Mast Failure - Old Air Command

[Chris Burgess]

Can someone fill me in on just which AC masts were suspect with "scored" holes in the mast? I would like to know because I heard of a recent failure on landing that I would like to know more about. I remember reading about this but don't remember what years were suspect. I want to "renew" the warning to check "old" mast for inside wall scoring. Thanks

[Timchick]

When in doubt, swap it out.

[GyroCFI]

If my recollection serves me correctly Chris, it was the versions that had a single locating bolt for the engine mount through the mast.

Don

[Riff Raf]

This might be what your looking for.

http://www.aircommand.com/safety2.php

[gyroplanes]

The very early Air Commands had a single 2x2 tube mast and all of the "pmemp" engine mount plates drilled through.

The mast was longer than Bensen's and the new Skywheels a good bit heavier than the Bensen blades. The increased mass above caused flexing of the mast, especially on grass runways, and lead to a tell tale flexure stress fatigue at a through bolt hole, just above the seat back. You could see the fatigue lines clearly from the outside on the forward edge of the mast.
The first reported case of this was on Ora Cooks Air Command......... ask him about it.

Air Command changed to a "triple hollow" extrusion, and later, to the 1x2x (2) configuration.

[Doug Riley]

In the failure-prone versions, all five of the bolts that secured the engine mount cluster plates to the mast passed THROUGH the mast. Five 1/4" holes behind and just below the top of the seat tank.

2x2 x 1/8" extruded tubing has quite a large industry tolerance for variations in wall thickness -- plus or minus .006, IIR. Bensen specified tubing with half this tolerance but, in any case, you have to hold the bolt holes in a bit more than nominal if you are going to avoid scoring the walls of the "plus" pieces.

Extrusions sometimes aren't quite square, either.

All this explains why the Gyrobee plans tell you to pull the holes 1/32" in from nominal.

[phantom]

some years back there was one that failed up here in canada and it was checked out by transport canada, they found that it was made from 6063 and not 6061 as it should have been.

norm

[Chris Burgess]

Thanks for everyone's comments. They are appreciated.

[Chris Burgess]

Some photos to ponder. Not great quality. No one hurt on this one. Lots of unknowns.

[Doug Riley]

Interesting that this one broke off at the compression-strut attachment blocks, not the engine mount. Also that it's a two-tube "redundant" mast.

It's scary to think of the upper section of the mast snapping off WITH A RUNNING ENGINE ATTACHED. Sheesh.

The "redundant" mast adds a good bit of strength in the fore-aft bending axis, but it's actually weaker side-to-side than a single 2x2 square tube unless the two 1x2's are structurally bonded. If they are, it's still not stronger laterally, just not weaker.

Omitting holes and using clamp-on fittings preserves more of the tube's strength than does the switch to the "redundant" design.

[hillberg]

After looking at the pixs:eek: The hole in the bottom of the seat would come in handy.:twitch:

[Jazzenjohn]

I was talking to someone at Oshkosh or Mentone this year and they kept insisting that it was no big deal having just one little hole in the mast. I couldn't convince him it was a bad idea, or that it isn't just one hole. If you look at the break, it is actually 4 holes, all in a row, almost like a perforation you might use to get a piece of paper to tear along a line...

Looking at the gyro from the front, it looks like there might have been a blade strike on the end of the left blade in picture 2.

[GyroCFI]

How old are these pics CHris? what was the disposition of the pilot after the incident?

[NoWingsAttached]

I wonder if it ever suffered some hard landing prior to the accident, which may have cause a later failure?

[DennisFetters]

I can chime in on this subject. The pictures are of a highly modified Air Command, using a KB 10 gallon seat tank, home-made mast and improper rotorblades. About all the no-no's you can do.....

Judging by the pictures, it looks like the rotors were not up to full speed, so it was probably a take-off blade flapping accident. The terrain is certainly conducive to help cause this.

To my knowledge, there has never been a in-flight failure of a mast that was supplied by Air Command when I owned the company. I still believe that is a true statement, although I have been out of that seen for quite some time and the aircraft are now considered as antiques.

After I sold the company, there were quite a few counterfeit parts, including masts that were sold to, or built by owners.

The only masts that had failed that were manufactured by me were two that I know of, and each owner had taken some bad advice and filled the tube with wood, and then drilled it out. This is a double no-no for two reasons;

1. I never drilled a mast tube all the way through. I had special fixtures and drill-stops and drilled from both sides, and only drilled enough to clear through the wall, so not to score the inside wall of the tube. When they drilled out the wood, they scored the tube and caused a weak spot and stress riser.

