Hornet build update [Archive] - Rotary Wing Forum

gyrodude

08-01-2004, 11:57 AM

I have had one in an older Air Command. The Air Command front steering linkage can become locked sometimes if you get it too far off center.
I was landing on a wet grass strip with a crosswind. The nosewheel being locked all the way over to one side meant that when it touched down I went into a skid sideways on the wet grass until the inside wheel dug in and the tipover started. Once the blades contacted the ground the mast broke off at the motormount cheekplates and departed the airframe ending some 50ft away. Think of the energy stored in the blades when they strike the ground. It ripped off the Wunderlich prerotator, the control rods, and everything else attached to the mast.
Ora Cook did the same thing in his Air Command at ROC a few years ago. Landing on pavement cost him a bit of road rash and an expensive rebuild. His mast was a redundant one done properly and it did not break off.

donshoebridge

08-01-2004, 12:22 PM

But the mast still had to be replaced, right?

If the common cause between your's and Ora's incident was the flawed design of the nose wheel assy, then the nose wheel on the Air Command should be fixed, not the mast.

Mike Jackson

08-01-2004, 03:35 PM

Don,

I have tried to understand the merits of a redundant vs single mast. I think Gary's 2 examples of rollovers are valid. Are two data points enough? Maybe not, but they are convincing with the data given. The design characteristics/flaws of the nose gear is not relevant to the dual mast debate. The point is these gyros did roll over and what happened to the masts is relevant.

It would be interesting to hear from those with similar rollover incidents.

Cheers,

Mike

donshoebridge

08-01-2004, 05:37 PM

No, I don't think two data points is enough, and no, I don't think they're convincing either. You two are trying to draw a connection between blade strikes and mast failures. A mast is not designed for blade strikes. Masts are designed to carry a useful load into the air, which is a great deal less load on the mast than a blade strike.

As far as I'm concerned, the information given by gyrodude is technically inconclusive. It's always unfortunate when a gyro rolls over and returns itself back to kit form. But if the root cause of the mast failing was due to a faulty design of a nose wheel assembly, and subsequent ground strike of the rotor blades, fault can not be aimed at a mast design.

The redundant mast design was not a product of blade strikes. A rotor blade of sufficient speed striking a heavy enough object, fixed or otherwise, such as the ground, will cause the mast to either twist to destruction, or snap right off, regardless if you have one, two, or three 1x2 tubes, period! Regardless if you have a redundant mast or not, simply because of the close proximity of the pilot to the mast the chances of pilot injury are pretty high.

The primary reason for a redundant mast is/was to provide greater strength for flight loads, and increased fatigue resistance. It was not the primary design intent to provide roll over crash survivability of either the mast, aircraft, or pilot.

Hence, the mast/diagonal combination on the Hornet is designed for strength in the lift and drag directions, ie, flight loads. I rather doubt that ANY square tube, or redundant mast structure flying today would ever survive a blade strike from a roll over event, to include a Magni which is made from 4130N.

I have ran FEA's till I'm blue in the face on the Hornet mast, and in the lift/drag directions, the Hornet mast is over-killed by a substantial amount. Computer models indicated that with a gross weight of 500 pounds, and rotor drag of 100 pounds, that the Hornet mast can withstand a G-loading in excess of 23 G's! Why? Because the diagonals are in tension during normal flight. In a vertical decent, the mast is in tension. With 2-per-rev pulses being present, the drag load provided by the rotor blades is increasing then decreasing, effectively shifting the RLV fore and aft, which then shifts the loads from the mast to the diagonals, and then back. Even if I were off by a factor of 10, I'd still be much better off than someone flying a mid-span supported mast, regardless if there were holes drilled in it or not. If you would like to see one of the FEA studies that I did, go check out the technical library area of my site.

If you take into account the total amount of material in both diagonals and the mast tube, the cross-sectional surface area of a redundant mast design is only slightly greater than that of the Hornet. But because I triangulated my aluminum, I've better distributed the loads and greatly increased the load carrying capacity and fatigue life of my mast.

