
    I did post the failure on Rotaryforum 4/28/17.  It got 120 looks, zero replies.  I also posted it on my Mohawk Aero Facebook group (Yamaha Aircraft Engines).
    I have built an adapter for only one customer using a Rotax C gearbox (RXC).  I don’t make enough money off gearbox sales to make it worth my trouble to promote any one over the other, they are all the same to me in that regard. 
    But I don’t use RXC not because this has anything to do with my competitor here, but rather because they don’t meet my spec - in any category.  If they met my spec, I’d use ‘em and sell ‘em till the cows came home, trust me.  I was building and selling RX1 adapters two full years before Teal Jenkins entered the scene as my direct competition.  No one, by any stretch of the imagination, can ever accuse me of sour grapes due to his intervention in my livelihood - the timeline simply doesn’t add up. 
    To date, my numerous choices for PSRU (including building my own Silent Drive) have been spot-on and I have been proven right, without a shadow of doubt, exactly 100% of the time with ZERO FAILURES of them in six years. 
    I say again, for the hard of hearing:  ZERO PSRU FAILURES in six years and countless hundreds of hours flying time by myself and my customers. 
    There have been no less than four documented accounts of Rotax gearbox bearing failure during the same period, at <50 hrs, 200 hrs, 300 hrs and 400 hrs. 
    Perhaps one of the most esteemed EAB folks in the country has suffered catastrophic failure on multiple occasions using Teal Jenkins’ combination of adapter, shaft connections and RXC.  One such failure was explained away as perhaps builder error.  I ask you, if a professional builder can’t put the RXC back on his adapter properly, how can Joe the Weekend Warrior be expected to do it every 75 hours?   
    Teal explained a RXC bearing failure on one of his kits to me this way:  The drive gear shaft bearings are so small that if the axial loading due to the helical cut of the gear faces loads only one of the two shaft bearings, that bearing is going to fail 100% of the time, guaranteed. 
    Other RXC experts and professionals have agreed, saying much the same thing. 
    The gearbox is an inferior design, plain and simple, for use with a 140 HP Yamaha RX1, let alone a 150 HP APEX.  You are required to tear it apart and remove the drive gear, shaft and shims just to install it.  Then you run the risk of not shimming the thing right when you put it back together, and that will destroy the bearings because taken alone they are too small to bear the load.   
    Perhaps you might not know that the gearbox was designed and assembled in the first place made to turn in the opposite direction that our Yamaha engines turn.    
    There have been countless other RXC failures, but they have largely been kept under wraps by self-serving dealers who fix and sell them.  There are so many accounts of RK400 excessive wear, spring failure, dog failure, screws coming loose and falling out that it is pointless to go into all the lurid details and list them individually here.  You all know what those issue have been, the risks involved, and you continue to use them anyway. 
    What I have brought to some eyes – including Teal Jenkins - is the intensive maintenance tear-down inspection requirements RK400’s carry with them.  Inspection requires an hour or two of labor, a new RXC gasket, and fresh oil (which needs to be changed anyway) and then WHEN – not if – it needs new shoes it will cost $150 (w/ S&H) for a new set, be that at 75 hours, 200 hrs, or whatever.  This is a certain eventuality much as any other friction shoe clutch or brake set requires. 
    The reason I don’t use Rotax C has nothing to do with Teal Jenkins or his tunnel-vision adapter.  My reasoning is/was/has been based on the facts below considered when first exploring various PSRU solutions, and should be the same facts anyone considers when building a high-performance aircraft conversion engine and installation.
    First, Rotax C (RXC):   
    1.     Numerous known bearing failures under 400 hours, on Yamaha and other engines
    2.     not rated for any horse power. 
    3.     rated for prop inertia mass of 6000kg/sq cm.  (Check with your prop dealer to make certain your prop is within this spec.)
    4.     Shaft OD and bearing ID size is 25 mm diameter
    5.     requires disassembly in order to install or remove it
    6.     Associated (alternative) RK400 clutch requires tear down inspection every 75 hrs, which requires RXC tear down.  RK400 clutch shoes will wear out, and replacement cost is $150.  Numerous documented failures when used with Yamaha RX1, though power to the prop has been maintained during flight in most cases. 
    7.     RXC cost is $1625 – 1750.  No longer generally available unless you provide a Rotax engine serial number and falsely claim you intend to use it on that, likely due to Rotax becoming aware of gearbox failures when used with Yamaha engines. 
