View Full Version : Mast Tube: Double or Single? Also tail boom question...

08-22-2004, 04:51 PM
I see that in the "documentation" Ralph calls for 2 - 1"x2" mast tubes bolted together. But from things I've read and looking at Starbees parts list, I see that they call for a 2"x2"x3/16" single tube. Whats the best to use?

Also, I see in the "documentation" that the tail boom isn't but on till phase 8. But I see alot of pics of bees that are under construction that have the tail boom on well before phase 8. Is it ok to go ahead and put it on sooner?

And lastly, since I will be building a StarBee Gyrobee, can I go ahead and paint my peices before "dry fitting" them since its highly unlikely that there will be a piece that doesn't fit?

Thanks all. I apologize if it seems like I'm being a pain, but I just want to ask these questions before rather than while I'm in the middle of putting it together and making mistakes!!!!

Brian Jackson
08-22-2004, 05:34 PM
From what I've read and heard, the 2 X 2 X 3/16 wall single mast is a little stronger than the 1/8 wall redundant mast. By how much I don't know. I'll let Ralph answer that one.

08-22-2004, 05:44 PM
Thats what I've heard, just want to be sure before hitting the order button...

Doug Riley
08-23-2004, 09:50 AM
The 2x2x3/16 is about 13% stronger in fore-aft bending than the double 1x2. It is about 75% stronger in lateral bending than the double 1x2. Either one is adequate, based on long experience.

Fore-aft bending strength is more important in a gyro with a vertical mast than it is in a gyro with a raked-back mast (such as a Bensen). Lateral bending is mostly an issue in the event of a blade strike in the back of the gyro.

Remember that, the stiffer the mast, the more rotor vibration is fed into the rest of the frame.

Jon B
08-23-2004, 10:43 AM
Taggert is building a new gyrobee (see last link on page below) using starbee gyro kit.


I thought you gyrobee nuts out there might find it interesting. Also, he mentioned having all the parts anodized before assembly. He has a section on his thought processes about paint and what-not, in case anyone is interested.


Brian Jackson
08-23-2004, 11:15 AM
Thanks for the info. How were those numbers arrived at? It makes sense because more material is concentrated on the outer edges of the mast, rather than in the "webbing" that's created internally when two tubes are mated.

The other nice thing about the single mast, besides machining simplicity, is that there's really no weight penalty. I did a quick x-section (attached) and if anything the single tube mast is slightly lighter, yet strength is increased. I'd say that's a pretty good trade :)

Doug Riley
08-23-2004, 11:37 AM
The only disadvantages I can see are (1) the 3/16 is even stiffer than the double 1x2, leading more vibration into the rest of the frame; and (2) on the remote chance that a crack or other failure starts in the tube, it will propagate clear through (with two separate tubes, the cracking won't automatically jump from one to the other). Extrusions are occasionally defective; I've seen some with extensive cracks on the corners right as they come from the extruder. Chuck Beaty reported a mast that split up thorugh the corners like a banana peel.

You can cull out defective corners by crushing a slice of the tubing in a vise. Th material should not crack diagonally through the corners, but instead should crack NEXT to the corners.

Brian Jackson
08-23-2004, 12:14 PM
Hi Doug.
The crack propogation issue is precisely why I've been leaning toward the redundant mast. I would expect a certain degree of material flexibility as opposed to a completely rigid structure. That's one of the reasons I've decided not to anodize the mast tubes. I know the effects are minimal, but it scares me a little, which is silly. However I might be using a Nickel-Plating process (http://www.pioneermetal.com/electroless.html) on them, which gives them a mirror finish and great corrosion protection without hardening the outer layer, or altering its chemical composition.

So between the redundancy of the mast structure and the nickeling process, it gives me a little more peace of mind. Hey, it's my first gyro so I'm gonna have enough to be nervous about already :eek:

Brian Jackson

08-23-2004, 12:46 PM
OOOOOHH! Nickel plating!!! Nice!! How much does that run? A Mirror finish would be AWESOME. Now about the single vs. redundant, you say you want to go redundant because of the strength/crack issue. But if the Single mast is stronger, won't there be less chance of cracking?
My first gyro, too.

Doug Riley
08-23-2004, 01:03 PM
I've never seen or heard of a gyro mast cracking unless (1) the blades hit the ground (2) the holes were drilled too close to the walls, scratching them on the inside, or (3) the tube was defective. Gyro masts are very conservatively designed with respect to their flight loads.