So in reply to gyrodude's initial posting about holes in the mast, the farther down the mast the FIRST holes are, the worse off you are. By placing the first set of holes in the mast where I did, I've greatly reduced the leveraging forces applied by the rotor blades during flight.

gyrodude

08-01-2004, 06:51 PM

Don You keep talking about flight loads. A 2x2 mast is plenty strong enough to carry any and all flight loads. We are talking about what happens in a blade strike now. First of all go back and read my post again. With holes drilled in my mast when a blade strike occured the first thing that happened was the mast broke off at the bolt holes!!!!! On Ora Cooks machine with a redundant mast and clamped cheek plates the mast did not break off but was bent considerably. There was reports of some Air Command masts drilled at the factory inproperly and the inner walls were scored meaning that the mast was a fatal accident waiting to happen. I checked mine and they were not scored so that did not cause the mast to break off.

CLS447

08-02-2004, 01:39 AM

I would say that this is not a faulty nosewheel on the AC, but improper adjustment of the steering spring tubes. One time, while doing sharp steering on the ground, my tubes got over-extended & popped apart. This caused wheel lock.

After readjustment, I can't make them come apart. If this still would have been a problem, I would have grounded the machine.

So , those with the older AC steering, check to make sure you can't get into this situation!

donshoebridge

08-02-2004, 04:21 AM

Don You keep talking about flight loads. A 2x2 mast is plenty strong enough to carry any and all flight loads.

I once thought that too. But after running FEA's on 2 x 2 tube, I can't say that now.

We are talking about what happens in a blade strike now. First of all go back and read my post again. With holes drilled in my mast when a blade strike occured the first thing that happened was the mast broke off at the bolt holes!!!!! On Ora Cooks machine with a redundant mast and clamped cheek plates the mast did not break off but was bent considerably. There was reports of some Air Command masts drilled at the factory inproperly and the inner walls were scored meaning that the mast was a fatal accident waiting to happen. I checked mine and they were not scored so that did not cause the mast to break off.

I'm not arguing the fact that a hole in the mast will weaken it. What I'm saying is that every mast is too weak right from the beginning, regardless of holes. A blade strike is bad no matter how strong or weak the mast, and it if breaks off and leaves the general area, good! What scares me more is having the mast wrapped up around me, pining me to the seat, or tearing my head off. To avoid either situation, you need to make sure your aircraft is functioning mechanically as it was intended, which requires good build, set-up and pre-flight practices. It's all part of the risk assessment, management and prevention process. But there's still a chance bad things are going to happen, and that fact alone scares enough people to keep them from building their own aircraft.

To expect a mast of ANY design to act as a roll bar, especially for ultralights, is asking more from the mast than is realistically possible, because all of the gyro weight would be in the mast design. This has about the same logic as flying a Mac powered Bensen across Lake Michigan from Ludington to Oshkosh. Besides, what would a strong mast buy you? So your mast doesn't break off from a blade strike... So what! The incident shouldn't have happened in the first place. To expect the mast to somehow make up for other failings is avoiding the root cause of the accident in the first place.

I've seen the product of "hole paranoia" before, and it's ugly, time consuming and eat up a bunch of money. But most importantly, it's HEAVY! Machined brackets and clamps come with a weight penalty, and for an experimental it's not so much of a problem. But for ultralights, all weight is bad!

GyroRon

08-02-2004, 05:01 AM

Don what is wrong with a plain jane 2x2 tube for a mast? Neither of my dominators had the so called redundant masts.

birdy

08-02-2004, 05:02 AM

Heh heh .
You stick with it Don.
Your simple,light weight frame has strength overkill.
This machine I flog around the scrub is much the same as yours,except for your diagonal angles.And I consider my frame to be more than adequate,seeing as it has over a thousand hours of rough strips and rough air.
But it don't matter,if you smack your blades on the ground,your go'n to damage the machine,end of story.
I'v wrecked two sets of blades and two masts with the same setup,and only BENT the masts.[with loaded rotors at high rpm]

donshoebridge

08-02-2004, 11:44 AM

Ron,

Did your seat tank bolt to the mast? And if so, how far down from the rotor head were the first set of bolt holes for the seat? Also, looking at one of the pictures I have of you and your Dom, it looks as if the mast is fairly short, which is a plus with regard to mast strength.