2. As first reported by Dr. Bensen many years ago, you should never install wood into a tube that has inside corners, like a square mast. The wood will expand and contract from changing moisture and flex the corners of the tube, causing failure over time. It will also flex at the scored drilled area and cause it to brake. True to Dr. Bunsens report, this happened to the two people that thought they were adding redundancy to their mast, and credited the wood for being the only thing that saved their lives, when it was the wood that almost caused their demise.

Next, I drilled the two holes at the top of the engine mast intentionally so to be the weakest spot on the mast... for a reason. Back in the early 1980's, I was fortunate enough to spend 4 months at the Bensen factory to obtain Bensen approval for my Rotax engine installation package. During that time a was able to spend almost every evening with Dr. Bensen and his right-hand man Charlie Elrod, and I was able to obtain a unique insight to why they did what they did.

Dr. Bensen originally designed the old off-set Gimbel head torque tube with a weak point at the forward and aft pivot, so if the pilot were to have a blade strike, the head would separate allowing the rotorblade energy to leave the airframe, allowing the airframe to come to a rolling stop, upright. This worked as designed, until a group of people got together showing this designed-in weak spot, and turned it into a issue for other purposes..... kind of like the CLT story now. Dr. Bensen gave into their hysteria and changed to the solid torque bar, which as he admitted to me, caused many more injuries and unnecessary damage to the machines after a blade strike.

I used the same theory as he, only I did it at the mast, and it worked perfectly. Not one person had ever been hit by the blades with my design. The blades would depart, and the aircraft rolled to a stop with the pilot wondering what just happened. Next week he was repaired and flying again.

I also didn't use a double tube redundant mast for the same reason. It worked like the leaf springs on your car, and when the blades hit the ground it would bend the mast like a spring, and release that force causing the aircraft to flip.

This is also why I came up with the triple-hollow mast that Tom mentioned. I wanted more straight because I was thinking to use the 120hp Arrow engines, and I didn't want the spring action of the double tube mast.

The fact is, that with the Air Command mast that I supplied, there was never anyone hurt from a blade strike, and the machine needed minimal repair. While, with a redundant mast with clamped engine supports on other designs, there were many people hurt and even killed, and the machine was almost always totaled.

Now you have the facts about this thread and Air Command mast failures.

[Doug Riley]

I tipped my original-issue Air Command over in deep, wet snow with the blades near flight speed.

Before flying it th efirst time, I swapped out the "perforated" single 2x2 mast for a double 1x2 one with a clamp-on engine mount that is kept from sliding by angle braces down to the vertical-strut blocks. My efforts to obtain the materials for this project resulted in my organizing AEROTEC, Inc. to supply them to others.

In this tip-over: (1) I was not struck by the blades and received no injuries of any sort (2) the McC blades were totally trashed (3) the torque bar and some other rotorhead bits were bent, smooshed or otherwise rendered useless (4) the mast and associated bracketry were not bent, broken or damaged in any way.

In the one specific type of accident that Dennis alludes to (a blade ground strike in back), a "breakaway" mast may reduce frame damage. In accidents in which the machine tips over thanks to other forces (cross winds or -- in my case -- digging into heavy snow with a 4" nosewheel), however, the continued presence of some kind of roll bar above the pilot's head is critical to preventing a broken neck.

Ideally, we'd have a roll bar that is independent of the mast. This would reduce the chances of the roll bar breaking off in a rotor strike. If the mast is going to have to serve as the roll bar, however, I think a breakaway setup is a gamble.

[Andrewka]

You saying Dennis is wrong in his idea of the AC lowrider??

[DennisFetters]

You saying Dennis is wrong in his idea of the AC lowrider??

It's not an idea; it was good engineering 25 years ago on a revolutionary machine that stood the test of time. Anyone can say whatever they want, but history proves me right, and you can't argue with history without being wrong.

A crash is a crash and there are no guarantees of the outcome, and there are many ways for a pilot to crash his aircraft, and there is no way to design an aircraft of this size to respond best to every type of crash or it would be so heavy that it wouldn’t fly…. or be of legal weight.

Fortunately for people like Doug that were supplying things like untested counterfeit masts, he didn’t have a Federal mandate to abide by. I was dealing with a strict legal weight limit imposed on the industry by the US Federal government to keep the aircraft under 254 pounds, and a double redundant mast and engine clamp would have put me over that weight limit. So, I elected to design the mast breakaway system for the most common type of accident. Since 99% of my customers were going to be new gyroplane pilots learning in their own machines with just some tow-glider experience under their belts, I knew that blade strikes from flapping was going to by far be the most common incident. I was right. But, thanks to my new revolutionary airframe design of all four ground contact points of the airframe being triangulated, it made the airframe not only light but stronger than other designs as well, so the overall crashworthiness was improved.