In theory, even the best aluminum mast will fatigue and break sooner or later. Aluminum will eventually break under a very light variable load if it experiences enough billions of cycles. A billion cycles is something like 2000 hours at 2/rev. None has failed at this kind of age that I've ever heard of.

If you drill the holes right, the remaining cracking danger would seem to be from unnoticed material defects. Of course, if you use two 1x2 pieces cut from the same length of extrusion, any manufacturing defect just might be present in both pieces!

IOW, the probability of you actually enjoying a real safety advantage with a double 1x2 in a given gyro is rather low, but greater than zero.

08-23-2004, 02:26 PM
Hmmm, now you guys have got me thinking. I'll be ordering from StarBee, so the quality of the cuts and holes will be excellent. But, given the fact that the redundant mast isn't that much more expensive, maybe it IS better to go redundant...for the peace of mind. However, I'd like to hear Ralphs opinion, as well as others....

08-23-2004, 02:33 PM
Here are some numbers with respect to loads on the mast of a gyrobee:

1. design loading: Not being aware of any US standard I will assume the loads mandated by the CAA (UK) Section T for gyros which are +3.5G and -1.0 G limit loads. This translates to +5.25G and -1.5G ultimate loading (limit load multiplied by 1.5 factor of safety).

2. Assuming a maximum AUW of 500 lb. The critical load condition on the mast is
at full stick back position ( 18 degrees b/w rotor head and mast on a bee), perhaps due to a suddent pull-up during a steep (75+ degree) steady spiral dive. The force bending the mast = 500*5.25*sin 18 degrees = 811.17 pounds.

3. The maximum bending moment will be at the point where the seat brace attaches to the mast . This is ~32.5 inches from the point at which the rotor force acts. Hence max bending moment is 811.17*32.5 = 26,363.01 lb. inch.

4. The stress on the mast is given by the beam bending formula fmax = My/I.
a) For the dual mast with 1/8 wall thickness I = .66 and the max bending stress on the mast = 39,943.95 psi ( just above the seat brace attachment)

b) for the single mast of 3/16 inch walls I = .75 and the max bending stress = 35,150.05

5. Based on data from ALCOA on a single 6061 T6 square tubes, the critical failure of the tube will be due to buckling at 35,000 psi. The dual tube may have a slightly different buckling stress; there is no way to know short of testing. However 35,000 would seem to be conservative. Either way the maximum stress the dual tube could sustain will not be more than 38,000 psi- the ultimate stress for 6061 T6 extrusions.

6. Conclusion:
a) factor of safety of dual mast = -0.14
b) factor of safety of 3/16 mast= -0.0043

well, neither of the options meet the standard ( at least based on the assumptions above), but the 3/16 thick mast is very close to meeting the standard ( certainly, well above the accuracy of the calculations) . + 3.5 G limit loading on a gyro does sound a tad conservative given that this load is unlikely to be seen by a gyro in gusts ( the duration of the gusts are too short to have a significant impact on RRPM change). My suspision is that the only way to pull these loads +3.5 Gs, if at all, would have to be during a sudden pullup in a very steep (cork screw) steady spiral dive. Ofcourse whether these loads are conservative or not is pure ( well, informed :-) )speculation on my part and as in all things aviation conservative is good.

Obviously, things are not as critical on a benson based design with a leaning mast. The maximum stress in that case is approximately half of what they are on the bee, resulting in a comfortable strength reserve.

That said, based on the all important 'service on the field' criterion, the dual mast would certainly seem OK. Perhaps Ralph could chip in with his thoughts and if a stress analysis was done while designing the bee, it would be great to see some numbers.

08-23-2004, 02:47 PM
the dual mast would certainly seem OK

Ok? That doesn't sound too good. And I think the point some of us were trying to make is that in the however unlikely event that one of the pieces should fail,due to DEFECT not stress, the other piece in the redundant mast would be able to carry the load.

08-23-2004, 03:41 PM
I suspect that some of you guys are approaching the point where you are calculating your way to stasis! I can certainly root around for the design notebooks, but a couple of practical points are worth noting:

(1) The original stress calculations were done for a 2x2x0.125 mast. The redundant configuration was added a few years later because everybody seemed to have one.

(2) The prototype Bee - like the Bumblebee - actually had a mast that was tilted FORWARD by a few degrees - in short, subject to higher stress than the present configuration.