Don what is wrong with a plain jane 2x2 tube for a mast? Neither of my dominators had the so called redundant masts.

GyroRon

08-02-2004, 05:15 PM

Don in either of the two Dominators I have much experience with there was NO holes - not a single hole - from the cheekplates at the rotorhead all the way to the cheekplates at the keel cluster.

Everything is clamped on the mast including the upper seat brace. I just want to make sure I am ok.

cgmg

08-02-2004, 05:35 PM

Chris,

Interesting you mentioned the steering tube problem with your Air Command. While my wife and I were practicing taxiing during our lessons, we had the same problem. The only times the tubes popped apart was in front of the hangar, where we were doing low speed tight turns, or pressing the rudder pedals while sitting still. It took our instructor a couple of attempts to correct the problem. Since then, we've never had the problem again, and we fly off of grass for 95% of our flights. And, like yours, our airframe is an older generation which has been upgraded to centerline thrust.

donshoebridge

08-02-2004, 06:22 PM

Ron,

Seeing how the mast isn't as tall as any of the Bee's, mainly above the seat, there wouldn't be nearly as much stress to the mast. With everything clamped on like it is, at various locations up and down the mast, I doubt there is a problem.

Don in either of the two Dominators I have much experience with there was NO holes - not a single hole - from the cheekplates at the rotorhead all the way to the cheekplates at the keel cluster.

Everything is clamped on the mast including the upper seat brace. I just want to make sure I am ok.

I need to set the recond straight. There was a small but important detail that seems to have been overlooked, mainly by me. The FEA’s that I ran were based on a worst case condition. In other words, the bolts have loosened, or were not properly tightened, and the result is that the bolts are carrying all of the loads. Any bracing, cluster plates, etc, have no contact with the mast and the bolts are being loaded in shear. Obviously, the bolts will not fail in this condition because the loads are not great enough. But the forces are now focused on one side of the hole(s) that are drilled.

Friction between the mast and cluster plates, cheek plates, and diagonal braces is really where the loads and forces are transferred. The nuts and bolts only provide the necessary pressure to hole everything together so that there is enough friction between everything.

Now, with that said, proper nut/bolt torques are critical. Too much torque and the tube compresses and flexes away from the plates or braces, resulting in reduced friction and decreasing the maximum load bearing capacity. Too little torque and the bolts are in constant shear against the holes of the mast tube and the plates and/or braces.

I wouldn’t think that too much torque is a common problem. However, too little torque I’m sure happens more than some people would like to admit. There are a lot of bolts on a gyro, and it’s easy to miss a couple.

With diagonal braces added, any mast will be stronger, and will better deal with loads. What’s important is the location of the diagonal braces. The attached images are sample studies to show that brace placement can effect the loading needed for a failure to occur. Neither study is completely accurate because of the absence of cheek plates, rotor head, seat, engine, etc. But you can still get an idea of what can happen.

Almost forgot... In the images, I cranked up the FOS value so you can see where the highest stress point is. The number above the input window is the important number (the number with all the digits behind the decimal point that is).

gyrodude

08-02-2004, 06:25 PM

What I did was the same thing. Adjust,adjust,etc. It always worked for a while and then unexpectedly it would lock up again. My machine was an old one with a lot of miles on it. Wear plays a big part in it.

willbow

08-02-2004, 10:53 PM

As a Hornet builder, I've studied the Hornet documentation exhaustively and have no concerns about the holes in the mast or diagonal bracing system nor the single tube v/s the redundant type mast tube systems.

