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Minimum stick force gradient

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Minimum stick force gradient

Old 2nd May 2012, 09:59
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Minimum stick force gradient

FAR 23.155 specifies the minimum allowable stick force per g as a function of aircraft gross weight and manoeuvre limit load factor. The latter linkage makes sense in protecting the airframe from over enthusiastic pilots. However, why should stick force gradient be linked to aircraft gross weight, other than to satisfy a general expectation that heavier aircraft require more muscle?
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Old 2nd May 2012, 11:02
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CS 23.155 Elevator control force in
manoeuvres
(a) The elevator control force needed to
achieve the positive limit manoeuvring load
factor may not be less than –
(1) For wheel controls, W/10N (where
W is the maximum weight in kg) (W/100 lbf
(where W is the maximum weight in lb)) or
89 N (20 lbf), whichever is greater, except that
it need not be greater than 222 N (50 lbf); or
(2) For stick controls, W/14N (where
W is the maximum weight in kg) (W/140 lbf
(where W is the maximum weight in lb)) or
66∙8 N (15 lbf), whichever is greater, except
that it need not be greater than 156 N (35 lbf).
I think that this links into 23.337 which also makes N1 a function of MTOW, where:

N1 = 2.1 + (24,000 lb / W + 10,000 lb)

[with a maximum of 3.8]

So given that a heavier aeroplane has lower g limits, you therefore want a steeper stick force per g gradient to ensure a minimum high stick force to pull the wings off.

Section S, a wonderfully simplified code used for microlights in the UK simply has a value of 7daN at N1, which is a lot easier to work with.

Good question!


Sidetracking a little, I personally believe that whilst this is important, more important to safety is the concept of PFTS - or "Pull Force to Stall", which exists in a sort of oblique way in part 25, and an even more oblique way in part 22, but is not anywhere in part 23 or VLA.

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Old 2nd May 2012, 12:11
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Thanks Genghis.

Point taken that heavier aircraft have lower manoeuvre limit load factors. However, the rule still allows pilots of lighter aircraft to pull the wings off more easily. It is as if pilots are assumed to be more aggressive when at the controls of heavier aircraft!

PFTS: Are you referring to stick force per knot from trim? If so, I find it equally surprising that Mil 8785C appears to be silent on the matter.
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Old 2nd May 2012, 13:04
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PFTS: Are you referring to stick force per knot from trim? If so, I find it equally surprising that Mil 8785C appears to be silent on the matter.
No. Apparent longstab, which is what you're describing, defines force (or displacement) per airspeed change largely provides ability to control and stabilise flightpath.

If you integrate apparent longstab between trim and the stall point you will get a total force - which I've termed PFTS (not a commonly used term, but it works). Very simply, it's how much pull a pilot has to make to stall the aeroplane.

This value arguably is an important factor in safety - stall avoidance being critical for obvious reasons. Research that exists suggests that a PFTF not less than 10lbf is appropriate to a light aeroplane, probably more for a big jet. An interesting bit of research that exists shows that the apparently very similar C150 and C152 have a factor 3 difference in PFTS, with the lower force C150 having a roughly 16 times greater rate of fatal accident with a contributory stall. In landing configuration mind you, both are well below the proposed 10lbf figure (around 0.5lbf for the C150 and around 1.5lbf for the C152).

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Old 2nd May 2012, 20:02
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Your PFTS criterion makes sense. It would be interesting to know if there is a wider correlation between PFTS and stall-spin accident rates amongst the GA fleet as a whole.

F.
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Old 3rd May 2012, 06:58
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It would. It's one of those bits of research I'd really like to do, but it would be a slow and expensive exercise to pull together.

Intuitively I believe that there is such a link, although I don't imagine for a moment that it's the only player in stall related accident rates.

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Old 3rd May 2012, 13:28
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While flight testing a modification on a Siai Marchetti 1019 last winter, I found it had a rather poor stick force gradient at the takeoff flap setting (30). I had a couple of rathe scary takeoffs, when a large pitch push force was required immediately after becoming airborne. It was controllable if you were ready for it, but a real surprise the fist few times.

