Chuck if your still reading, somewhere on other post's you have been talking about how benson necked down the hub bar to what we have now.
With that being part of the in plane stiffness problem.
Do you know why Benson did it in the first place, is there some reason he did it other than maybe ease of construction?
Wolfe, my guess is that the 2.5” hub is a carryover from the wood rotor days; with limber wood blades, a 2.5” aluminum hub is more than stiff enough.
Also, remember that Bensen’s designs date from the 1950s and 1960s, long before there was an internet and the free flow of information so that Arthur Young’s research on underslung, teetering rotors wasn’t generally available outside of the Bell Aircraft Company.
Nowadays, all one has to do is go to YouTube and look up “Birth of the Bell Helicopter” and get a complete rundown on Arthur Young’s research, narrated by Bartram Kelley, Young’s assistant and later, VP of engineering at Bell-Textron.
I didn’t have the foggiest notion of the the 2/rev solution of seesaw rotors until after watching Birth of the Bell Helicopter: stiff inplane rotor and limber mast.
Bensen had at least some understanding of seesaw rotors; cautioning his customers to avoid braces to the top of the mast, saying the gyro had a “tuned airframe” and braces would mess up the tuning. On the other hand, Bensen messed up the tuning himself by going to the double 1x2 inch “redundant” masts.
I stumbled on to 2/rev solutions without understanding how they worked until after watching “Birth of the Bell Helicopter.”
Thanks Chuck I wonder if Benson new about the in plane stiffness problem back then with his wood blades or he just got lucky and then just stuck with it.
I have also watched the birth of the bell series several times, bloody interesting films.
Here's what started it all. Bensen managed to get his hands on one of these things while employed by GE at its flight test center in Schenectady, NY.
That 2-blade rotor must have been an eyeball rattler but even worse, the Brits had fitted a similar rotor type to a Jeep and towed it behind a bomber to several thousand feet.
Gyro designers, as a general rule, have been individuals without formal engineer training and to their credit, have stuck to designing fancy pods to hang from Bensen rotors; staying away from monkeying with the rotor.
A good example is the rotor airfoil, the NACA 8H12, apparently believed to be a special autorotational airfoil. It is in fact a failed helicopter airfoil, designed by the NACA in the 1940s that didn’t make the cut. Any modern helicopter airfoil would perform better on a gyro. However, with the efficiency of a gyro being as low as it is, a small improvement in rotor efficiency is nearly irrelevant.
Bottom line: gyros are fun to fly and great for chasing feral hogs.
The SkyWheels rotor with its wide hub was free of 2/rev vibration but the blades were tailheavy which made the gyro unstable; mildly so in a light gyro but dangerously so in Dreadnought class gyros.
Skywheel users were very fond of their blades, misinterpreting their behavior.
The noseup pitching in an upward gust is believed to be the result of high lift and the extended float during landing is believed to be the result of high inertia rather than unstable blade twist that occurs during a load increase.
I’m not sure how the chordwise balance could be corrected; the skins are quite heavy and the spar is a solid aluminum extrusion. A brass spar?
A gyro owner who went by the name “Madman Mike” used extruded aluminum 8H12 blades plugged into a Skywheels hub with satisfactory results.
Dar, there was a US rotorblade vendor that was selling extruded blades at one time that evidently were dimensionaly the same as Skywheels; can’t remember the name but think it was located on the East Coast.
Once the die is paid for, an extruded blade should be the cheapest. I would use a modern helicopter rotor blade airfoil such as the Boeing VR-12.
The question often arises; “how soft should the rotorhead mounting be for a seesaw rotor?”
The answer is; “how stiff is the rotor inplane?” The rotorhead mounting needs to be soft enough not to force the rotor out of pattern. The rotor undergoes a 2/rev drag variation in forward flight and needs to be on a mount that flexes enough not to force the rotor out of pattern.
A Skywheels type hub is stiff enough to tolerate a stiff mount.
A rotor with drag hinges, depending upon drag hinge offset from center of rotation, requires a soft mount. (Drag hinges don’t do anything once the rotor is up to speed; several thousand pounds of centrifugal force tends to keep the rotor in pattern.)
Rotors with Bensen type 2.5” hubs are somewhere in between.
After MMM smashed his Skywheels hub and complained about 2/rev vibration when the same type extruded blades were mounted on a Bensen style hub, at my suggestion he installed parallel drag links and stated that the 2/rev vibes had again vanished.
Now that I think about it, weren’t the one that found the original posts about MMM?
Yep, went back and looked it up and post #22 shows MMM's drag links.
Woops: goofed again; on a second look, that was Joe Pires' DW rotor.
Hi again Chuck, I am have some concerns with my decision to stay with a vertical mast (as it was before converting to round tube).
Do you think my concerns are valid?
My mast is 2" x 0.065" and about 50" from the top engine support to the bottom of the cheek plates.
I estimate my AUW at about 850 pounds.
My teeter bolt will be about 5" behind the mast.
Going on the formula you posted already I would be looking at a bending load of about 735 foot pounds, (calculating with the mast length being from the teeter bolt to top engine mount of about 60")
As you said there is no doubt the mast will be bending under normal flight loads, but how much is acceptable?
I have not been able to find a calculator to predict bending amounts other than a load between two points, as apposed to the load at one end.
Naturally a vertical mast is not the best solution for reducing two per rev also.
Mast bending will be zero (or near a minimum, for an offset rotor head) if you make the resultant rotor force of thrust T and in plane rearward force H, be aligned with the mast axis in cruise condition. This is why a Bensen mast is tilted rearward. The maximum bending force then depends on the flight state. A complete analysis would probably show that the bending load is not the leading failure criterion but rather the fatigue load.
Thanks Kolibri, you are absolutely correct. I am thinking of changing my mast to what is should be and tilt it rearward.
Having a limber mast and then mount it vertical is less than ideal on a few levels, but before I go changing things I would like to know weather it is actually unsafe or just less than ideal. I have seen several machines with a vertical mast.
Assuming that the straight mast was designed according to the applicable regulations and using proper engineering it is as safe as an inclined one. It will be just a bit heavier, which will probably not really make much of a difference though. It will also be a bit stiffer and if you want a limber mast an inclined one is to prefer. Please note that the modulus of elasticity of the material will also be important regarding transmission of rotor shake to the air frame. Here bamboo would, in my opinion, be a good choice. It has been used extensively building the first airplanes from Lilienthals gliders to Santos Dumont's "Demoiselle" but has gone out of fashion basically a hundred years ago. Additionally there is, from what I know, no aircraft grade bamboo available, so you'd have a hard time convincing authorities and insurance companies of your design. FRP also has a low modulus of elasticity.
Thanks Kolibri, I have decided to remove all my doubt and tilt the mast back at 7 degrees anyway.
The job is half done already but I should have just done it to start with, My whole reason for going to a limber mast was to reduce the two per rev so no point contradicting the issue with a vertical mast.