50 foot-pounds on the yoke...lots? not so much?

[stepwilk]

I'm writing for an aviation-history magazine about a Diesel Eight that actually went supersonic in a dive in 1961, during a Douglas test flight. The test program called for trimming the stabilizer to a certain degree up before starting the 15-, 20- and 25-degree descents, and this meant a force of 50 foot-pounds was needed on the yoke.

How would one characterize such a force? Big push, not a big deal, enormous, whatever?

I've never flown anything bigger than a Constellation, but I don't think I ever pushed 50 pounds...

[VinRouge]

Depends how long the yoke was!

[ChristiaanJ]

stepwilk,
I'm NOT trying to be funny or anything.

But how long is the yoke? Very roughly 2.5 feet, I would say.
So 50 foot-pounds - which is NOT a force , but a moment - corresponds to about 20 pounds force.

Now get your bathroom scale, and push it down until it indicates 20 pounds.
Certainly enough to be significant, but not enough to be "a big deal", I would say.

It's certainly more than you would want to have to maintain for any length of time during a 'normal' flight without trimming it out, but to me it sounds perfectly normal for a test flight of the kind you describe.

CJ

PS And yes, I know the story. The supersonic dive of the DC-8 was perfectly deliberate, and part of the test programme, not an unforeseen incident!
I wonder if anybody on the ground did hear the bang...

CJ

[stepwilk]

Well, the official Douglas report of the test flights says--and this is the reproduced original, not a transcript--"The dive and recovery techniques to be employed were to include pretrimming the stabilizer on dive entry, which resulted in application of approximately 50 foot-pounds of push-force to maintain the stabilized dive."

should I assume that whoever wrote that report was being a bit careless in his wording, which is quite possible? Because as a writer, I certainly interpret that to mean that no matter what mechanical advantage pertained farther along the system, somebody had to push on the horns of the yoke with a force (not a moment) of 50 foot-pounds.

Or does the "push-force" refer to what the resultant force was at the fulcrum of the yoke and the elevator controls?

[Mr Optimistic]

It would certainly make more sense to specify the force than the moment however foot-pounds is a moment, as distinct from pounds force, which is equivalent to the weight of one pound mass.

50 pounds force is a lot and approaching the limits for a 'safe' single man lift. OK for 10's of seconds I would have thought but I wouldn't like to do it for minutes judging by the weight of the 25kg sacks of coal I lug around.

[sycamore]

There would normally be a special set of `grips` on the control column and rudder pedals for measuring the forces when doing stability and control test manoeuvres,as well as displacement gauges. They would also be recorded at the same time,and telemetered to the ground station,so that if one was approaching a particular `limit` in terms of force/displacement,or a `reversal`,one was aware of it.The limits would have been specified in the Airworthiness requirements and/or the Design limits/Stress engineers, something like 75lbs pull/push for longitudinal control/30 lbs for aileron/150 lbs for yaw.Also depends on whether it is for `stick/wheel`,fighter/transport,manual,or powered controls..

[ChristiaanJ]

Should I assume that whoever wrote that report was being a bit careless in his wording, which is quite possible?I would think so.

When talking flight control engineering, the significant figure is the moment, because it is valid all along the 'chain', from the control column to the elevator hinge, while the forces vary all along the way because of the different lever arm lengths.
Since it is a technical report, it would be the more relevant figure to describe what was happening.

Where it says "... application of approximately 50 foot-pounds of push-force ...", I would say it would have been more exact to have said "... application of approximately 50 foot-pounds of nose-down command ...".
In other words, I think that when the author wrote "push-force", he only meant to refer to the direction of the force needed to provide the 50 foot-pounds of control input.

When sycamore mentionsThe limits would have been specified in the Airworthiness requirements and/or the Design limits/Stress engineers, something like 75lbs pull/push for longitudinal controlthose limits are the requirements for the handling qualities for the pilot, and those would be indeed specified in terms of force exerted by the pilot on the controls.
Strictly speaking, the Airworthiness requirements, as such, couldn't care less how that control force is translated to a hinge moment at the level of the elevator !

CJ

[ChristiaanJ]

IGh,
I agree, what's measured is force and displacement, not the moment.

stepwilk,
Any chance of a link to the official Douglas report?

IGh quotes the AAHS Journal article, which is obviously derived directly from the report, but may not be exempt from a possible typo.