I did some data gathering, and the following graphs are extracted from my report. The aircraft was near gross weight and forward C of G for all this testing, control forces trimmed to zero at 70 KIAS, and all other things remained unchanged. "pounds" is stick force in pitch, with a pull force being positive pounds (you'll see some negative ), "Tq" is engine torque in percent (Allison C250-C17 turbine). The rest is self evident.









And here is the view of the elevator I took from the back seat while training the company pilot. Unaccelerated flight, trimmed to 60KIAS, flaps 60, and no more nose down pitch control available! (If you want to nose down, you have to either remove some flaps or some power).

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Old 3rd May 2012, 16:02
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Pilot DAR


I flew a couple of flights as passenger on the turbine SM1019 many years ago in Italy ; I remember it had a feature in the power lever which, during a go-around, reduced automatically

(i.e. without any pilot input) the flaps setting from 60 to 45.
If the graphs are for the fwd CG position, with very low stick-free static stability , I suppose that with aft CG you can fly with max torque at all stick positions but with almost zero force ! ! Is this correct ??
D
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Old 3rd May 2012, 23:27
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Daniel,

I'm not aware that this SM1019 had such a feature, though with the flap switch right on top of the power lever, you had what you needed right there.

During my flight testing I did encounter conditions where the pitch forces were near zero through a large range. It required changing one's technique to not "feeling" the plane, but rather "telling" it!

I also did find this aircraft rather poor in directional control at low speed on the runway. Using the flight manual technique for a maximum performance takeoff was an invitation to directional disaster! Landing on wet snowy grass made me very happy that aircraft had reverse thrust available, as a few times that was the control to remain on the runway after landing, not rudder and brakes!
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Old 30th May 2012, 12:34
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Dr. Guy Gratton gave a lecture recently into research he had done to determine the wide difference in accident rates between the Cessna 150 and 152, which are broadly very similar aircraft.

As I recall, the conclusion of his research was that very light controls of the Cessna 150 allowed pilots to inadvertently stall the aircraft when distracted by navigate /communicate when they should really aviate.

http://bura.brunel.ac.uk/bitstream/2.../Fulltext2.pdf

Last edited by Mechta; 30th May 2012 at 12:35.
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Old 1st Aug 2017, 17:42
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From : http://www.pprune.org/flight-testing...ml#post7167083

"An interesting bit of research that exists shows that the apparently very similar C150 and C152 have a factor 3 difference in PFTS, with the lower force C150 having a roughly 16 times greater rate of fatal accident with a contributory stall."

Can anyone point me at the research documentation?
TIA
A
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Old 1st Aug 2017, 20:35
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re http://www.pprune.org/flight-testing...ml#post7167083
Can anyone point me to the research referenced:
An interesting bit of research that exists shows that the apparently very similar C150 and C152 have a factor 3 difference in PFTS, with the lower force C150 having a roughly 16 times greater rate of fatal accident with a contributory stall.
Many thanks

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Old 1st Aug 2017, 22:41
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Mike Bromfield, Guy Gratton & Mark Young, The Effects of Stick Force Gradient on Pilot Mental Demand, AIAA Atmospheric Flight Mechanics Conference, AIAA SciTech, (AIAA 2015-0751 Jan 2015 Kissimmee FL, USA). http://dx.doi.org/10.2514/6.2015-0751

MA Bromfield & GB Gratton, Testing the relationship between stick force gradient, workload, and loss of control in light aeroplanes, SETP Cockpit Jul-Dec 2012, pp6-32.