IMHO, the holes are not there for the sake of having holes here and there but to accomodate necessary fasteners. Presuming that one did not score the tube wall when drilling through-and-through holes and that all fasteners are present and tightened properly, structural integrity should not be significantly compromised.

Also, IMHO, one has to look at the integral structure of the airframe. The seat is more than just a seat bolted to a structure but indeed is part of the structure itself. An appropriate description of the airframe itself (main tube/mast/diagonal braces/seat/engine mount) might be a "pre-stressed truss". I could easily believe that it would take forces in excess of 23g to force a catastrophic failure of the airframe. Or at least the main mast system. Way before that point though, I would think that some other vital component failure would have doomed the pilot to whatever his/her fate was going to be.

As far as a blade strike goes, it's gonna be bad news whether it be a single tube mast or redundant tube mast. Consider this notion, you have a 20+ foot dia rotor directly over your head rotating at 200+ rpm, a 40+hp motor directly behind you swinging a 60 inch or so 2 bladed prop spinning about 4000 rpm. No mast tube of 6061 single or double configuration is going to fare very well in a blade strike.

I think that all considerations have to be weighed with respect to the performance envelope expected of an aircraft of this type. At least in, for lack of another term, this "mk 1" version, Don wants to keep the craft within part 103 specifications which is very wise IMHO, personally I think that the craft is well capable of performing well outside those limits but were talking about sanity here :) Point being that, as Clint Eastwood would put it .. "A man has to understand his limitations". :)

Without a doubt, especially where it comes to flying machines, I hope not to ever take things for granted and will endeavor to take a close look at my bird, stem to stern, before each and every flight.

Really though, whether you are talking about the Hornet, RAF, SparrowHawk ... the ultimate question comes down to "are you willing to bet your life on it?"
Well, I dont expect to ever see 23g :)

Sort of OT but I was watching some WW I stuff on The Histor Channel and often wonder of those who survived a day of suicidal charges in the face of enemy mahcine gun fire. If you managed to survive that day, what would it take to really bother you and make you feel really put out on after that?????

And though it might not be scientific ... I dont think it's my fate to die in a gyro :)

GyroRon

08-03-2004, 04:17 AM

" And though it might not be scientific ... I dont think it's my fate to die in a gyro "

You better be knocking on wood! I would never say something like that, it is like placing a Spell on myself.

scottessex

08-03-2004, 04:27 AM

Sounds like the same kind of thing all of those 44,500 people killed in car accidents last year thought.

Dean_Dolph

08-03-2004, 01:27 PM

Yeah, and the rest of us keep on driving!

birdy

08-03-2004, 07:39 PM

There's only one other place ,besides me gyro,I'd rather die, :) and it don't concern you lot. :D

willbow

08-03-2004, 08:33 PM

I'm not sure what that means David .... Your bird has built-in handles just in case 6 of your best friends need to carry you around??

Just kidding :)

Will

CLS447

08-04-2004, 01:47 AM

Hey Birdy, If she's lucky, rigormortis(?) will set in quickly !

birdy

08-04-2004, 02:03 AM

I'v heared that rigormortis lasts longer than Viagra to.

CLS447

08-04-2004, 02:07 AM

Birdy, it's 6AM here, Good Morning mate! What time is it "down under"?

birdy

08-04-2004, 02:18 AM

Bout 7;45 pm now,then allow 5 mins for me to type this out.
Where's Reading P A.??

Udi

08-04-2004, 09:36 AM

Some of you may have forgotten that crash worthiness is an important design feature of any aircraft. It goes without saying that crashes should be avoided at all cost, but having an aircraft with good crash worthiness is like having good insurance.

Don - you have said you don't mind that your mast would depart as a result of a blade strike. Would you mind if it takes your head with it? Are you planning on having a shoulder harness on your gyro? How is the harness going to be attached to the gyro?

Aircraft should not only be designed to fly safely – that’s a given. They should also be designed to protect their passengers in the unlikely crash. Unfortunately, in gyroplanes, a blade strike is not very unlikely.