    8.     RXC requires major machining in order to adapt it for suitable use on conversion engines and keep installed lengths to a minimum.  Cost for machining at a typical shop $200.00
    9.     RXC flex shaft coupling is on a 76 mm bolt pattern, and was designed in 1960 for use on small cars producing ~100 HP category, as a half shaft joint.  They are intended to be replaced periodically and when they fail they fail completely.  They are round in sectional profile, like a donut.  Known failures have occurred when used with Yamaha RX1 conversions running 80-85% power in cruise and WOT climb.
    10.  RX1 adapters available:  Mohawk Aero GTA (2012) collar-plate; Skytrax (2014) uni-part
    11.  APEX adapters available:  Mohawk Aero GTI (2016) collar-plate
    Comparison:  Mohawk AK7 (and Air Trikes SPG4) gearbox
    1.     No known bearing failures to date
    2.     Rated and guaranteed by manufacturer to 180HP
    3.     Inerital mass rating 9000kg/sq cm
    4.     Shaft OD/ bearing ID 40 mm
    5.     Gearbox is not disassembled for installation/removal from engine
    6.     Associated (alternative) BMW clutch requires tear down inspection w/ costs similar to RK400.
    7.     PSRU cost is ~ $1800.00.  Usually in stock, and always available from Mohawk Aero.  
    8.     No machining required, total final cost of PSRU is the same as RXC
    9.     Flex shaft coupling used is newer BMW part intended for damping driveshaft harmonics on 300 HP autos. Bolt pattern 78 mm.  Sectional profile square, giving the part twice as much load-bearing material as RTX “Hardy disk”. No known failures on any propeller drive installations to date up to 200 HP
    10.  RX1 adapters available:  Mohawk Aero GTA (2012) collar-plate and Mohawk Aero uni-part (replaces crankcase cover)
    11.  APEX adapters available:  Mohawk Aero GTI (2013)   
    In closing, I developed a new clutch in 2017 to be used on my Yamaha conversion kits.   On paper, it should see 1000 Hrs TBO.  The first design  (January 2017) had some flaws.  First the seal blew out of one end at WOT.  Then the gearbox harmonics forced the clutch roller race to walk on the outer drum and become misaligned after 8 hours at 80% power.  I fixed those problems by redesign, and have tested it for over 20 hours without any problems.   The earlier failures did not decouple power to the prop at any time, and were easily detected upon simple sound, visual and touch inspection (turning the prop) after shutting the engine down.  We will continue to test and put hours on them, and report the total time accumulated and any/all successes or issues.  
    2018 GT4 Roller clutches are being readied as of this writing, and are expected to be available late December.  Of course, as always, all of my products are covered by a 100% money-back guarantee of satisfaction for at least one-year or 100 hours, whichever comes first.  That includes all engines I sell with my kits.  GT4 clutch guarantee works like a tire warrantee:  First year/100 hours 100% satisfaction money back or replacement guarantee. After the first year, for 4 more years or a total 500 hours, value is reduced by useful life wear. Cost is $695.  No tear-down inspections required, all pre-flight and inpsection can be done by listening, looking through the PSRU inspection holes, and turning the prop by hand.   
    Mohawk Aero will not sell GT4 clutches to anyone who has not purchased a gearbox and adapter from us.  We  originally also sold sprag clutches, and although we have suspended testing and production on them in order to do a better job putting hours on the GT4  we may at some point return to offering sprags for sale at a later date.  
    All of this being said, here's the bottom line:  You might be able to run a RXC with a hardy disk or RK400 and never have any trouble.  You might go for 200 hrs and not even have to replace clutch shoes.  It all depends on your particular build, maintenance and operation of the aircraft.  Some equipment is better than others - there can be no doubt about it, and the choice is left to you to decide what you find most desirable.  In the end it is all "Experimental", and that is why we are not flying certificated aircraft.  I did not come here to dump on anyone or anything, but in my view some things have been said that are personally damaging and seriously misleading, and other things have been left unsaid which are also misleading as well as potentially dangerous to limb, limb and aircraft if left unknown and unaccounted for.  No one can make an informed decision concerning anything if you don't first have all the facts before you, presented in an unbiased manner - to at least some acceptable degree.  I hope I have done justice to remaining open-minded here, and that you perceive my long-winded, open letter here as such.  
    Thanks for reading this.  I leave you to consider all viable options now in peace, goodwill and harmony, and sincerely hope you join us on Facebook  where we explore the possibilities for the future of Yamaha aircraft conversions and share our  builds, to the exclusion of all else.   With warm regards to one and all, and may the force be with you...  