MA Bromfield & GB Gratton, Factors affecting the apparent longitudinal stick-free static stability of a typical high-wing light aeroplane, RAeS Aeronautical Journal, vol 116 No.1179, pp467-600 (May 2012)

M Bromfield & GB Gratton, Supporting the investigation of factors affecting loss of control of light aircraft, Proceedings of the 40th Annual International Symposium - Society of Flight Test Engineers, Linkoping, Sweden, Sept 2009

M Bromfield & GB Gratton, Factors affecting loss of control in general aviation aircraft, Proceedings of the 39th Symposium of the Society of Experimental Test Pilots, San Diego, California, USA (March 2009) Brunel University Research Archive: Factors affecting loss of control in general aviation aircraft

Michael Bromfield - PhD Thesis

J Brownlow, N Everett, G Gratton, M Jackson, I Lee, J Thorpe, A study of fatal stall or spin accidents to UK registered light aeroplanes 1980 to 2008, General Aviation Safety Council 2010.


If you want to track them down, Mike Bromfield is at Coventry University, Guy Gratton sits between Brunel and Cranfield Universities and the National Centre for Atmospheric Science - both have webpages that makes them easy enough to contact. I think that the GASCo report you can get from them directly.

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Old 2nd Aug 2017, 06:15
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Many thanks, just what I wanted
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Old 6th Aug 2017, 15:09
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May I please add one more consideration into this discussion. All references above have been to the type or class or aeroplane. I think that the need for a minimum ALSS gradient is driven more by the experience level of the pilots flying them. Typically, they are student pilots or GA private pilots with low overall experience and, probably, recency. If you put an experienced and current professional pilot (who routinely flies second order aircraft by hand) into this class of aeroplane then the same need for a minimum ALSS gradient may not exist. It would be interesting to relate stall and spin accidents in GA aircraft to the experience and recency of the captain as much as to the criteria being discussed here.
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Old 6th Aug 2017, 17:35
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Very valid observations there Lomcevak.

Classes of aeroplanes that are routinely flown manually by high ability pilots to achieve high accuracy manoeuvres probably don't, as you suggest, need much consideration of safety related handling qualities minima of this nature. Picking two pertinent types - the Typhoon or Extra 300; both will mostly be handflown, by high ability pilots, whose primarily objective is accurate manoeuvering for operational purposes. Therefore the use of operational HQ criteria can reasonably supercede safety oriented HQ criteria.

However, I think that you're on shakey ground assuming that the range of either transport or GA pilots have a particularly high lower threshold of pilot handling ability. Any GA instructor or airline training captain should be able to offer some entertaining anecdotes about the pilots at the mininum threshold of continued activity. Whether you absolutely need aLSS gradient minima is debatable, but there will be significant value in the ability to fly the aeroplane accurately at reasonably low workload levels. You could on the other hand make a much stronger case for the PFTS minimum value Mike Bromfield proposed in his work, for the obvious reasons that not stalling inadvertently is much more important than holding speed accurately away from the stall.

"Role relation" ! - I recall somebody teaching me all about that once a concept much easier to introduce overtly in military than civil flight test, although certainly both should be include it fully in their test planning.

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Old 19th Aug 2017, 06:25
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An interesting bit of research that exists shows that the apparently very similar C150 and C152 have a factor 3 difference in PFTS, with the lower force C150 having a roughly 16 times greater rate of fatal accident with a contributory stall
Previous discussion is a little out of my range, being a simpleton. Might the 150 having 40° flap available compared to the 152 with 30° have anything to do with the results? Is that the reason Cessna reduced flap travel across most/all its range of high wing aircraft? Flew the 150, 172, 182 for many hours in my early days and never had a my God moment using 40°. Fat, dumb and happy perhaps?

The attachment states 40.1% of the accidents occurred during a go around, and a Cessna needs a BIG push on the yoke during that manoeuvre in the initial stages.

http://commons.erau.edu/cgi/viewcont...&context=jaaer
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Old 19th Aug 2017, 22:54
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If you read the papers, most of the C150 testing was done at 30 flap to compare like with like. The 40 flap case did give a more extreme issue however.

Good luck ever getting a straight answer out of Cessna on why they went from 40 to 30 maximum flap with the 150 to 152 development !

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