Langley AFB. Possible crash a few minutes ago.

nauga said:
No idea if that's what it was intended for or not; but accepting it for the purpose of discussion, changing the balance of a control surface is a really odd way to tailor stick force per g (i.e. "maneuvering stability"). A real bobweight or downspring can be pretty f'n simple to implement and modify safely.

Nauga,
and the "Why learn aerodynamics" thread
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I agree, my choice would be a bobweight as well. What do you think the counterweight intended to accomplish in this setup, if not pilot-preferential stick force?
 
nauga said:
No idea if that's what it was intended for or not; but accepting it for the purpose of discussion, changing the balance of a control surface is a really odd way to tailor stick force per g (i.e. "maneuvering stability").
Click to expand...
I believe in a conventional sense you are probably correct. However, as I heard years ago, at this level of flying, like the unlimited class at Reno, everyone is looking for that special non-conventional mojo to give them that extra .001 MPH or that quicker, cleaner snap roll. And based on his record, he may have found that mojo in those elevator counterweights.
 
I am pretty sure that the "Plug" is the weight. The plugs are cut to the weight desired, for the balance of G force versus stick force.

On turbines, those plugs are cut to the exact weight for a balance, screwed into the correct position around the wheel, and secured immovable. Each one is cut from a long piece, and filed to exact weight, and referred to as a "Shot".

I did not cut or insert them, but did assist in defining weight and location, from electronic balancing sensors, at 3600 RPM.

Securing such weights is critical, whether in elevators or turbines.

WW 2 fighters had bob weights, and command officers decided where they were to be, to avoid pilots exceeding G forces. Pilots demanded that their mechanics reduce the arm, to allow violent defensive or offensive maneuvers. After the sides realized that the pilots were going to pull for the turn they needed, no matter how hard that was, compromises were made, and short bob weight settings became normal.
 
geezer said:
WW 2 fighters had bob weights...
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Pilots on POA are flying airplanes with bobweights (e.g. T-38 and Cessna 310, two I have personal experience with). Bobweights for maneuvering stability tailoring are not the same as mass balance weights on control surfaces and I can't for the life of me imagine why one would try to make one serve as the other. I also believe it's possible that this was *not* the intent, that something has been lost in translation, but we'll have to wait and see.

I hope that more information on the mod and what it was intended for comes to light in the investigation. I'm genuinely curious.

Nauga,
longitudinally
 
nauga said:
Pilots on POA are flying airplanes with bobweights (e.g. T-38 and Cessna 310, two I have personal experience with). Bobweights for maneuvering stability tailoring are not the same as mass balance weights on control surfaces and I can't for the life of me imagine why one would try to make one serve as the other. I also believe it's possible that this was *not* the intent, that something has been lost in translation, but we'll have to wait and see.

I hope that more information on the mod and what it was intended for comes to light in the investigation. I'm genuinely curious.

Nauga,
longitudinally
Click to expand...
 
Bacho said:
Well, if that’s not a design fault, I don’t know what is.
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Poor choice of words on my part. If it failed as indicated, clearly not a good design. My point that anyone that thinks they can make even what appears to be a simple design change to an aircraft without there being a risk of it affecting the operation of the aircraft is kidding themselves. The best we can do is try to understand all of the change's failure modes and coupling to other systems to predict how overall risk is effected.

I'm willing to bet that there were multiple people involved in the design change, and looks like nobody caught this.
 
hindsight2020 said:
@2:44 oh look at that, the guy who flies it repeating my 'uneducated' WAG.
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Thank you for saving me the time of watching the video. If you're referring to my "Why learn aerodynamics" tagline, it was directed more to the implementation, not the WAG. I still think it's a terrible idea, and an appeal to authority does not change my opinion.

Nauga,
keeping the lead in
 
nauga said:
Thank you for saving me the time of watching the video. If you're referring to my "Why learn aerodynamics" tagline, it was directed more to the implementation, not the WAG. I still think it's a terrible idea, and an appeal to authority does not change my opinion.

Nauga,
keeping the lead in
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And I agree with you! That setup is not a great idea for the intended purpose considering the existence of other alternatives like bobweights, which we've both flown enough hours to have an educated opinion on.

I only made the appeal to authority snark because you imputed my assertion that control force could be their intended motivation. IOW, you can disagree with their design choice without implying a lack of aerodynamic understanding on my part. That's all, it's not deeper than that. Cheers.
 
Hate to say it, but poor design IMO.
Screws can-and will- vibrate loose without threadlocker or other compound (such as Rocksett) which prevents that.
As mentioned, threadlocker is a poor choice for screws that will commonly be removed and replaced. Aluminum being so soft, with fine threads are subject to stretching if over-torqued which would again make it a poor choice for repeated removal and replacement. A locking screw set into the side of the access screw would have been a better design in an area with such limited clearance.

What a terrible and absolutely preventable tragedy.
 
Omalley1537 said:
Thoughts on a milled groove to hold an internal C clip as a failsafe for the plug? Too susceptible to the C clip somehow coming out?
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I think if the material is there to support it, that would be an elegant solution.
 
Safety wire, cotter pin, lots of ways to keep threaded stuff from unthreading on an airplane. Whoever else signed off on the design is going to feel lousy about this, but perhaps they didn't know that the thing becoming undone could jam something else. Because if it's just the counterweight falling out, I'd guess - just guess - that it would mean the control forces might be higher and unbalanced and a PITA to fly, but that it would still fly. Could easily be wrong, don't know anything about the forces involved.
 
Lots of bandaid solutions being proposed; but the failure mode remains the same, only the probability of occurrence changes. Seems to me if what you need is a bobweight the real corrective action is to put it somewhere where bobweights are typically mounted, far upstream of the mass balance weights.

Nauga,
and his mechanical advantage
 
Albany Tom said:
Whoever else signed off on the design is going to feel lousy about this, but perhaps they didn't know that the thing becoming undone could jam something else. Because if it's just the counterweight falling out, I'd guess - just guess - that it would mean the control forces might be higher and unbalanced and a PITA to fly, but that it would still fly.
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A whole mass balance weight coming loose on a surface as big as a typical elevator is bad news from a fouling standpoint before you even get to a change in handling qualities. Weight or screw, I can't imagine a designer with any familiarity at all with the design would not grasp the significance of the limited clearance and a removable part within. Given the visibility of the accident, I'm sure the docket will contain a summary of the design and acceptance process. I'll be reading with interest.

Nauga,
whose weights are cast in place, but he still preflights them
 
nauga said:
Given the visibility of the accident, I'm sure the docket will contain a summary of the design and acceptance process. I'll be reading with interest.

Nauga,
Click to expand...
Isn't/wasn't this an experimental? The design process and review could be "That's where I want it".
 
kyleb said:
Isn't/wasn't this an experimental? The design process and review could be "That's where I want it".
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it was an experimental-exhibition factory built aircraft. rob worked with the factory during the build to incorporate modifications he wanted. appears that there was more than one person involved that missed a possible single point failure possibility. in the end, there will probably be a few lessons to be learned from this accident.
 
kyleb said:
The design process and review could be "That's where I want it".
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Not likely for a factory build, but even if so there is still something to be learned from application of that process, both by the casual observer (all of us) and those who ostensibly hold the design details and likely some level of shared responsibility for the final product (the factory). Certainly the community that is using or considering using this type of setup.

Nauga,
and another set of eyes
 
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