CJ

[stepwilk]

Yes indeed, there's an excellent link to the official Douglas report in its original typescript form. Go to Fred Cox DC-8 Jet Collection Index (http://www.dc-8jet.com) and click on "supersonic DC-8," or whatever the exact wording is.

[stepwilk]

Aha! Finally an answer to my original question: "How would one characterize such a force?"

And the answer is, "Not a huge amount of force." Thank you, sir.

[ChristiaanJ]

stepwilk,
Thanks for the link! Just read it through.

Page 3 mentions the "application of approximately 50 pound push force to maintain the stabilized dive".

Page 5 also mentions a "pull force increment exceeding 100 pounds", presumably from the initial 50 pounds nose-down.

1) I was wrong, obviously. The report says 'pounds force'.

2) Where did you find the 'pounds-feet'?

3) As ASFKAP says, it's not a huge amount of force, but it's getting significant....
Not "struggling with the controls", as the journos love to say, though.

CJ

[stepwilk]

The "foot-pounds" was my misteak--too casually using the phrase most familiar to an American non-technician.

[ChristiaanJ]

The "foot-pounds" was my misteak--too casually using the phrase most familiar to an American non-technician.All problems solved the, it seems.

AND you have an answer to your original question.

AND, thanks for what turned out to be an interesting thread !

CJ

[stepwilk]

"Not 'struggling with the controls', as the journos love to say, though."

Careful, now. I don't style myself a "journo," but I am a writer. Which is why posed my original question on this forum.

We do our best, as do many of the journalists.

[ChristiaanJ]

"Not 'struggling with the controls', as the journos love to say, though."
Careful, now. I don't style myself a "journo," but I am a writer. Which is why posed my original question on this forum.
We do our best, as do many of the journalists.I'm sorry if you read that in my remark.... it was just an aside, and not aimed at you at all.

I was serious when I made my remark about using a bathroom scale to try it out, and no doubt so was Mr Optimistic about his sacks of coal.

Pushing or pulling 50 lbs from a sitting position (not lifting it, which is why the sacks of coal or potting soil sack analogy doesn't qute hold) is still quite significant, and not something you want to do for any extended period of time.

CJ

[stepwilk]

Christiaan, I know you weren't aiming it at me, you may well not even have noticed that I was a writer in my original post. It's just that I react to the fact that all of us, including me, are constantly criticizing each other's professions, whether I'm making crude jokes about lawyers or SLFs are groaning about bozo pilots or--as it sometimes seems--the entire world is unloading on journalists.

"Those that can't do, teach." "If you're a journalist, you never let the truth get in the way of a good story." "Goddamn cheapskate lawyers will have us all in the poorhouse as soon as they're done with their class-action suits." "Who can be dumber than a plumber?" "Farmers, all they do is live off their damn subsidies."

How's that for thread drift, but can't we all just get along?

[ChristiaanJ]

stepwilk,
I saw in your first post you were a writer - one of the reasons I answered !

As to your comment, I think we agree: in every profession there are good 'uns and bad 'uns.
The "struggling with the controls" is just a very old PPRuNe 'in' joke, nothing more.

And I hope to read your magazine article sometime !

CJ

[John Farley]

Going back to your original question it is surprising what forces one can apply to a yoke (especially in the push sense).

100 lbs of push is no big deal if you can lock your arms straight.

A long time ago I was asked to measure the yoke forces on a totally manually controlled CN-235 in the cruise (215 kt) following a runaway electric trim.

I chose to do the nose up case first because that does not rely on your finger strength to pull on the wheel. We had an instrumented yoke and when the force got to 100lb I quit and returned to the hangar to do some checks of the seat locking and rail system. (I was not keen on pushing hard only to have the seat shoot back due to the locking or rail system not coping - as that seemed to me a certain way to have the wings off).

In the hangar I was able to push 300 lb with both arms straight (I am a weedy bloke) and was satisfied that before taxi one could do a visual check that the seat locking pin was properly engaged.

Thus reassured I did the full nose up case which turned out at 235 lb of push.

Since the trim range available in the cruise was equal I declined to do the nose down case beyond a 50 lb pull (roughly 1/4 trim) as I reckoned it would be all too easy to have the yoke slip out of your fingers at higher pull forces. Wings are great for flying but they do need to stay attached.