    Yours very truly,
    GT Mills
    PS:  When I reduced chain tension on the Mohawk Silent Drive, it became impossible to start the engine.  When I properly adjusted the chain tensioner to remove all slack, it started up just fine.  With a gearbox, backlash is not adjustable, and must always be present.  You can never get a gearbox to run without it, whereas you always remove all of that slap on a chain drive.  I proved that the less slap, the better the YG4 starts, and the more the slap the worse it starts.  Two gears is x amount of slap at start up.  Three gears is twice as much slap as two, not just 1/3 more.   I suspect that Teal is about to find out that his three-gears are going to exacerbate the rough starting we have to deal with when using a 12:1 compression Yamaha thoroughbred.  Three gears is twice as much harmonic vibration, twice the noise, twice as much loss of HP due to axial loading, and more weight.  Anyone wanting to weigh in on why I chose to build a Hy-Vo chain drive 3 years ago, and then finish and fly it 2015, instead of going with a three-gear solution to raise the prop shaft on a Yamaha conversion kit, be my guest - I'm all ears.    
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  10. Mohawk Aero added a post in a topic YAMAHA APEX (EFI) 150HP 118# Engines   

    Time to update this thread!  Mohawk Aero's 2018 GT4 Roller ramp clutches are coming out very soon. I am done with the drawings, and converting them for CNC machining as of this writing.  Updates (should I go with geek-speak and call this the GT4.2 update?) include, among other things:             
    Dual idler bearings (front & rear)(Steel-sleeved) rubber-bonded drive-stud pockets to damp vibration and allow for minor misalignment in all directions: radial, axial, and angular.  Kinda like merging "the rubber donut" with a clutch!     
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  11. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    Running a RK400 clutch on a Rotax C (RXC) gearbox requires inspection and/or wear maintenance such that every 75 hours (like about once a year for most weekend warriors) the mechanic must disassemble the RXC by first draining the oil then splitting the case halves, and then pulling the back half of the PSRU off of the engine mount or adapter. There are other solutions to this type of power plant conversion than this time-consuming, vexing and messy adaptation.  
    First:  Why use a clutch at all?  Easy starting.  Prevent cracked welds.  Some YG3 and YG4 conversions with certain prop combos will absolutely not start w/o one.  
    What else is there?  Mohawk's GT4, a maintenance-free roller ramp clutch that is a little bit smaller and exactly 1/2 the weight of an RK400.  (RK400 weighs in at 7 pounds, the GT4 clutches weigh 3.5 pounds.)  
    Next:  Why use an RXC gearbox?  I've worked with them.  We all know of Steve Henry, and he uses it - right?  Well, here's the thing - He uses them for one reason, and one reason only:  It is shorter than other mass-produced gearboxes that are readily available, and thus allows it to fit under the cowl of his Highlander w/o mods.  
    That's it.  There is no other reason.
    The first time I split one open and looked inside I walked away and looked to other solutions w/o a second thought. That was 8 years ago.  Since then I have read on other forums and heard first-person accounts of RXC bearings failing after as little as 12 hours, and others at 300 to 400 hours on high HP applications.  These horror stories are kept under tight wraps for the most part, no one wants you to know how fragile the RXC can be.  
    Guys, I don't know what you "know" or don't, I don't know what you've been told, but there are other options available.  First in 2012 came the universal Mohawk YG4 adapter which fits any PSRU ever built.  That super light-weight adapter has been flawless from day one - tested and flown for at least 500 hrs with many different types of PSRUs, including an amazingly smooth, robust, versatile and light weight Hy-Vo chain drive, the Mohawk Silent Drive which includes a prop-strike protection clutch.   
    Next, a couple of years later, Teal called and inquired about my adapter, but decided he didn't want a "stacked" adapter, instead he wanted something to replace the crankcase cover and went on to fabricate and sell (much to my chagrine) his own adapter, limited to the RXC and YG4 (RX1).  I ahve no idea why he required an engine cover replacement.  If we were dealing with my old 500 HP 500 Ft Lbs torque Porsche GT1 race car I guess I could see the point, but in my extensive motorcraft experience and knowledge of engineering practices I decided - quite rightly - that adapter design was more than adequate. The proof is in the pudding: the design has been a perfect, a flawless performer right out of the gate and from then on for well over 500 hours to date and counting.  Both types of adapters use exactly the same mounting points,  removing the engine cover is pointless with only 150 HP and 90  FPT involved.  The way my adapter is built, fitted and attached it simply cannot move under these forces, and any difference in performance of the two designs for this application has been well proven to be a moot point.     