(3) A number of times the upper seat braces were loosened up to check the bolt for corrosion. At NO time was there ANY deformation of the hole for the upper seat brace attachment noted. Deformation of that hole would certainly be noted if the mast were stressed to anywhere near the point where buckling of the extrusion was an issue. That was true for the single (0.125 wall) mast at 2 years and the redundant mast after a total of 10 years of service.

(4) The mast was checked with a steel straight edge at every annual inspection, looking for any sign that the mast was "taking a set". No bending was ever noted.

Now all these inspections were not done for the benefit of any of you. They were strictly to benefit US since we were flying the machine.

The bottom line is simple - either build it or pass! If you do build AND fly it, it will not fall apart on you. I would certainly not be building another if I thought that was even a remote possibility.


08-23-2004, 04:00 PM
Thanks for your input Ralph. I guess the real point is that unless your doing some crazy sh*t in your bee or pull up hard in a corkscrew dive as one pointed out, your not going to break EITHER mast. I suppose if your that worried about it, you shouldn't build it. Since I will be buying from StarBee, I can trust their single mast, since I've heard nothing but good things about them...

08-23-2004, 05:16 PM
just to reiterate, I think the service on the field record is the critical one and with out doubt the bee has an excellent record.

(3) A number of times the upper seat braces were loosened up to check the bolt for corrosion. At NO time was there ANY deformation of the hole for the upper seat brace attachment noted. Deformation of that hole would certainly be noted if the mast were stressed to anywhere near the point where buckling of the extrusion was an issue. That was true for the single (0.125 wall) mast at 2 years and the redundant mast after a total of 10 years of service.


Ralph, re. your comment on the seat back holes, the holes are not critical in fore/aft bending and are not stressed highly. The peak stress is just above the holes but on the fore and aft faces of the tube ( not the sides with the seat brace holes).

The reason for me bringing this up was to get some insight into what limit loads are realistic for gyros. There is a long standing belief that the mast is over designed, it may well be, but the question is: over designed for what loads? It is in this regard that I am interested in the stress calculations.

Brian Jackson
08-23-2004, 06:44 PM
I think Ralph just summed it up nicely. Past a certain point these may be silly concerns.

There was a time not long ago when such concerns didn't exist, and people put blind faith in other's non-researched designs, buying up ultralights in an '80s Mall-style "impulse buy". A lot of good people died, assuming their ships were safe.

I'm thankful for this forum, and I ask a lot of seemingly ridiculous questions. Never once have I been told that I'm an idiot for asking them. Newbies (like me) bring an unorthodox way of understanding things, thus keep the high-timers on their toes, or so I'm told. :) The fact that we now question and research minute details, rather than jumping in blindly, is a testament to the efforts put forth by the aviation community. Education is paramount to safety.

Like GyroRon says, "Just my 2 cents."

Brian Jackson

Brian Jackson
08-24-2004, 12:09 PM
This negates my previous posts.

Nickel plating is NOT an option for a couple of reasons. I just got off the phone with the engineer. True it's a build-up layer and doesn't affect the substrate material. But the nickel itself is very hard,with a Rockwell value around 50~55... much harder than traditional anodizing which is around 40.

Nickel plating is subject to fracturing called "crazing", which could easily be misinterpreted for metal fatigue. Antiquing of old china stoneware often uses "crazing" to fulfill the effect. A crack in an aircraft component is an entirely different issue. How would you know if it's cosmetic or structural?

So it comes full circle. Mr. Taggart is totally correct in his decision to anodize the tubes. There is a reduction in fatigue life, but no worse than leaving the material untreated.

For those interested, here are some links:
link 1 (http://www.finishing.com/153/54.html)
link 2 (http://www.matronics.com/rv-list/hovan/tips/AlAnDef.html)
link 3 (http://www.findarticles.com/p/articles/mi_m0HRR/is_2001_Summer/ai_75477957)

When you read them you will understand my concern, and why I investigated the subject.

Brian Jackson

Chuck Irby
08-24-2004, 12:26 PM
Wouldn't powder coating be the best of the alternatives then?

Brian, in your research did you learn how, or why, is it detrimental to leave 6061 T6 untreated? I have some that has been exposed on my machine for about 5 years and it still looks like some I recently purchased.

08-24-2004, 03:46 PM
But if the Single mast is stronger, won't there be less chance of cracking?