25 years later there air Commander Gyroplanes flying today that have accumulated thousands of hours on the original masts that I supplied, and none have ever failed if installed and flown correctly. Many people have walked away from an accident with minimal damage to the aircraft, and all thanks to my mast.

What more could you expect or ask for and still be reasonable?? Hindsight is 20/20, and anyone can throw out judgment-calls 25 years later. But, 25 years ago I had the guts to make a decision and go forward with a project that changed gyroplane history. Where were these judges back then with their opinions when decisions had to me made?? Most were still just a sparkle in their daddy’s eyes or digging up cat-turds in the sandbox.

[Timchick]

I know a pilot who purchased a used Air Command with a 2x2 mast and inspected the aircraft as best he could and started flying it. I'm not sure how many hours of flying he had on it when it started shaking during a flight so he made an emergency landing. The mast was bending at the motor mount hole and would've probably broken off in flight if he'd continued to fly it. Anyone who purchases a used Air Command with a 2x2 mast should plan on replacing the mast to make sure it is air worthy. If you believe a 2x2 mast is sufficient replace it with a new 2x2 mast.

[DennisFetters]

I know a pilot who purchased a used Air Command with a 2x2 mast and inspected the aircraft as best he could and started flying it. I'm not sure how many hours of flying he had on it when it started shaking during a flight so he made an emergency landing. The mast was bending at the motor mount hole and would've probably broken off in flight if he'd continued to fly it. Anyone who purchases a used Air Command with a 2x2 mast should plan on replacing the mast to make sure it is air worthy. If you believe a 2x2 mast is sufficient replace it with a new 2x2 mast.

Absolutely!

It was probably crashed and repaired to be sold.

Don't trust a pre-owned Air Command without knowing all the owners. After a crash there are people that purchased counterfeit masts or made their own that were made of 6063T3 aluminum, or that were drilled through and the walls scored. If it's not original equipment, don't trust it.

[Redbaron]

[QUOTE=DennisFetters;445311]
Fortunately for people like Doug that were supplying things like untested counterfeit masts, he didn’t have a Federal mandate to abide by. I was dealing with a strict legal weight limit imposed on the industry by the US Federal government to keep the aircraft under 254 poundsQUOTE]

Nice!:first: this is so aweful, I fell over laughing. opps

[C. Beaty]

Drilling square tube at the corners is the worst possible place if the goal is to avoid breakage.

Years ago, I was performing first test flight on a Bensen B-8 owned by a young Coast Guardsman, Tom, when the complete vertical tail fell off.

No other option but to switch off the engine and do a vertical descent.

Hit square but flipped on the bounce.

Much to my surprise, the single 2x2 mast didn’t break or even bend.

Tom’s father was a machinist at the Honeywell plant in St. Petersburg and had made clamps rather than through bolts at the seatback-engine mount.

I reported this in a Sunstate newsletter; Bensen picked this up and soon all Bensens were clamped at the seat back rather than bolted.

In my experience, single 2x2 masts give fewer problems with 2/rev vibration than double 1x2s.

[Andrewka]

Sorry Dennis. I was remarking on some one elses post.

[Doug Riley]

The weight of the clamp-on engine mount did not put the 447 Commander over the 103 limit, as the stock, stripped machine was over the 103 limit to begin with -- unless maybe you left off the prerotator or put lighter blades on it. Neither of those tactics was desirable. Light blades would increase the HTL (which was already 5-6" with McCutchens). McC's are in the midrange of ease for hand-starting: a prerotator isn't essential, but is mighty nice.

Bensen used a flexible "pole" mast in lieu of more elaborate rotor hinges. This represented a huge design compromise right out of the gate. Such a mast must be unbraced to flex properly; hence it makes a decidedly marginal roll bar.

Reasonable people can differ about whether it's better to let the "roll bar" break off at the pilot's shoulders with a fairly light overload, in the hope that this will keep the machine from rolling in the first place. If it breaks off and the machine rolls anyway, you are at greater risk of turning up dead of a broken neck.

I read and heeded Chuck Beaty's 1970's article in the SRC newsletter about various people's experiences with masts that did, or didn't, break off at the seat back in blade strikes. I calculated the strengths of 1x2 and 2x2 masts, both drilled and undrilled, before changing mine out. I wrote, and submitted to the PRA mag, an article detailing these calculations, back around 1987. Among other things, it pointed out that (1) the drilled 2x2 mast is plenty strong enough, even with holes, to do its primary job of holding the rotor on in flight (2) omitting the holes increased a mast's bending strength MORE than going to double 1x2 and (3) double 1x2 is about 20% stronger fore-aft and about the same amount WEAKER in side-to-side bending.