    Regardless, there is now a third option which, like Teal's YG4-to-RXC adapter, replaces the crankcase cover.  Unlike Teal's model which is designed around the diminutive RXC, this one fits a heavy-duty SPG or Mohawk SP gearbox, rated at 9000 kgm (compare to RXC at just 2/3 that, 6000 kgm).  I do not fabricate these adapters.  I do, however, sell all three - Mohawk universal GTA, The latest uni-cover adapter for the Mohawk SP and the Air Trikes SPG, and the Sky Trax uni-cover adapter for the RXC.  They all sell for $1500.00.  But, tell you what:  I am in a generous mood, and will sell you one of these new uni-cover adapters for just $1195 for a limited time.  Again, that is for a limited time, only, so if you see this post next month and want this deal, I doubt I will still be feeling so generous.  
    This third adapter is for use with a special Mohawk SP gearbox (MSP, which is the same as the Air Trikes' SPG version but built to different specs to accommodate a shorter installation and clutch of your choice).  Comparing to an RXC:  The shafts are much larger diameter, the gears are much wider, and the bearings are 50% larger.  All of this means more weight, of course, and the proof is in that the MSP comes in around 18 pounds compared to the little RXC which weighs  around 14.5 lbs.  
    But connect the RXC to a RK400 clutch and you get 14.5 = 7 = 21.5 LBs, whereas the Mohawk SP with GT4 weighs 18 + 3.5 = 21.5 LBs.  It's a wash. 
    You don't have to "settle" for the high-maintenance RK400 nor do you have to "settle" for the RXC which was spec designed to a 90 HP Rotax 582 or 617 engine.  Want numbers?  (Again) The RXC is rated at 6000 KGM, the MSP (and SPG)  is rated at 9000 KGM.  
    I have been building YG4 and YG4i (Apex) adapters for many PSRUs, including RXC, Arrow, SPG, Mohawk SP, and my own Hy-Vo Silent Drive since 2011 - several years longer than anyone else - and you will not find anyone, anywhere, who has built YG4 adapters for more than one, single type of PSRU - let alone five very different types, for both the 140 HP YG4 and the 150 HP YG4i (Apex).  
    Did you catch that last line?  You don't have to "settle" for a carb 140 HP YG4.  The 150 HP YG4i Apex gearbox and adapter conversion kit is here - and has been here since January 2017.    
    Mohawk Aero is the only source for the YG4i - Yamaha Genesis 4 (Apex) 150 HP fuel injected motor aircraft conversion kit.  This isn't some dream, this isn't some prototype to be built somewhere down the road in the future.  The future is now, and it is here.  
    Personally, I have found the Hy-Vo Silent chain drive to be far smoother, lighter, quieter, and efficient than any gearbox I have ever tried regardless of brand, and I've run a bunch of them.  They don't compare to the Hy-Vo silent chain, and the big difference is most notable in flight with this type PSRU being far smoother than any gearbox could ever hope to be.  As goes efficiency, bear in mind that with every helical gear & shaft you lose 1-2% power through heat generated by the axial loads produced against the bearings and housing by the angular-cut gears pushing them sideways.  For a two-gear set that is 3-6 HP on a 140-150 HP engine.  For a three-gear set, that works out to 4.5 - 7.5 HP lost (this can be determined with an infra red heat sensor, converting the PSRU heat measured to HP converted/lost), not to mention added, weight, noise and vibration transferred throughout the power plant and airframe. 
    For more info and help with YG4 and YG4i conversions, using RXC, SPG, MSP, Arrow, & Silent Drive PSRU applications, or any of another type you come up with on your own, email Greg at
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  12. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    The guts of the YG4 is heritage legacy R1 motorcycle which turns in excess of 15,000 RPMs.  Connecting rods, valves, pistons, wrist pins, all the goodies.  The YG4 (short for the true name of this motor, Yamaha Genesis 4 - RX1 is a sled and no one calls a Mustang engine a Mustang...maybe a 351 or something) has cams that are ground to peak out at 10,250 engine RPM (ERPM), so that with the internal spur gear @ 1.19:1 the final countershaft RPM, or shaft RPM (SRPM) matches the YG3 Nytro speeds and the same CVT clutching can be use on both sleds.  These engines put out a good, honest 140 HP starting at 9750 ERPM, peak out at 143 HP at 10,250, then it starts to drop off and after 10,700 you're really wasting your gas.  
    Don't worry about redline, there is no such thing on the YG4.  The cams won't ever let you get that high with a prop attached.  I suppose if you took off your prop or your RK400 clutch goes south you could free spin the engine up past 17,000 and hit a valve... 
