Hornet build update

A little Birdy (not David) told me my name was being taken in vain here.

The load that a mast can take in bending is very easy to calculate. Look up "beams" in any engineering handbook (such as the excellent Marks), follow their formula to calculate the section modulus of the tubing you're using, and the apply the very simple bending-load equation for cantilever beams.

The direction of the load on a mast in flight is a function of the rotor's flight angle relative to the horizon. On typical small gyros at cruise, this is 10-14 degrees (it equals spindle back-tilt, plus flap-back). During a flare, of course, it's more for just a moment after the pilot yanks the stick and before the frame noses up in response.

Back to the 10-14 degrees. This angle, not coincidentally, is very near the mast rake angle of a Bensen. If the mast is raked 10 degrees back, the gyro flies with its keel level and the rotor cruises at 10 degrees angle of attack, there is ZERO bending load on the mast. The "drag," whatever number it may be, is not imposing a separate bending load on the mast... "drag" is simply the vector component (of the total rotor thrust) that happens to pull straight back. IOW, the 10 degrees takes "drag" into account.

In the two gyro tipovers I've had (both with multi-tube, undrilled masts), the mast was undamaged. Both masts are still flying.

I have read of a couple fatalities in which the mast broke off at the seat back bolt holes. In one case, the stub of the mast ripped the pilot's helmet off and inflicted a fatal injury; in another, the gyro flipped upside down and, with the pilot's head being the highest point, broke the pilot's neck.

Bensen specifically announced the dual-tube mast as an improvement in crash protection -- though of course it's only an improvement with respect to blade strikes on the side of the gyro, not ones in the back (or front!) .

Personally, the dual-tube concept gives me a bit of extra warm-fuzzy in that cracks won't automatically propagate through two separate tubes. They will go all the way through a single-tube mast, of any wall thickness, once they get going. They are, however, a lot less likely to get going in a tube with thicker walls, no holes and no scratches. These latter factors probably have more to do with the mast's "survivability" than wall thickness or the use of one vs. two tubes.
 
Doug,

Doug Riley said:
...Back to the 10-14 degrees. This angle, not coincidentally, is very near the mast rake angle of a Bensen. If the mast is raked 10 degrees back, the gyro flies with its keel level and the rotor cruises at 10 degrees angle of attack, there is ZERO bending load on the mast. The "drag," whatever number it may be, is not imposing a separate bending load on the mast... "drag" is simply the vector component (of the total rotor thrust) that happens to pull straight back. IOW, the 10 degrees takes "drag" into account.
Click to expand...

I must somewhat disagree. If lift and drag were constant variables, I would agree, but they're not. Assuming lift is a constant, drag would vary at a rate of 2-per-rev. So the force vector would always be changing.

Am I wrong?
 
Not sure what you disagree with, Don. My example is just that... one set of possible numbers. With different blades, different hang test values or different thrust line location (among other things) the bending load won't be zero.

It certainly isn't in a Gyrobee with a vertical mast, to take an extreme example. Because the 'Bee does have a vertical mast, I wouldn't suggest using just a 2x2x1/8 in its mast. Either a Bensen-style redundant mast or a 2x2x3/16 single tube seems more appropriate. The section moduli for both of these latter masts are virtually identical fore-aft.

That lift and drag aren't separate things is not open to debate. They are analytic values, derived by applying vector geometry to the single net reaction force experienced by an airfoil when it accelerates air. The airfoil's force can just as well be divided by vector analysis along other axes. For example, the reaction force can instead be considered to be made up of two forces, normal and parallel to the airfoil's chord. This alternative might be handy when analyzing wing spars and such. As long as you obey the rules of vector geometry, you can artificially chop up any force into any number of vector components (along any axes) that you find convenient.

Yes, the thrust of a 2-blade rotor varies cyclically. I imagine (though it would take some fancy equipment to measure precisely) that the thrust varies in magnitude pretty much along an axis that's perpendicular to the tip path plane (or the rotor "disk"). A variation of magnitude along this axis would affect both the lift (vertical) and drag (horizontal) components. It would be a mere coincidence to find that it varies only in "drag" and not at all in "lift," since these values are based on breaking up a single force into its vector components along more or less arbitrarily chosen axes.

