From: "Al Gietzen" To: "Rotary motors in aircraft" Subject: [FlyRotary] 20B - torsional vibration Date: Wed, 5 May 2004 16:00:15 +0000 Content-Type: Multipart/alternative; boundary="NextPart_Webmail_9m3u9jl4l_25115_1083774470_1" --NextPart_Webmail_9m3u9jl4l_25115_1083774470_1 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Subject: [FlyRotary] Re: 20B =20 Al, =20 Same here on my 20b. Out of a 94 Cosmo. It has the counter weight under = the accessory cover, no harmonic damper in the front or back. =20 Barny =20 The reason I had asked this question is because in doing some = development testing on a 20B, Powersport determined the need for a harmonic damper = on the shaft at the front to reduce a torsional vibration. The test were = done with the Powersport re-drive, which, if you are not familiar with it, is = a tight tolerance spur gear arrangement which is designed to be "stiff" in order to keep any resonance frequencies above the top end of the = operating range. =20 I have been looking (again) at the torsional vibration issue, and = wondered in Mazda had determined there may be a problem with the 3-rotor, either regarding the engine internals, or the driven accessories. Upon further thought, I realized that Mazda only used this engine with a torque = converter coupling to an auto trans; and the torque converter is about the best vibration damper you could find. Putting a stiff drive on the output = end changes the picture entirely. =20 Tracy's re-drive design takes the opposite approach; i.e., a flexible coupling with some play in the gears which lowers the resonance = frequency to keep it below the low end of the operating range. This obviously has = been quite successful in using with the 2 rotor rotary. Use with the 3-rotor = is not yet proven. Going to a 6-planet system obviously can handle more continuous torque, but if you do have a resonance in the operating = range; 6 vs 4 doesn't really make much difference, because you can get torque amplifications of 10 or more. =20 On first cut, it seems to me likely that picture with the RD-1B on the 3-rotor may be even better than on the 2-rotor because of a further separation of the driving frequency (higher because of more pulses/rev) = and the natural frequency of the re-drive system (which stays roughly the = same). Unfortunately, until you test, you don't know, because the entire = (geared) rotational system from front pulley to prop is extremely complex analytically. =20 Now; if you are not asleep yet, I will tell you that I was concerned = about the amount of belt vibration (alt and pump drive) I observed on my 20B during the dyno runs. This could be caused by some non-uniformity in = the pulleys, but there seems to be no measurable runout. It is likely that = it was the result of torsional vibration. This could result in reduced = belt life, or a failed alternator mount, or nothing. One thing is pretty certain, a harmonic damper on the front would reduce the risk of a = problem. =20 The torsional vibration characteristics of the system will be different = for the engine, with the re-drive and the prop, than it was on the dyno. It won't be long (crossed fingers and toes) before I can observe what = happens on the airplane. =20 If I could find a harmonic damper that will mount to the front end of = the 20B e-shaft, I'd look at putting it on (especially a viscous damper). = It could reduce the torsional vibration for the entire system. (Anybody = know of one?) =20 BTW; I'm not trying to be an alarmist, it's just that torsional = vibration in pulsed drive engines is a fact of life - auto engines, airplane engines; whatever. Any torsional vibration resonance, which is what can get you, = is something manufactures have always had to deal with. People have in the past wrecked dynos and broken drive shafts when unknowingly running into = a torsional vibrations resonce, and one topic-du-jour example is the MT = prop on 4-cylinder Lycomings. Seems that there is mysterious cracking of = prop blades, and MT has restrictions on ceratin RPM ranges. I find evidence (although limited and may just be my interpretation) of problems with a 3-rotor and the Ross drive. So we need to be observant when going into uncharted territory. =20 Al --NextPart_Webmail_9m3u9jl4l_25115_1083774470_1 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Subject: [FlyRotary] Re: 20B

 

Al,

 

Same here on my 20b. Out of a 94 Cosmo.  It has the counter weight under = the

accessory cover, no harmonic damper in the front or back.

 

Barny

 

The reason I = had asked this question is because in doing some development testing on a 20B, = Powersport determined the need for a harmonic damper on the shaft at the front to = reduce a torsional vibration.  The test were done with the Powersport = re-drive, which, if you are not familiar with it, is a tight tolerance spur gear arrangement which is designed to be “stiff” in order to keep = any resonance frequencies above the top end of the operating = range.

 <= /font>

I have been = looking (again) at the torsional vibration issue, and wondered in Mazda had = determined there may be a problem with the 3-rotor, either regarding the engine = internals, or the driven accessories.  Upon further thought, I realized that = Mazda only used this engine with a torque converter coupling to an auto trans; = and the torque converter is about the best vibration damper you could find. =  Putting a stiff drive on the output end changes the picture = entirely.

 <= /font>

Tracy’s re-drive design takes the opposite approach; i.e., a flexible coupling = with some play in the gears which lowers the resonance frequency to keep it = below the low end of the operating range.  This obviously has been quite successful in using with the 2 rotor rotary.  Use with the 3-rotor = is not yet proven. Going to a 6-planet system obviously can handle more = continuous torque, but if you do have a resonance in the operating range; 6 vs 4 = doesn’t really make much difference, because you can get torque amplifications = of 10 or more.

 <= /font>

On first cut, = it seems to me likely that picture with the RD-1B on the 3-rotor may be even = better than on the 2-rotor because of a further separation of the driving frequency = (higher because of more pulses/rev) and the natural frequency of the re-drive = system (which stays roughly the same).  Unfortunately, until you test, you = don’t know, because the entire (geared) rotational system from front pulley to = prop is extremely complex analytically.

 <= /font>

Now; if you = are not asleep yet, I will tell you that I was concerned about the amount of = belt vibration (alt and pump drive) I observed on my 20B during the dyno = runs.  This could be caused by some non-uniformity in the pulleys, but there = seems to be no measurable runout.  It is likely that it was the result of = torsional vibration.  This could result in reduced belt life, or a failed = alternator mount, or nothing.  One thing is pretty certain, a harmonic damper = on the front would reduce the risk of a problem.

 <= /font>

The torsional vibration characteristics of the system will be different for the = engine, with the re-drive and the prop, than it was on the dyno.  It won’t = be long (crossed fingers and toes) before I can observe what happens on the airplane.

 <= /font>

If I could = find a harmonic damper that will mount to the front end of the 20B e-shaft, = I’d look at putting it on (especially a viscous damper).  It could reduce = the torsional vibration for the entire system.  (Anybody know of = one?)

 <= /font>

BTW; = I’m not trying to be an alarmist, it’s just that torsional vibration in = pulsed drive engines is a fact of life – auto engines, airplane engines; whatever.  Any torsional vibration resonance, which is what can get = you, is something manufactures have always had to deal with.  People have = in the past wrecked dynos and broken drive shafts when unknowingly running into = a torsional vibrations resonce, and one topic-du-jour example is the MT prop on = 4-cylinder Lycomings.  Seems that there is mysterious cracking of prop blades, and MT has = restrictions on ceratin RPM ranges.  I find evidence (although limited and may = just be my interpretation) of problems with a 3-rotor and the Ross drive. =  So we need to be observant when going into uncharted = territory.

 <= /font>

Al

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