    There is no need for a pre-oiler pump.  The only time time this engine will start w/o choking is when they have recently been run.  I can start mine up to three days after the last flight w/o choke, but most people must use the choke the first start the very next day.  I also use an electric fuel pump, this way I can prime the carbs before hitting the ignition switch.  The Yamaha pulse pumps make great back up pumps, and a single one (they run in tandem on the sled) will provide enough fuel for all manner of flying except full throttle climb.  
    To oil-prime the engine before starting after a long spell of not flying, or in very cold weather (an engine's worst enemy is extreme cold starts), I do not hit the fuel pump switch, and leave the choke open.  I crank the engine for 15-20 seconds during which it never fires, and is plenty of time to run the oil throughout the motor.  Then flip on the fuel pump, choke it and fire her up and away she goes.  
    Being a true snowmobile engine, not a motorcycle engine, the YG4 operates at all manner of extreme angles and conditions - except inverted.  Those cylindrical oil reservoirs I keep seeing on conversions here are pretty, but how well are they baffled inside?  The stock RX1 oil tank is probably much better.  It has a very complicated system of baffles to keep oil where it is supposed to be in all angles of attack.
    To be serious about power, especially in the mid range, converter mechanics really need to figure out a way to work your engine frames around the intake airbox, not the other way around.  When you scrap your YG4 silencer you are guaranteed to lose 5-10 HP across the power band.  One big mistake I see here when fabricating your own airbox: the oil breather hose must be placed in such a manner that the closest carb cannot suck from it, or else that cylinder will be choking on oil and lose power and you will have different output across all 4 cylinders - an unbalanced engine.  Take a look at the inside of a YG4 silencer, and first understand why it is designed like it is with the oil tube venting FAR away from the #4 carb, and where all 4 carbs will pull from the vent evenly.  
    To make a point:  One day I took just the plastic shelf out of a stock YG4 silencer and ran a quick test.  We lost 500 RPMs!  I pulled the foam air filter out and lost another 500 RPMs.  Removed the silencer altogether and the YG4 wouldn't top 5800 RPM.  In other words, the engine is perfectly tuned to the whole system, and changing anything is a step backwards.   
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  13. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    You're thinking Rotax, which is an oil-cooled, air cooled engine with water-cooled heads.  Yamaha is not a 50-50 air-cooled/ liquid cooled engine, it is 100% water-jacketed - no oil cooler.
    We stock dual core, 1.75 x 14.5 x 16" aluminum radiators with 1/4" burp line inlet, $90 + S/H.  These lay at about 25 degrees, angled to catch the wind, below the engine.  Also have silicone elbows and adapters, all-metal drain cock valves, silver-plated copper 1"x T flared hose couplings, flared 1" aluminum pipes and bends made to order. 
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  14. Mohawk Aero added a post in a topic Yamaha RX1 Engines   

    This just posted to the Yamaha Apex thread next door, worth mentioning here as well since most folks want/need a clutch to ease starting with these high-spirited, thoroughbred engines:
    Production Mohawk GT4 RAM clutches (roller ramp) in-stock today, $695.00 comes with lifetime guarantee. 
    Same 2" width as RK400, 3-3/4" diameter (compare to 4-1/2" for RK400).  3lbs 5.8 oz.  Available in 75mm stud bolt input flange pattern for Rotax C or 78mm stud bolt flange pattern for SPG4.  Fits Skytrax RX1 Rotax C adapters and machined (shortened) gearbox housings.  No additional machining required, ready to plug-and-play.  All necessary hardware included.
    If you want the beefier SPG4 gearbox, please order through Mohawk Aero as the Air Trikes SPG4 has clearance issues.  If you already have one call and we can work it out. 

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  15. Mohawk Aero added a post in a topic YAMAHA APEX (EFI) 150HP 118# Engines   

    Here is an assembled production run GT4 RAM clutch ready to install on Kurt Carleson's Yamaha RX1-powered single place Air Command Gyrocopter, which is currently in my hanger in Savannah, although he lives way over yonder, t'other side of Atlanta. 
    Here's a Youtube link of him flying his gyro with the Yamaha installation I did for him last year using the Mohawk GTA adapter and Arrow 2.58:1 gearbox with internal sprag clutch.
    Kurt decided he was ready for more (one of those former biz jet pilot guys who needs a fix) climb thrust, and asked me to install a 3:1 ratio PSRU.  I had a beefy new SPG4 redrive on the shelf I wanted to try out on the awesome YG4 powerplant, but decided I would make my own clutches instead of going with centrifugal clutches like the RK400 or Air Trikes BMW. 
    Steve Henry's APEX is running, it is mounted on a stand and he has all the wiring sorted out.

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