Swedish yoke

C. Beaty

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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.
 
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DarDow101

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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.
I found this Van Craft has some interesting rotors that seem to have the composite rotor hub like skywheels and metal blades kind of like what you described Madman Mike had tried out.
History of the Gyroplane - part 10 the calm before the storm.mp4_snapshot_04.18.911.jpg
 

C. Beaty

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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.
 
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wolfy

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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.

Cheers wolfy
 

wolfy

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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.

wolfy
 

kolibri282

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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.
 
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wolfy

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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.

wolfy
 

kolibri282

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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.
 
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wolfy

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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.

wolfy
 

Smack

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'Ol Otto probably used bamboo because..... that's the best material he had from which to choose.
If he had aluminum available...... I bet he would have used it. Heck, maybe Otto would have used a 3D printer.
Bamboo is not homogeneous; you're right, no 'aviation grade'.
 

kolibri282

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Aluminum is probably not the best choice for small aircraft. There are several foot launchable gliders and none of these so far (as far as I know) is made from aluminum. There are some wooden ones (ULF-1, Hall Vector 1) and the rest is FRP. The problem with aluminum for small aircraft is, that, to be light enough, you have to use tubes with very low wall thicknesses which makes them very prone to fatigue failure. I would feel very uncomfotable in an aluminum glider. For gyros things are a bit different, they are so draggy anyway that the additional weight you need with aluminum does not matter. There are some projects using aluminum tubing but I think these will turn out to be inferior at least to the FRP gliders. Even for "real" soaring aircraft the aluminum Pilatus and Blanik aircraft are the odd ones out.
 

dinoa

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Bamboo has nodes so the fibers are not all continuous. It may have been used for millenia, but it's not optimal.
 

kolibri282

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The nodes might be weak spots but they are thicker than the segments in between, so my guess is that a bamboo rod has an equal strength all the way down. Don't forget that mother nature has had a few million years to optimize this and she never wastes anything. The main problem is, that there are not enough data and design codes. You will probably be able to come up with a better (lighter) design using FRP but you'll need a very careful design and manufacturing process. For the Pietenpol Aircamper there is a choice of using wood or welded steel for the fuselage. The wooden variant is a few percent heavier than the steel tubes, I made a comparison just for the fun of it, but if you'd use wooden parts slightly thicker, cut them in half, mill a channel along the center line and glue them together, giving you a sort of tubular wood section, you'd beat the steel variant hands down for lightness and any time if we start to talk fatigue. There is a good reason why the Cap series of aerobatic airplanes is a wood designed.
 
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