Metals are either hard or soft; brittle or ductile. When an alloy is formed or treated in such a way to make it hard (strong) it will become more and more brittle at the same time; meaning it will take an applied load up until a certain point with minimal deflection until it cracks or breaks. If it is left soft (weak) than it will be ductile; meaning it will bend and deform under smaller loads, with not so much a tendency to suddenly fail.

The softer a material is the longer fatigue life it will have (number of cycles, or number of times it can bend back and forth before it fails). The harder it is, the more resistant it is to bending, but the shorter the fatigue life.

So it comes full circle. Mr. Taggart is totally correct in his decision to anodize the tubes. There is a reduction in fatigue life, but no worse than leaving the material untreated.
Aluminum oxide is harder than regular aluminum (therefore more brittle too!). When you anodize aluminum, you are turning the outer five ten-thousandths of an inch or so into aluminum oxide, to give it a uniform, hard, resistant to the elements finish. The aluminum oxide is porous, so you can dye it colors at the same time.

The outer surface will be harder and more prone to fracture, yes.. but when you are talking about 5/10,000 of an inch the difference is so negligible that you should not be worrying about it at all.

If you brush by your gyro with your belt buckle you could create a gouge (scratch) in the aluminum that is deeper than 5/10,000. This would in theory create a weak point in the aluminum where stresses would be concentrated, but to worry about something like this is insane, because it is so negligible. Much the same with the weakening effects of anodizing.

If you don't do anything to your aluminum and just leave it bare.. it will naturally oxidize over time itself anyway.

08-24-2004, 04:03 PM

I'm certainly not suggesting any cause for concern in this design, but I would challenge the assertion that experience of flying examples proves anything in this case.

The assumption that the 50 or so flying 'Bees have accumulated enough hours to have encountered the maximum stresses which can occur doesn't seem valid. These aircraft are flown in light recreational use, nearly always in good weather. Has anyone ever wrecked one?

If not, there's no data on which to base an evaluation of airframe failures.

A design such as the Cessna 172, which I believe just celebrated its 150,000th unit, is different. It is a popular design for training, and often equipped for IFR, so the fleet has hundreds of millions of hours, and many examples have been pushed till they've broken. Ditto for bicycles or automobiles produced in volume.

It may be only Bensen and RAF which have enough units out there to start making assumptions based on field experience.

08-24-2004, 04:40 PM
I guess were getting worked up over nothing then, huh? It seems to me from this discussion that either setup is just fine and the odds of some type of failure are too low to drive ourselves nuts over.

08-24-2004, 05:25 PM
Yea, but I still want to know how much the nickel plating is.

Chuck Irby
08-24-2004, 06:08 PM
Hey Tim, if I am reading your mind, that thing, on a bright sunny day, would blind anyone who looked directly at it. :D

Doug Riley
08-25-2004, 09:38 AM
The +3.5 G does not represent a load that can actually be achieved by the rotor.

Take a typical 'Bee with a 24-foot rotor that turns 325 RPM at one G. RRPM changes as a function of the square root of the change in G load. The square root of 3.5 G is 1.87.

The rotor blades' tip speed is 408 ft./sec. Tack on another 73 FPS to get the advancing blade's tip airspeed if the gyro is doing 50 mph airspeed.

The tip speed would have to increase, applying the square root rule, to 763 ft./sec. Add on the gyro's forward speed at 50 mph and the advancing blade would see an airspeed at the tip of 836 ft./sec.

This is deep into the transsonic* range and our blade airfoils (which are low-speed types, not much different from Cub wing sections) simply won't fly at that speed without an impossible amount of power and all the instabilities that Chuck Yeager had to deal with.

Bottom line: 3.5 G represents a good margin of safety over the maximum G that a rotor can be made to pull. That max G is probably more like 2.5.

It's better from a design efficiency viewpoint to rake the mast. It reduces the bending loads. The 'Bee's vertical mast makes mounting the Rotax upright much easier, though. It seems to me an acceptable compromise in a Part 103 type gyro. A 'Bee can be expected to flown in roughly the flight envelope of a Quicksilver (though with more tolerance for wind). If you really want to rip it up and do aerobatics at 100 mph, buy something else.