Udi

quadrirotor

08-04-2004, 09:57 AM

good remark... to be meditated!...

donshoebridge

08-04-2004, 10:00 AM

Udi,

There are a lot of things in a gyro accident that can kill you. Having the mast wrap around you is only one possibility. But the fact still remains, gyro designers can spend thousends of hours digging throught NTSB reports, and cause of death reports, and short of building a gyro like and A-10, there isn't much that you are going to be able to do to stop the energy stored in a rotor system.

So other than making the mast out of solid A2, what would you suggest as a designed in safety feature?

scottessex

08-04-2004, 10:13 AM

Let me tell you, I am REALLY GLAD my mast stayed on! I don't think I would have liked the rotor blades departing company with the gyro, or the mast breaking off. A jagged sick of aluminum in the back of the head is no good. After a blade strike the blades are usually damaged and now out of track and out of balance, you do not know which direction they will go. Kind of like breaking the stick off a bottle rocket and hoping it still goes straight.

gyroblackwell

08-04-2004, 10:14 AM

Ok guys,

Lets put things into perspective here. Don puts a design that he has worked on alot. He shows FEA data that supports his design. Along come the "arm-chair" designers and pick at one little point that is made "off the cuff" without having anything to really back it up, and you are all over it! "BAD DESIGN" "BAD DESIGN" ...... I don't have a problem with constructive critizism.
I do have a problem with empty comments with little to no regard for the person that has the documentation to back the design.

The gyro bee has drilled holes in the mast. It can be a single tube or double tubes. I prefer the double tubes, because the mast support ends at the top of the seat.
Rick Martin did a number on his gyro bee not long ago. We saw pictures of the end result. Was his mast laying on the ground? is his head now missing from his body?

No!

Now if your intent is to scare anyone away from designing a gyro and or destroy further development of the sport, then by all means keep up the good work.

I like to have my design picked on as long as the picker gives me positive information. If you think it is wrong, then give an example of a fix. (not just saying build it like a dominator ron! ) anyone can copy a design. I dare any of you to go out on that limb of "true design" and do better !

clamping everthing to the mast IS a better way of doing things. BUT the parts count goes way up, and the weight goes right along with it. Now if you want to fly an illegal craft without the proper ticket as some here have done, then by all means build it beefy, and stick a huge motor on it!

If you want an ultralight, then you need to comprimise between strength, and weight.

We are trying to do this. I have made a number of changes to my design based on others input. That kind of response is what makes a design safer.

Just my opinion!

scottessex

08-04-2004, 10:26 AM

That is why it is Experimental aviation. It is truly hard to get everyones opinion to be the same on anything.
I am anxious to see both designs fly! That is the biggest problem strength vs weight.
always a compromise somewhere.
By the way my mast bent right at the top of the motor mount, the keel, right in front of the seat mount.
I wish you both success in your new designs, that is what makes this great. I meant no critisism.

willbow

08-04-2004, 10:40 AM

Crashworthiness???? How do you measure crashworthiness????

In ANY gyro made today .. the rotor and the tailfeathers are the only control surfaces a gyro has.

In the event that any of those components catastrophicly fail, especially at altitude - you are essentially a stone!!!!

How crashworthy is your gyro?

I can understand Scott's mast bending just above the motor mounts (btw - am very glad you were able to walk away from that one Scott)

Not to pick on the Bee, but it's mast is unsupported beyond the diagonal braces and engine mount and constitutes a 3 foot or so long lever. On the hornet, this same span area is less than 5".

It would take a tremeandous amt of force to make that 5" section of mast fold and even more force to exceed the tensile strength of the diagonals.

Sounds crashworthy to me

Udi

08-04-2004, 01:02 PM

Don,

I apologize if my previous post came through as negative criticism.

Actually, I was impressed by your stress analysis. I think you are doing a very thorough job. I was alarmed, however, when you said:

A mast is not designed for blade strikes. Masts are designed to carry a useful load into the air

And

A blade strike is bad no matter how strong or weak the mast, and it if breaks off and leaves the general area, good!