In any event, I agree that the mast is subject to cyclic loads and it better be designed very conservatively to give a safe fatigue life.

I don't bring up the lift-drag vector business just to nag. The whole spurious "theory" of high-thrustline gyros is based on the mistaken notion that drag is some separate force all its own, yanking away at the teeter bolt. Drag as an isolated, separate thing is an invented notion. If you want to talk about drag, you can't do so and get accurate results without simultaneously accounting for the OTHER invented component of rotor thrust -- lift.
 
Doug Riley said:
Yes, the thrust of a 2-blade rotor varies cyclically. I imagine (though it would take some fancy equipment to measure precisely) that the thrust varies in magnitude pretty much along an axis that's perpendicular to the tip path plane (or the rotor "disk").
Click to expand...

Doug - are we taking into account the rotor profile drag? I would think that the profile drag is cycling only in the direction parallel to the flight path, thus posing a 2/rev back and forward bending cycle on the mast.

Udi
 
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Doug, this is a little blurry for me, too. If you try to think your way through this calling lift and drag abstracts, that's fine, but then don't you have to concede that the net load on ther mast changes its vector at a 2-per-rev rate?

If not, what causes the shake?
 
I have to disagree with you on a minor point or two, Doug.

There is no periodic variation of rotor thrust unless the blades are out of track which you feel as the 1/rev "thump in the rump." The low blade develops less lift when on the advancing side than does the high blade when its turn comes. The thrust vector, normal to the hub, rotates in a conical pattern that stirs the stick, mast and everything else in a 1/rev circle.

Any periodic variation of rotor thrust would be felt as a vertical thump whether at 1/rev or 2/rev.

The 2/rev cyclical drag variation that is characteristic of see-saw rotors, referred to in the textbooks as "H" force, is unrelated to the thrust vector. Also, it's relatively small compared to the resolved horizontal component of rotor thrust.

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I believe few things are more important than having the mast act as a roll bar.

I can recall 7 sets of rotors being smashed on my gyro with 2.5" round 2024 mast. The mast and axle are still original; the keel tube was once replaced. 2.5" x 0.120 wall round tube is a hair lighter than 2x2x0.125 square tube and drawn tube always has superior mechanical properties as compared to extruded tube.

(1) After about 50 hrs. on a partnership plans built Bensen, I thought I was an ace and could fly anything. I originally built Old Thud with an overhead stick and learned I wasn't an ace after all. Smashed a set of cloned Bensen metal blades.

(2) I put my first pair of Hughes helicopter blades down in the Gulf of Mexico; too low, too slow and too far off shore when a wire fell off the electric fuel pump. The blades didn't curl up but looked as though they had been trampled by a herd of elephants.

(3) I had one Hughes-269 blade left over in addition to the pair I'd put down in the Gulf. What does one do with a single blade? Build a 1 blade rotor with balance weight of course; another dumb trick. I realized immediately that I couldn't fly with a one blade rotor; -once the rotor is up to speed, the single blade follows cyclic input but the counterweight doesn't. I put daylight under the wheels and decided it wasn't for me. A friend of mine wanted to try it so I let him after warning him that the stick could only be moved very, very slowly. Gary didn't heed my advice and the machine hopped over after he began stirring the stick.

(4)* Busted a set of homebuilt symmetrical airfoil blades when the aluminum stub axle on a main wheel broke as a result of a hard landing. These were 4½" chord blades using 0.032 skins with leading edge radius formed in a press brake and trailing edge riveted shut. The spar was a stick of ½" square 2024 bar with internal lead nose weight pinned to it at about 70% radius. The blades had originally been built for a 3-blade rotor that never got finished. Performance was mediocre at best.


(5) Lit down in a scrub palmetto patch when the crankshaft broke on my trusty Mac. That time, Old Thud had his keel tube replaced. Hughes OH-6 blades.

(6) One of the partners in the sod airport where we kept our gyros wanted fly a gyro. He was able to fly it just fine but got too tricky with his landing. He tried to come in under some power lines and land on the grass apron just outside our hangar. He misjudged his available space and whacked a power pole with the rotor blades. Hughes OH-6 blades.