P.S. Paul Plack: Yes, people sure have wrecked them. As kit maker for awhile, I got several sob stories about blade strikes and rollovers. The guys who did the rolling can comment about whether their masts broke; sorry I didn't keep statistics, beyond noting that some did and some didn't.
* Upon reflection, I'm watering this down a bit. The precise meaning of "transonic" is a speed high enough that local speeds on the body are supersonic. With the speed of sound around 1100 fps, that's probably not happening on a rotor blade at 836 fps. However, low-speed airfoils become ridiculously draggy at those speeds, so the basic conclusion remains the same: an autogyro rotor of our type isn't going to rev up that high; the power required is too great.

08-25-2004, 11:48 AM
Thanks Doug! I agree 3.5 G does seem a stretch but my basis for using it was section T ( the UK gyro standard).

From what I am aware of, the subsonic airfoils typically start performing poorly when compressibility effects come into play which is typically at ~750fps. I too did the sums to determine if 3.5 Gs where feasible. I came up with tip speeds a tad below 750fps and power required ~ 50 hp ( this ignores compressibility effects). However, I ignored the forward speed and so my numbers are a little more optimistic. Factoring in the forward speed, as you say +2.5G limit load and +3.75 ultimate load ( with 1.5 factor of safety) seems reasonable, though personally I think 3.00 G and 4.5 (ultimate) may be an all round good standard for single seat ultralight/ ultralight++ gyros. Wonder what the bee was actually designed to.

What about gusts though? Can they as in FW result in high loads? I think not as the rotor inertia damps any sudden increase in G forces ( the rotor needs to accelarate first before the G loads can be transfered to gyro).

Doug Riley
08-25-2004, 12:13 PM
Raghu, this got kicked around on the old forum awhile ago, back when Craig Wall used to participate (I don't know if your tenure and his overlapped!).

Someone pointed out that Jim Vanek (king of the modern gyro loop) reports G-meter readings in the high 3's or low 4's (forget which). The business about compressibility and power-required was duly trotted out to show that you can't get a rotor to generate those G's by RPM increase. I think it was Craig, though, who suggested that an up-gust does the same thing as suddenly yanking the stick back -- and that, in turn, is the same as suddenly increasing collective pitch: the AOA of all the blades goes up at once. The effect isn't as dramatic as it is in a FW plane, because the AOA increase is tempered by the (relatively) great inherent airspeed of the blades due to rotation.

So, yes, the bump from a thermal has less effect on a rotorcraft than on a FW plane, but, yes again, it's there -- apart from the RPM effect. What G load that can give us, I don't know. Jim's loads seem high, however, and are the result of violent maneuvers that no sane person should be doing in a Gyrobee (or anything else with a similar mast).

C. Beaty
08-25-2004, 12:19 PM
Raghu said:

"From what I am aware of, the subsonic airfoils typically start performing poorly when compressibility effects come into play which is typically at ~750fps."

That's local velocity, Raghu, not freestream velocity.

I have a copy of a Boeing rotorcraft airfoil handbook that shows considerable compressibility effect on thick airfoils at high angles of attack at speeds as low as M= 0.35 or so based on wind tunnel data.

Brian Jackson
08-26-2004, 07:43 AM
Brian, in your research did you learn how, or why, is it detrimental to leave 6061 T6 untreated? I have some that has been exposed on my machine for about 5 years and it still looks like some I recently purchased.

Hi Chuck.
My actual "research" thus far is more like question-asking from an ignorant (meaning uninformed) newbie to the field of metalurgy. I hope that my concerns and relevant postings on the subject weren't misconstrued as being untrusting or doubtful of those whom have vastly more experience than I. It's just my personality I guess. I need to understand complicated subjects in depth before I feel confident with a decission to proceed. I'd like to believe those are traits of a good engineer.

Regarding your question about leaving the aluminum unfinished, I think Nick made a valid point that coincides with everything else I've read... the aluminum will oxidize on its own over time anyway. Weighing that against the sheer number of bare-AL Bensen's still flying since the '50s & '60s, I'd surmise that it's a moot point to debate the safety concerns of anodizing. From where I sit, it appears that the oxidation/hardening is inevitable and unavoidable, and based on a lengthy track record has no provable detrimental effects.

There may be other corrosive effects (salt, skin acids, etc.) that factor in over prolonged exposures to bare-AL, but that's way out of my league. What's interesting (and humbling) though is the newfound respect I've gained for those well versed in metalurgical study. The science is fascinating.