As a potential customer, I want to know that the designer gave some thought to accidents. Doug Reiley, in another thread, wrote a convincing explanation why certain holes in the mast are not going to weaken the mast significantly in the event of a blade strike.

Saying that a gyro cannot be made strong enough to protect me in the event of a rollover or a blade strike does not make me feel warm and fuzzy about your design.

I hope this is taken as constructive criticism.

Will – when a Cessna 172 loses a wing in flight it would crash and burn. Is that a good reason not to make it crash worthy? You are an extremist, my friend. Making a gyro more crash worthy means that you pay attention to the consequences of small accidents like a rollover, a blade strike, a drop from 10 ft, etc. No aircraft is survivable from a catastrophic failure in altitude.

Udi

willbow

08-04-2004, 01:34 PM

Sorry Udi .. didnt mean to come across as an extremeist .. it's just that no mast will survive a blade strike very well and I think what Don meant was that if the rotor does break off, better it fly away from you than turn you into deli meat.

Anyway, what would you like to see changed?

Udi

08-04-2004, 02:05 PM

I don't want anything changed, Will. I have no issues with Don's design.

Udi-

donshoebridge

08-04-2004, 03:17 PM

I was wondering when Doug's name was going to come up.

I've seen the thread/post that you are refering to Udi.

First of all, the mast is ridiculously over-strong for the flight loads it encounters.

Later in the same post he stated the following...

With respect to withstanding blade strikes, the mast is essentially a cantilever beam stood on end. Cantilever beams by their nature are not very strong for their weight, and that holds true for gyro masts. Bending loads that will break a 2x2 (or double 1x2) mast fall between 300 and around 650 lb. Not that much in the best of cases!

I fully agree with the numbers stated above - 300-650 lbs. But when taken in context with the earlier statement, some very important facts and questions come to mind;

1) With 500 lbs of lift and 100 lbs of drag, that sure puts the mast real close to the 300 pounds in my book. I wonder if the 100 lbs of drag is an average? I wonder where it peaks at with 2-per-rev? What happens to the drag component when you start pulling some G's?

2) Assuming rotor blades weight 60-70 pounds, with them near flight speed, say around 250 RPM, and a diameter of around 23 feet, how much energy do you think is going to be applied to the top of the mast if a blade were to hit the ground? Now I haven't figured it (because I'm lazy), but I'll bet a HELL of a lot more than just 300-650 pounds! Depending on where the blade is in its rotation (advancing, retreating, crossing left or right) the forces applied to the mast will decide which way the mast will likely bend/break. In all roll over cases, how many gyros have had the retreating blade strike the ground first?

Dean_Dolph

08-04-2004, 04:41 PM

Don, some comments and observations from a non-engineer, unless the shade tree degree counts!

First of all, in my opinion, it is ludicrous to even worry about crash worthiness of an open frame machine. I'm perfectly satisfied with one that is designed to not have any in flight failures of frame and components. If a pilot has crash worthiness at the top of his gyro criteria list then the Little Wing or similar machine offers the best option.

And secondly, because, or in spite of, all the number crunching & analysis, empirical data/experience (Close to 50 years worth!) should convince us beyond reasonable doubt that there is very little problem with the mast, even with ground strikes. However, the supporting structure(s) have been a problem from time to time. The masts that got the most attention were on Fetter's original Air Command design. And even then there was/are two camps.

One wants the mast/rotor to depart and the other wants it to stay intact. The first camp believes there is less chance of injury if the mast/rotor leave the immediate area while the second believes there is less chance if the mast/rotor remain intact and somewhat limited on where it can go. In either case we can't predict what will happen in a blade strike although we might be able to suggest that the majority of the time it will........

I have witnessed a couple of ground strike incidents and it seems that the only similarity was that the machines all rolled to the left so maybe something can be made out of that.