(7) A friend was flying my gyro when the snap ring retaining one of the main wheels came off and the wheel swan dived from ~1,000 ft. He was unaware of it until he landed and of course the wheelless axle dug in and the machine groundlooped. Hughes OH-6 blades. Didn't hurt the wheelbarrow wheel though.

*Forgot this one in the initial tally. There could have been others; it was a long time ago.

My personal observation is that nearly all masts with holes drilled at the top engine mount or seat back break as a result of a rotor strike. I have never seen an undrilled mast break. Anecdotal of course.

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And Don, no matter how fancy a computer program, the old adage, "garbage in; garbage out" hasn't been repealed. If you don't know the imposed loads, you don't know what will break when.
 
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Chuck: You've studied the literature much, much more than I have.

Paul W. Plack -- yes -- the total force applied to the mast changes 2/rev. The discussion is really about whether it changes its angle, its size, or both. My money's on the "both."

I suspect we do have a small 2/rev "thump in the rump," though the guys with the vibration detectors may make me a liar. It seems to me that it would be sheer happenstance that the rotor would develop the same net thrust when in the fore-aft position as it does when abeam. When fore-aft, you have air flowing at a slightly diagonal angle to chord of each blade, effectively making the airfoil a thinner, stretched version of itself. In the abeam position, the rotor is being rotated about its feathering axis by the teeter bolt, up-pitching one blade and de-pitching the other. The thrust on each blade gets evened out so the seesaw balances at any given moment, but does it follow that the total thrust in each of these peculiar situations is the same?

Years ago, the Mass. PRA chapter put some gimbal heads on a fatigue tester that pulled straight up-and-down on the heads with a varying load. The PRA mag article that covered the experiment pointed out that they used a ridiculously high load variation -- 100 lb. -- while the real up-down variation per Bensen was on the order of 6-8 lb. Maybe Bensen was just making an allowance for a nominal amount of out-of-track...

By the way, I was inaccurate in stating earlier that the section modulus of Starbee's 2 x 2 x 3/16 mast section was the same fore-aft as a Bensen "redundant" mast. In fact, they contain the same amount of material and WEIGH the same, but the thick 2x2 is about 13% stronger in bending parallel to its faces. The modulus for the Bensen unit is .66, and for the Starbee unit it's .75. For 2 x 2 x 1/8 it's .55. The Starbee section uses the material more efficiently by putting more metal out away from center of the beam, where it does the most good. An obvious bonus of the thicker square tube is that it has the same additional bending strength in the sideways direction, while the Bensen unit is actually weaker than a 2x2 x 1/8 in that direction (modulus = .42), unless the two 1x2's are strongly bonded together. (They generally aren't.)
 
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Could be Doug. I've just done some ballpark playing with the numbers and it appears rotor thrust could reach a maximum with the rotor at 0º-180º and be a shade less at 90º-270º.

In any event, it's not much and on a gyro with limber mast and well tuned rotor, I've never felt a 2/rev vertical thump or the resulting 2/rev fore/aft stick pulse that would result from rotorhead offset.

I suspect that whatever stick pulse which exists results more from feathering axis moment of inertia than anything else.
 
Doug & Chuck,I'v had a theory bout the inevitable shake,or more like wobble that is present in every 2 blade rotor and I'd like your oppinions.

I'v always thought it was friction or I think you call it paracitic drag.
When the blades are in the for and aft position,theres not much difference in the two blades airspeed[and the profile change,caused by the change in direction of the oncomming air, is almost equal for each blade],but when they are left and right,the retreating blade is seeing less airflow than the advancing blade.Greater airflow means more friction with the air,on one side of the disc,meaning twice per,theres a friction dragging on the disc on the advancing side,feeding back through the machine and stick.
It's not a lift difference coz the teeter cancels that out,but the teeter dosn't cancel out the airflow[friction] differential.
Or dose the pitch differential equalise the friction differential,wich would mean neither would cause a shake coz they are equal and opposite forces in tandem??
Remember,I'm still a SCG,and probably rambling now.
 
Since we're still on the subject...

What is the basis for a slider head? And what is the basis for having springs in-line with the vertical push-pull tubes on the 2-place Butterfly and other gyros? How about the counter weight on the front side of Ernie B 's rotor head?
 