Doug Riley
08-26-2004, 07:57 AM
Speaking of corrosion: The type of corrosion that's more of a worry in the real world is galvanic corrosion. This is the type that occurs when two dissimilar metals touch in the presence of an electrolyte. They form the two poles of a battery and one eats the other up. The electrolyte doesn't have to be anything as potent as battery acid; acid rain or salt water (live near the beach? Or in road-salt country?) will do it. In fact, I suspect that a local male dog "did it" to my Air Command over the years, as I got bad corrosion on the outer ends of the axle of my gyro where it sat in an open hangar. You could see tracks under the paint that looked like the veins in your arm; when you picked them open, white powder blew out.

Moral: Steel and aluminum parts should be electrically separated by paint, plastic gaskets or other means, even if you go for the bare-aluminum look for the rest of your frame. Stainless steel, strangely enough, is even more chemically active when it touches bare aluminum tha regular steel -- those parts on my Air Command were stainless-against-6061-T6.

Brian Jackson
08-26-2004, 10:36 AM
Mr. Riley,

Wow. I knew dielectric unions were preferred, but didn't realize Stainless Steel was such an instigator. Though proven safe, the GyroBee has many such stainless-to-AL unions, for example the Shock Plate/Mast union, which I believe is direct-bolt. There's also a direct stainless (high carbon) cluster plate to 2X2 AL union specified in the original plans, though StarBee Gyros are utilizing a 3/16" AL cluster plate as opposed to the 1/8" stainless. I assume the 50% thicker material is to compensate for strength.

There are many other occurances of dissimilar metal contacts in most gyros. Certain stresses demand materials of higher tensile or fatigue/cyclic values. That was what originally piqued my interest in the subject, at least where masts were concerned. Seems reasonable to spec a finish with the highest surface flexibility and the least reduction in fatigue life. But from my chair such a process remains the "Holy Grail" and doesn't yet exist.

Until such time, it seems good engineering will continue to be a series of educated and tested trade-offs between form and function. I hate to use that old design clichet', but until Star Trek HoloDecks become a reality for product design, we're stuck with the tried and true methods.

In brief conclusion, gyro airframes appear to me to be quite forgiving. Why else would so many of them still be flying safely after this many years?

Brian Jackson

Doug Riley
08-26-2004, 10:51 AM
Brian: We've kicked this around a lot on the forums. In theory, all those aluminum-to-stainless joints are pure poison. I don't have any unpainted stainless touching aluminum on my Gyrobee, for what it's worth.

To be honest, I pooh-poohed the whole subject as overblown for a long time. Getting into sailboats (which have lots of stainless parts attached to aluminum), plus the corrosion-tracks-under-the-paint incident I mentioned, made me a little more cautious. Galvanic corrosion is a huge deal on boats.

There are still lots of gyronauts who'll tell you it's all theory and doesn't matter much on gyros. It's worth noting the issue, though.

Brian Jackson
08-26-2004, 11:34 AM

Agreed. In the grand scheme it's probably a tivial issue. But for some guy years from now, who knows. I love this forum because chances are if you've thought of it, somebody out there has researched it. That's precisely why I freaked out when I started a thread several days ago asking about Glide Ratios. Who would have believed active pilots would set their alarms to get out of bed in still wind just to investigate the subject and post their findings?

To me, as a new PRA member, it's comforting beyond words to know first hand the level of support other gyro pilots bring to our craft. It adds a whole new level to the term "sweating the details". Though I have a sincere and deep-rooted respect and admiration for everyone whom flies gyros, especially those with larger testicles (Birdy!) Many newly introduced to rotorcraft will undoubtedly contemplate such things. Just yesterday I introduced a good friend to rotary wing aviation. This sharing of knowledge, I believe, is crucial to furthering our sport.

Doug, I firmy agree with you, as you said "it's worth noting the issue." You've been involved with sailboats, which is a whole artform unto itself. Do you know of any current data (forgive the pun) online that deals with corrosive issues or dielectfic properties of dissimilar metals? At some point this will come up again.

Brian Jackson

Doug Riley
08-26-2004, 11:46 AM
Brian: My info comes mostly from boat books. Many of them reprint the galvanic series, which is a table that classes metals in a way that lets you know how vigorous the inter-metal corrosion will be if it gets going. Sorry I don't have an online reference for you. Maybe Google "galvanic series." Or maybe someone else can jump in...?

