----- Original Message -----

From: Tracy Crook

To: Rotary motors in aircraft

Sent: Friday, April 15, 2005 10:33 PM

Subject: [FlyRotary] Re: Rx-8 Rotors

Here's my take (FWIW as usual).  Ed's answer is essentially correct.  But there is another reason for the greater depth of the 2nd - 3rd gen apex seals.  The increased contact area in the groove is needed to transfer heat from seal to rotor.  So no milling down the RA seals to fit RX-8 rotors in RX-7 engines.  (but hell, it might work anyway). 

 

Also, the center of mass of the rotor DOES rotate in a perfect circle.  No monkey motion is ever involved. 

 

Another question was about why counter weights were required if the rotors ballanced each other out.  They are not in the same plane of rotation which creates what is known as a "rocking couple".  The counterweights make another rocking couple (out of phase) to cancel out that of the rotors.

 

Tracy (still at SnF)

Hi Doug,

 

Some good thoughts and questions, of which I would like to know the answers to as well.  I believe the reason the experts are recommending milling the RX-8 rotors for the older seals (when used in the older rotor housings with peripheral exhausts) is that the RX-8 seal was not designed to withstand the forces of crossing the exhaust port (in the Peripheral wall) opening unsupported. Its rather skinny and long.  Mazdatrix reported the RX-8 seals warped as a consequence of the the hot exhaust gas blowing over them and primarily the lack of wall support at the exhaust opening(on the older housings).  The combination of the additional heat and lack of wall support appears to be more than the 8 seal can take.

 

However, I am in basic  agreement with you why machine the rotor when perhaps a new/modified seal is the answer.  I want to check with Tracy Crook since his seals are reportedly 800% stronger than stock apex seals to see whether the seals could be machined down/created so that they fit in the RX-8 rotor  standard apex slot and still be strong enough - say 300% stronger than stock {:>).  Since they are not made of typical "gray iron" alloy that the stock and most other seals are made and chill/case hardened, they may be amenable to machining.

 

Yes, having airports scattered around in just about every county and sometimes three or more in a county, the geography is much kinder to engine-out excursions here on the east coast.  Fly High, Glide long!  Better yet, keep running on both rotors.

 

Ed A

 

 

----- Original Message -----

From: Shearbond@aol.com

To: Rotary motors in aircraft

Sent: Thursday, April 14, 2005 10:10 AM

Subject: [FlyRotary] Rx-8 Rotors

So.....since the rotors weigh the same within measurement error, the ability to rev to the 9,000s range vs. 7,000s range is not related to the orbiting mass of the rotor.  (Note: the motion is NOT truly circular, it is an epicycloid path that the center of mass of the rotor takes.  That 10 lb. rotor flops around plenty (technical term) and 10 lb. is lots heavier than an aluminum piston!

 

It is apparent from an article on the "other" site, that the wall thicknesses and casting detail are lots more refined on the RX8 rotors than on earlier models...for more uniform and desirable heat transfer and uniformity.....oil cooling the rotor, etc.

 

The mass being basically the same, it becomes a high probability that since centripetal force is F = (Mass x rotational velocity squared)/radius of rotation, {F=(m x w^2)/r}, it must be the weight of the seals themselves that are the critical element?  The force at 8,500 vx. 6,500 is (8.5/6.5) squared or 1.71 times greater.  Might this be why seal wear on the original seals goes up substantially at around 6,500 rpm?

 

Tell me....why would one increase the depth of the seal groove of an RX8 rotor to allow a heavier seal????  On the RX8, 3.25mm/9.5mm => approx. 34% reduction in apex seal mass!!  A lighter seal means a whole lot less force of the seal against the housing at 8,500 rpm!!  Anyone want to go back to old seals and rev to 8,500 rpm? 

 

Note:  the seal force against the housing at 8,500 rpm on the RX8 is still higher than the old seal at 6,500.... (.66 x 1.71 = 1.129).  Not much more, but there are probably even more very small details that we are not at first glance able to know and understand?  Seal material/housing material compatability is probably one significant factor??  (I used 9,000 vs. 7,000 and the numbers still come out about 10% higher seal force on the RX8, even with the lighter seals.) 

 

Having been an R&D engineer at FoMoCo, it is hard to explain to most persons the creativity, detail, imagination, trial and testing and testing and testing and testing and ...... which goes into making an engine acceptable for production in quantities of XXX,XXX's and higher.  (One simply can't be wrong...it could bankrupt even the largest OEM.)  And....those Mazda engineers have done what many OEM's gave up on many years ago.  How?  Through their persistence and incredible insight into the issues.

 

Before one gets the wire EDM out and starts going counter to what Mazda no doubt spent many $$$$ (more than all of our annual incomes put together??) on how to increase HP in an RX8, more information is needed before we start mixing and matching just because the parts will fit.  I know we are experimenters here, but lives of some who may not understand the "physics" limitations will ultimately be at stake.

 

Above all:  Just because something works for 100 miles in a sprint race DOES NOT mean it will work while hummmming along for 5 hours at a crack at 75-80% max. HP on a cross-country over and over and over again....hopefully.  So far, my Lyc is a piece of 30's era design and materials...but it has run for 2,000 hours (350,000 miles) at 75-100% without fail!!

 

Most of us aren't as lucky as Ed and his "on airport" emergency landings.  Perhaps it is that his green carpet (Carolina's) is lots more friendly than our granite peaks out here?

 

Doug in Colorado