You're right, Birdy. There is a periodic drag variation of the blades endwise Vs. crosswise.

The best solution is to have the softest possible rotor pylon and let the blades go where they may.

The slider and RAF's "magic" rubber bushing are solutions to this problem where a limber mast hasn't been or can't be used.

The configuration of my gyro with tail boom running from the rotorhead over the top of the prop requires a completely triangulated pylon and a slider was the single approach out of many different schemes tried to solve the 2/rev shake problem.

Surprisingly, the actual motion of the rotorhead is quite small; judging from the scrub pattern on the slider bearings, motion is only ±1/16".

Without some sort of flexibility, a resonant mode of the rotor/mast combination is likely to be excited and shake could be be very severe.

There are 2 main resonant modes of the rotor by itself. If you grasp a stick of welding rod between thumb and forefinger at ¼ of its length from one end and thump in the center, it will vibrate at its natural resonant frequency for quite a while. That's the xylophone mode and your thumb and forefinger are at a nodal point where there's no translational motion.

The other primary mode of rotor vibration is the tuning fork mode. With the welding rod clamped in a vise at its center, deflecting the tips permit them to vibrate in unison.

The xylophone mode will normally be above the frequency of the 2/rev aerodynamic input and won't be excited.

The tuning fork mode will be below the aerodynamic input and won't be excited either.

That's for a free rotor.

Mount the rotor on a spring at the center, (the mast) and the frequency of of the 2 modes tend to coalesce into a single frequency that's more often than not at the same frequency as the 2/rev aerodynamic hit. Under conditions of resonance, rotor shake can become dangerous.

Arthur Young and his associates at Bell encountered the same problem with the precursors of the B-47 helicopter that has affected most anyone who has flown a gyro with see-saw rotor and solved the problem by mounting the engine/transmission/pylon assembly on soft rubber biscuits and adding a yoke at the hub to stiffen up the blades in an in-plane direction.

My personal observation is that a round mast with equal flexibility in all directions is smoother than a square mast.

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Don, when you say springs parallel to the push tubes, I assume you're referring to the trim springs.

The pitch pivot of a Bensen style gimbel rotorhead is set forward of the rotorhead axis. Rotor thrust would tip the rotorhead forward without the trim springs.

The offset provides stabilizing feedback into the control system that permits some pretty awful designs to fly "hands off."

In steady trimmed flight, the trim springs balance the nosedown torque applied to the rotorhead gimbel by rotor thrust. With an upward gust for instance, rotor thrust increases, overpowering the trim springs and tending to tip the rotor nose down and keep the machine headed into the relative wind. Even if the pilot has an iron grip on the stick, the feedback provides a stable feel and serves as a guide in avoiding disturbances.

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The counterweights on Ernie B's rotorhead serve the same purpose as the "harmonic balancer" weights on an engine crankshaft. Some skyscrapers have massive weights on springs on the upper floor to dampen harmonic vibrations.
 
Chuck,mate,we gota hit the rum one day,I'm sure we could solve all the worlds ills. :D
 
I imagine, Birdy, in a face to face meeting, you could talk circles around me. Especially after the rum takes hold.
 
Chuck,

I know what a pitch spring is. The springs I'm referring to are in the control rod path. They are very similar to what you might see on a nose wheel control system, except with stiffer springs.

I don't have a picture to post, or I would. The arrangement I saw utilized a rod end with a shaft passing through the ball. The shaft was threaded on the ends, but smooth in the middle where the ball would slide. There was a compression spring on either side of the ball held in pre-load with a control rod on one side and a nut on the free end of the shaft. The rod ends were in the control fork (proper name?) at the aft end of the joystick control assembly. Control forces at the stick would move the rod ends in a typical manner, but the forces would have to pass from the rod ends, through the compression springs to control rods to the rotor head. Basically, the joystick was soft coupled to the rotor head.
 
Then you’ve got me Don. I don’t have the foggiest notion of why someone would soft couple stick to rotorhead.

Perhaps it's to soft couple just the student's control so he can be over ridden by the instructor.

Bill Parsons used a soft coupling on the student's stick on his trainer.
 
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I imagine, Birdy, in a face to face meeting, you could talk circles around me.

Somhow I dought that Chuck.
 
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