Brian Jackson
08-26-2004, 11:54 AM

I took your advice and came up with THIS (http://www.corrosionsource.com/handbook/galv_series.htm). I'll read it through. Thank you for the tip.

Brian Jackson

P.S., If anybody else is reading this thread, I'm truly not neurotic... just an information junkie :)

08-26-2004, 12:47 PM
Well, I'm painting my bee, and all the plates n such, so I'll be in good shape there...I'm starting to get nervous, I'm about to order my first set of parts....

08-31-2004, 08:49 AM
The assumption that the 50 or so flying 'Bees have accumulated enough hours to have encountered the maximum stresses which can occur doesn't seem valid. These aircraft are flown in light recreational use, nearly always in good weather. Has anyone ever wrecked one?

If not, there's no data on which to base an evaluation of airframe failures.

I wrecked my Bee once trying to master flying it with an overhead stick... long story not relevant to this thread. The gyro basically rolled over to the right on a concrete runway at or near liftoff speed (~25 mph). It came to rest inverted on the right side.

First I should mention again that I have a folding mast. The upper redundant mast consists of 2 each 24" long 1" x 2" x 0.125" wall 6061 tubes secured to the lower redundant mast via 2 each 0.125" thick SS plates. There is an approx. 1/4" gap between the upper and lower masts when in the raised (upright) position.

The damage to the rotor head / mast assembly was as follows:

Pretzeled Rotordyne blades... There were some tears in the aluminum skin, but no failures of the bonded seams. The cast tip caps separated from the ends of the blades and were never recovered.

The hub bar was twisted slightly about its long axis... not visible to the eye but measurable with dial indicators.

The main bolt in the rotor head that held the teeter tower / bearing assembly to the torque tube (bar) was stretched and warped. The torque tube (bar) and teeter bolt were warped.

The aluminum cheek plates directly under the rotor head were warped and as a result, the rotor head was twisted slightly in a counterclockwise direction (when viewed from above) in relation to the upper mast.

The upper mast was undamaged.

The SS plates between the upper and lower masts were warped, and as a result, the entire upper mast was additionally twisted counterclockwise (when viewed from above), to the left, and backward in relation to the lower mast. Some of the bolt holes drilled through these SS plates exhibited some elongation as the plates stretched and twisted.

The upper few inches of the lower redundant mast (above the seat) was bent slightly to the left.

Nothing on the mast assembly failed.


John L.

08-31-2004, 11:05 AM
A little history may put this issue into a bit of perspective.

(1) When we first flew the Bee we were using a single 2x2 mast with 1/8 inch (0.125) walls. The Bumblebee had such a mast, as the the Aircommand machines. We had no problems, but I wanted just a bit more margin.

(2) During the second flying season I located a source for the 1x2x0.125 extrusions and we replaced the mast with the current redundant configuration. The result was an incremental improvement in load factor, but not as much as one might predict since the pieces were bolted together - not bonded. However, the psychological value was high! Just for the record, the new mast was about 47% heavier than the original single-tube version.

When it came time to work up the documentation, the redundant mast was obviously the one specified.

Now fast-forward 12-13 years and we find ourselves building a StarBee kit. One of the options is a single-tube mast with a 3/16 (0.1875) wall. This option:

(1) is simpler than the redundant mast

(2) is stronger than the redundant mast

(3) is lighter than the redundant mast (45% heavier than the original single-tube mast as opposed to the redundant mast at 47% heavier)

If I could have laid my hands on this extrusion back in 1990 I would have used it on the original machine. There is nothing "wrong" with the redundant mast, but this decision should be a no-brainer!


09-01-2004, 08:22 AM
Straight from the man himself!!! I think that should settle it!!! Still waiting for my parts to arrive... oh, the agony!!!!

10-16-2004, 11:47 AM
Ralph, re. your comment on the seat back holes, the holes are not critical in fore/aft bending and are not stressed highly. The peak stress is just above the holes but on the fore and aft faces of the tube ( not the sides with the seat brace holes).

This probably may be interesting (and may be not as well):
I tried to estimate mast longitudal bending loading using CosmosXPress. It's actually not professional programm, just a trainer, but it may show some basic things and it shows them overscaled for better pictoriality. I wonder how many things in stresses and design became clear for me since I started to play with SolidWorks and this XPress stuff...
This small animation (http://delta.wtr.ru/files/stress.WMV) shows how would a mast bend if stressed by rotor drag. Note that the most stressed areas are quite around strut holes.