Thursday 23 May 2013 19:33 UTC
The place for technical discussions about GA and flying.
Technical discussions about GA only, please.
Oh ya, I would say, big enough:D
Interesting it has a big surface on one side, instead of two small on each side. Being on one side, using it without elevator, it would also give a roll moment, right?
Thank you very much! Maybe some practical tests in the air might put this long debated issue at rest for ever.
This is stretching somewhat beyond the realms of everyday PPL'ing, and isn't something which readily requires thought on the part of a GA pilot, however the effects of friction (or more often 'stiction') and backlash in the control circuit can presents some handling challenges.
When considering the pitch stability of an aircraft (which directly impacts on the ease by which a pilot can control an aircraft), engineers and Test Pilots will define this in a few different ways. Relevant to this discussion is a commonly used measure called the stick force gradient, or stick force versus airspeed.
Let's say you were flying an aircraft that was perfectly trimmed out for straight and level flight. If I now asked you to hold an airspeed 10 knots higher without retrimming, you would find yourself (in a well designed aircraft), having to keep a constant force pushing the stick forward. If I asked you to go another 10 knots faster, you'd have to push even harder at a constant force. If I then asked you to come back to your original airspeed, you should eventually reach a point where you can take your hands off the controls and it holds your trimmed airspeed again.
Now in a well designed and certified aircraft, it is required that this force between changing the speed from trimmed, to 10 knots different, to 20 knots different, is reasonably significant (for large FAA type aircraft, the requirement is for a minimum of one pound-force per six knots), and linear (that is, the force to hold 20 knots is double that at 10 knots).
A well harmonised aircraft that exhibits these characteristics will tend to be easier to control and trim out.
Now in reality, the aircraft controls are a mechanical system, so will experience some freeplay, friction and so on. The effect of this is a deadband around your trimmed airspeed.
(Borrowed from the interwebs)
If you look at this graph, you will see that to reduce the airspeed, a pull force is needed, and to increase the airspeed, a push force is needed. However around the middle of the trim speed, you can push or pull the controls a little bit and nothing happens. This is where you are 'taking up the slack' in the control runs.
So bringing this all back to the original question - If you have a control jam like that you're describing, you may find that the initial movement of the trim wheel takes up some of this slack in the controls. My expectation is that in the event of this very rare type of control jam, you would take up controlling the aircraft in pitch using the throttle.
(P.S. For those that have made it this far without falling asleep - Well done, and don't worry, none of this is on the PPL theory exams!)
Very technical and useful post. Thank you so much!
You mentioned freeplay, read about some sloppy controls problem, even some flutter occurences due to stretched cables beyond limits. Also, how much freeplay is allowed in a flight control system? From this point of view, I think it would be useful, personally I would like to know how to check them doing full preflights when time allows or at least what to look for beyond full free and correct deflection travel check.
I flew some aircrafts which have pretty much flaps freeplay, what's your experience regarding this aspect? A local instructor told me there's no problem given the fact that flaps are kept up inflight due to airflow. Also, they are a little assymetric on ground, don't know if they align inflight as he said. What do you think about this aspect?
Is a disconnected freefloating surface, elevator, aileron(s), prone to flutter? Do flight surfaces usually streamline themself due to the way they are designed (balance) or they are more kept in place by cable tension? Or they are mainly kept streamline inflight due to airflow? Basically, what keeps them aligned inflight? Mainly the flow force which is big enough to counteract any weight or the way they are rigged? What about surfaces which are not mass-balanced?
What about fabric covering surfaces? Are they prone to this problem?
I don't have any experience in the airplane design part of aviation and even if I have some flight hours, I'm somehow confused about the above mentioned aspects, so I would really appreciate your help, thank you very much guys!
I found many light sports and I think also some standard FAR 23 certified don't have mass balanced ailerons. (American Champion series: Scout, Citabria,..) Well, are these airplanes more prone to flutter in a flight control malfunction?
Do you think that in these aircrafts, for example, in an aileron cable failure will be impossible to control the airplane due to freefloating surface flutter and leading to inflight breakup?
Sorry, haven't had the time so far to write a comprehensive reply.
In short, my conjecture would be that no, a cable break should not result in flutter (lock in or divergent oscillations) occurring for lower speeds (say nominally below Vno). The lack of mass dampers isn't indicative of damping occurring elsewhere (i.e. in the control cables themselves), simply that the design of the surface didn't require one.
In fact, if the mood takes you, have a read about control surface hinge moments. The following a nice introduction (you may need to read the preceding chapters to gain familiarity with the nomenclature - google it); http://www.dept.aoe.vt.edu/~lutze/AOE31 ... istics.pdf
Having got to grips with that, this NACA report on control surface flutter may be useful to you: http://naca.central.cranfield.ac.uk/rep ... m/2824.pdf
Edit: Changed the link to a more relevant NACA report.
Good theory, read it entirely despite the fact that I was lost in some calculus
There were some occurences, for example, in the Zodiac 601 LSA aircraft structural failures crash reports, the NTSB recommended that the aircraft must be mass-balanced instead relying on cable tension. It seems that even slack in cables might trigger control surface flutter and considering that many aircrafts are not mass-balanced, I wondered how would they perform in a cable failure emergency, that's why I asked about it. (However, in those accidents there are also others design aspects involved, so the aileron flutter problem might not be the single causal factor.)
I think if they are prone to flutter, the cable seems more like a structural failure trigger. I wouldn't be really satisfied flying in these planes and considering this potential problem would be true. Also, it would be absolutely wrong to avoid these airplanes if they are not more likely than others to have this problem. I'm not too worried about it because a cable failure is a rare occurence, but I think it worths to wonder about it considering the potential consequences.
I would be really satisfied to know that the manufacturers take care about these aspects, but the ASTM standards, as found by the NTSB, are not so keen on flutter issues. There were even some FAR 23 certified aircrafts which had flutter-induced problems near out of control after a broken trim tab linkage and, personally, I haven't expected that such a small tab linkage failure might have such a significant effect. So, if a freefloating trim tab triggers flutter, how a free surface, especially without mass-balance weigths, would behave?
That was what I worried about, considering the above aspects.
So, if in normal operating flight control there is no flutter issue, maybe thanks to low LSA operating speeds or other design aspects and due to that there is no requirement to be mass balanced, I figured out things might change in a freefloating scernario, a lot of aircraft design recommandations state that even some freeplay in the surface increases flutter probability.
Of course, I'm interested in all this aspects regarding normal operating envelope speeds, I don't even think about a high-speed dive above Vno or other particular examples.
Thank you so much!
Any new thoughts on the flutter issue?
What I've also found about disconnected aileron malfunction is if the ailerons are free, they might deflect upwards as you slow down (high alpha), but I never thought being so, I always thought they will fair, streamline in position almost neutral.
No, ailerons generally float up, because of the pressure differential. Slight up deflection is the minimum drag position, too, for most airfoils. If you look carefully on many types of aeroplane with cable operated ailerons you will see that both aileron trailing edges ride slightly higher in flight than they do on the ground (even with the control connected).
The EAA have just recently uploaded another of their webinars on this topic, which is defintely worth watching.
Particularly the bit about aileron/rudder crossover speed, which I don't think I've ever seen mentioned anywhere else.
'Flight with broken flight controls'
Who is online
Users browsing this forum: Rich T and 1 guest
Login / Register