Hmm. I had a tandem Dominator with a 2-bearing head. It had pleasantly light control forces. The pushrods were 1" dia., and the rod-end bearings were the rather expensive aircraft-grade models. My 1986 Air Command OTOH had 7/8" pushrods and commercial-grade rod ends. Neither setup presented any problems in these areas.
Flexing of the teeter towers in the "chordwise" direction was not an issue, because the towers were joined together by two rectangular gusset plates (one above each blade). The plates represented extra bolts to tighten and loosen when mounting/dismounting blades. I didn't find that task burdensome.
The reason for two-bearing heads has nothing to do with the bearings' pure thrust ratings. Rather, it's the fact that the rotor disk does not fly square to the bearing's rotational axis in any flight mode except a vertical descent. In forward flight, the disk "blows back" 2-3 degrees. The rotor's thrust is then not pulling straight up the bearings' rotational axis -- it's pulling at an angle (it would bend the spindle bolt back if the bolt were soft enough). IOW, the the rotor's thrust is apply a "prying" action the outer race of the bearing, not unlike prying open a paint can. Ask the bearing manufacturers about their bearing's ability to tolerate this type of loading. They'll likely call it an "overturning moment." Turns out that our head bearings aren't especially good at resisting large overturning moments. The heavier the gyro, the higher the overturning moment.
As with so many aspects of the Bensen gyro, Igor Bensen had an exquisite sense of what simplifications he could get away with in a 500 lb. gyro's rotor head. The overturning moment on a single bearing from a gyro of this weight was tolerable.
When you scale up a Bensen, many of these "informal" simplifications just won't do anymore. Everything from using a plywood scrub brake to letting the mast tube do all the flexing is fine on a B-8M -- and not so fine on the half-ton and 3/4-ton models.
Hey, Doug, how's it going?
First: Air Command (no idea what OTOH means) TANDEM, not single place, is the topic of my earlier post. These were sold in the 1990's from what I understand after being corrected elsewhere by Mike Boyette. Once again, these were sold with Skywheels mounted on a SINGLE BEARING head and performed remarkably well with very nominal stick inputs. The yolk and cross tubes are VERY NARROW compared to any Dominator, with the latter having much wider arms thus increasing torque which overcomes the added height of a double bearing head.
When replacing these original Air Command heads with newer double bearing heads, like RFD, e.g., the added 1" height increases required input forces dramatically, resulting in slower response and
SIGNIFICANT decrease in maneuverability.
Second: You write, "IOW, the the rotor's thrust is apply a "prying" action the outer race of the bearing, not unlike prying open a paint can. Ask the bearing manufacturers about their bearing's ability to tolerate this type of loading. They'll likely call it an "overturning moment." Turns out that our head bearings aren't especially good at resisting large overturning moments. The heavier the gyro, the higher the overturning moment.
Wrong, Doug. Wrong, wrong, wrong, and...yep, I checked, and STILL wrong. I
did ask the bearing manufacturers. MRC, Timken (FAG) and NTN. "Overturning moment"? Can openers? LOL, Doug you are simply repeating something Chuck Beatty wrote or said, you never called a bearing manufacturer, obviously. If you have spoken with any of the major bearing manufacturers, PLEASE send the contact info so I can straighten out MY errors. I welcome to be corrected, and only seek the truth and the FACTS, with no care for personal pride or standing in any such case in which I may be wrong. I don't give a rat's ass what anyone thinks about me, which should be quite obvious to any and all by this late date.
When considering a single deep row bearing this notion of your so-called "can opener" "overturning moments" is true. Absolutely.
When discussing a DOUBLE ROW ANGULAR CONTACT BEARING the argument false. Absolutely.
In fact, this is
EXACTLY WHAT THESE BEARINGS ARE DESIGNED FOR. THEY ARE RATED FOR
ALL FORCES APPLIED AT VIRTUALLY
ANY ANGLE FROM AXIAL (zero degrees) to RADIAL (90°) , AND/OR
ANY COMBINATION OF the two, at any time, momentary or constant,
SUMMED TO THE MAXIMUM RATED LOADS GIVEN ON THE SPEC SHEET FOR ANY PARTICULAR DOUBLE-ROW ANGULAR-CONTACT BEARING.
Read...my...lips: There is NO DIFFERENCE REGARDLESS OF THE ANGLE OF LOADING. Period. End of argument. You want to argue with SKG, MRC, FAG, NTN whatever, be my guest. Pick the dang phone up and CALL THEM if you will. But PLEASE stop repeating BS 2nd hand rumors just because they originate with Chuck Beatty, whom we all love, trust and respect with great devotion.
Anyone desiring an authentic MRC and/or NTN and/or Timken bearing technical handbook please feel free to email
[email protected] and I will GLADLY send you the PDF files I have on hand, no charge.
Anyone else who just wants to go on believing in and repeating second-hand rumors about can openers and other nonsense, be my guest. It does no good to beat a dead horse, after all.
And when you discover the FACTS - after READING THESE HANDBOOKS AND SPEAKING directly with a real, live, major bearing mfr's tech advisor or engineer
please come back here with those FACTS, with names and phone numbers, so that I may re-educate myself by speaking with the very same reps you would quote and upon whom you would found your claims.
Please do not belabor the point senselessly with 2nd hand rumor from anyone, including our beloved master of gyrocopter engineering Chuck Beatty, just because we love and do respect him so. Look, I am sorry if this flies in the face of "conventional wisdom" as you think you know it, but this is just wrong, and someone needs to put an end to this silly myth that's been spreading about for the past 15 years or so. Yes, when you get over 1200 lbs TOW you need a two-bearing head. But under that it is unnecessary and in the case of narrow-coupled control systems (Air Command e.g.) it puts unnecessary strain on critical control system parts such as rod end "Heim" joints and is a plain old bad idea.
Finally: Doug writes, "Flexing of the teeter towers in the "chordwise" direction was not an issue". Look, just because one guy did not have a problem does not mean the problem does not exist. In math this is known as a false proof, a failed method to make a point.
Please see photo. THIS IS MOST DEFINITELY AN ISSUE ON SINGLE BEARING RFD DOMINATOR HEADS LIKE THE ONE IN THIS PHOTO, WE HAVE SEEN THIS PROBLEM ON NUMEROUS SIMILAR HEADS both single place as well as tandem rotors.
THE TOWERS WILL NOT STAY PERFECTLY VERTICAL BECAUSE THE TWO AN4 BOLTS ARE INSUFFIENT TO KEEP THEM ALIGNED. THESE HEADS NEED TO BE DRILLED AND PINNED IN TWO additional PLACES NEXT TO THE AN4 BOLTS IN ORDER TO PREVENT CORD-WISE MOVEMENT. It's not in all likelihood not dangerous, but it is certainly irritating and frustrating in tuning to eliminate as much stick shake as possible.
I'm going back to my cave. This is no place for me.