Bensen, in his literature, described these two last characteristics in quite deceptive lingo: The redundant mast (he said) had "double strength in the fore-aft direction and a lateral resiliency calculated to yield without breaking."

When the double 1x2 tubes are clamped together,and bolted together, at several places (as they are in practice on a gyro), their lateral strength re-approaches that of the 2x2 (but is never greater).

"Yield without breaking"? Forget about it -- 6061-T6 is not especially tough and breaks at loads only a little higher than those that cause permanent bending. 4130 is better in this regard.

I did not offer my clamp-on mounts until years later, well after my little winter tipover. My main purpose at the time was to offer a complete set of components for the Gyrobee, although I also offered a kit for retrofitting the Air Command to the design that had served me well on my Air Command. Many of the materials and parts were common to both.

[Steve McGowan]

Does these things reallllllllllllllllllllllllly fly.. Skeeres the ****tte outta me..

[DennisFetters]

The weight of the clamp-on engine mount did not put the 447 Commander over the 103 limit, as the stock, stripped machine was over the 103 limit to begin with -- unless maybe you left off the prerotator or put lighter blades on it. Neither of those tactics was desirable. Light blades would increase the HTL (which was already 5-6" with McCutchens). McC's are in the midrange of ease for hand-starting: a prerotator isn't essential, but is mighty nice.

All else put aside, I will allow 25 years of unarguable history prove me correct in my decision of why I used the single mast with drilled holes, and why I needed to use it. It simply worked as designed for its intended purpose, facts are facts and history makes for facts.

Finally, you are wrong. The 447 Commander is a legal ultralight as I sold it. It weighed 252.5 pounds. It was weighed by the Kansas City FAA office at their location. One of their inspectors had a hair up his a$$ and wanted to prove that the 447 was not a legal ultralight. They even busted one of my fly-in's with a set of truck scales and started weighing our aircraft, with inaccurate results, so I volunteered to bring a complete 447 Commander kit to their office, assemble it, let them weigh it with certified aircraft scales, and I would fly it. They accepted, and I did as promised. I assembled the machine out of the box in 3 hours in their office, folded the axles back and rolled it out their front door, stuck the blades on and let them weigh it. Yep, 252.5 pounds as advertised! I jumped on it and flew it down the ramp. They offered thanks and apologies, and a letter certifying it was a legal ultralight. There you go..... No more argument, it was just under the weight limit.

In fact, after that, that FAA inspector became a big supporter of all my other aircraft designs, and always came to my factory to inspect and certify them.

If you added options meant for the licensed Commander versions, like a prerotator, then it would be overweight. But, that was up to the owner to do after I legally sold him an ultralight, as I had to do by Federal mandate. It was between him and the FAA after that.

As an aircraft manufacturer, I was regulated on what I can do and what I could sell, unlike someone building and selling stuff out of their garage under the radar.

[hillberg]

Off the subject, Dennis how hard is it to get aircraft quality materals in China?

Aluminum,wood, steel, odds & ends?

[DennisFetters]

Off the subject, Dennis how hard is it to get aircraft quality materals in China?

Aluminum,wood, steel, odds & ends?

Solid aluminum is not much of a problem, you can get 6061, 2024 and 7075 all day long, but tubing can be a problem.

The 4130 tube is also a problem; in fact I can get it out of Taiwan but only as thin as 1mm. I don't have any problems getting it out of USA; just shipping cost is a bit much.

Hardware….. forget about it. Its pretty much crap, rod ends and stuff very sub quality. I have to import all that from USA.

It's possible to build a very high quality aircraft here in China, but you have to make sure that you have complete control over the project, show them absolutely everything to do no matter how simple that is, and you have to import all critical hardware and some materials.

Once you have them all set up in production and all the training and procedures in place, then they can do a pretty good job duplicating what you want. But if you let them try and do it on their own, it is always junk. They don't even know what they don't know.

[Doug Riley]

No argument that, as I mentioned above, if you left off the prerotator, you could make 103 weight. The no-prerotator model was never mentioned in any promotional literature that I ever saw. In fact, I've never seen an A.C., or a photo of one, without a prerotator.

Much the same happens with the marketing of all sorts of 103 craft. The manufacturer will strip one and make weight, but upsell you to the model that you actually see in the promo photos.

We will have to disagree on the judgment call about the mast-mount structure.

[C. Beaty]

I imagine, Doug, that the breakaway mast, as you lawyers might say, was an ex post facto “feature.”