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

[PW_Plack]

Raghu,

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.

 

[KDOG]

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.

 

[barnstorm2]

Yea, but I still want to know how much the nickel plating is.

 

[Chuck Irby]

Hey Tim, if I am reading your mind, that thing, on a bright sunny day, would blind anyone who looked directly at it.

 

[Doug Riley]

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.

 

[raghu]

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]

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]

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]

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]

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]

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?

Thanks,
Brian Jackson

 

[Doug Riley]

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]

Doug,

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.

Thanks,
Brian Jackson

 

[Doug Riley]

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]

Doug,

I took your advice and came up with THIS. I'll read it through. Thank you for the tip.

Respectfully,
Brian Jackson

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

 

[KDOG]

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....

 

[gyropilot]

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.

Paul,

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.

Regards,

John L.

 

[Ralph]

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!

Ralph

 

[KDOG]

Straight from the man himself!!! I think that should settle it!!! Still waiting for my parts to arrive... oh, the agony!!!!

 

[twistair]

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 shows how would a mast bend if stressed by rotor drag. Note that the most stressed areas are quite around strut holes.