I say again, give me a stable machine that will not fail in flight and access to the proper training and I think my odds of surving are as good as driving the Houston freeways.

birdy

08-04-2004, 10:08 PM

I'v said before ,if you want to crash test an aircraft,make it a gyro.It's the safest machine to crash. :D
For an outsider this may be hard to accept[heavy rotors spin'n,prop,no protection]but I had a bit of practice at crash'n. :mad:
Left and right.
The time I fell left,with loaded blades["J" stick broke] the armchair desigener would say I would git me head ripped off coz the retreating blade striking the ground would force the mast forward.It did,but only bent it bout 15 degrees,the blades were written off[one broke off 18" in from the tip].
This mast isn't supported anywere above the top seat mount,and that's only a clamp.

I personaly can't see how anyone should git hurt if they are still alive when they get to the ground,no matter if you land on your wheels or your door.If you land on your roof,your already dead ,so this thread don't concern you. :D
Providing you fly it as a gyro,not a cessna. :)

bones

08-05-2004, 02:11 AM

Scott,
Please explain why you ended up denting your bird that bad when you landed there, seems to be lots of open space..

scottessex

08-05-2004, 02:24 AM

Basically I blew a head gasket on takeoff, I was unable to gain airspeed or altitude to turn back to the runway, I couldn't get over the powerlines either. So I put 'er down, hard, and bent the right axle. Everything seemed good, untill the weight started coming off the blades and settling on the airframe, the axle broke, and I had a roll-over. The full story and pics are in the general discusion section, under bad news from georgia.

donshoebridge

08-05-2004, 04:02 AM

It's starting to sound like we're all preaching to the choir. For the most part, we're all in agreement.

The reason tempers flared in the first place was because there are a few people that are actually trying to design a gyro without stealing too much from other gyro designs, such as myself, and Tim Blackwell. Call it honor and integrity if you want. Both of us having been design engineers for many years figured (like many engineers) that we could do it either ourselves and/or better than previous engineers. This may or may not be true. Only getting off the ground will tell.

I have confidence in my design, as do others, and I will fly my Hornet when completed. Everyone here has what I call the "self preservation gene". Those that don't, usually crash trying to self train in a hurry, or put their aircraft together with wood screws.

But any ways... I think this topic/thread has about had it. Let's leave on a middle-note.

bones

08-05-2004, 08:17 AM

Ouch sorry Scott, that would make ya just a tad cranky

scottessex

08-05-2004, 08:25 AM

Yeah Bones it sucked! :mad:
The new rebuild thread is under the Bensen KB section.

And Don, I love your suspension set up! And all of the kids in my neighborhood better not leave thier bikes outside at night, or my gyro may have a new front wheel! :D

donshoebridge

08-11-2004, 09:53 AM

Scott,

It was a pretty easy design. It's real cool to stand on the keel tube and jump up-and-down and watch it work.

Doug Riley

08-16-2004, 08:08 AM

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.

donshoebridge

08-16-2004, 09:26 AM

Doug,

...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.

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?

Doug Riley

08-16-2004, 10:33 AM

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.

Udi

08-16-2004, 01:13 PM

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").

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

PW_Plack

08-16-2004, 03:12 PM

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?

C. Beaty

08-16-2004, 03:58 PM

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.

donshoebridge

08-17-2004, 04:11 AM

At least my garbage is neat and tiddy.

Doug Riley

08-17-2004, 05:35 AM

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.)

C. Beaty

08-17-2004, 07:50 AM

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.

birdy

08-21-2004, 04:20 AM

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.

donshoebridge

08-21-2004, 05:27 AM

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?

C. Beaty

08-21-2004, 04:34 PM

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.

birdy

08-21-2004, 11:08 PM

Chuck,mate,we gota hit the rum one day,I'm sure we could solve all the worlds ills. :D

C. Beaty

08-22-2004, 08:41 AM

I imagine, Birdy, in a face to face meeting, you could talk circles around me. Especially after the rum takes hold.

donshoebridge

08-22-2004, 02:32 PM

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.