posted 08-20-2005 01:52 PM            

However in practical terms a pilot tends to be much more cued in to stick forces than stick position, and it may sometimes be difficult for a pilot to be aware of exactly where the stick or yoke is in relation to the stall angle-of-attack position, especially when we change the "feel" of things by varying the trim setting etc. Any time that you are exerting back pressure on the stick or yoke, be mindful that you may be approaching the stall angle-of-attack!

Steve
visit www.aeroexperiments.org

posted 08-27-2005 02:31 PM            

The difference between the yoke position at stall in wings-level flight and in a steep turn may only be a few cm but it is enough to create some very interesting relationships when the yoke is fixed in one position.

Like I mentioned, in hang gliding the effect is strikingly obvious--the control bar has to be way forward (like moving the control stick or yoke way aft) to produce a stall in a steep turn.

Tom Knauff is an excellent author of training guides for sailplanes and he actually advocates rather steep turns in the pattern, because the stick has to pulled further back to produce the stall angle-of-attack when the bank angle is steep (and also because in a steep turn, inside rudder yaws the nose downward so pilots are less likely to be tempted to over-rudder (skid) their turns.)

Also, see http://www.av8n.com/how/htm/aoastab.html#sec-long-tail-pitch from John Denker's excellent "See How It Flies" website.

Steve

visit www.aeroexperiments.org

posted 08-29-2005 09:16 PM            

PS though it's great to study up on theory, and there's no better source for that than the "See How It Flies" website ( www.av8n.com ), any pilot with access to an airplane with a control yoke can replicate the experiments I described above. Use a small-size vicegrip so not much gripping force is needed to hold it in place even when pulling a couple of G's--(obviously we don't want to put any significant stress on the control yoke torque tube), and pad the jaws with duct tape or some sort of cloth friction tape or better yet some sort of adhesive foam. The challenge will be to keep it from sliding on the torque-tube considering the gentle grip pressures you'll want to use. Or for a more rudimentary experiment just fly in a 60-degree bank with the stall horn just on the edge of sounding, and grasp the control yoke torque tube where it enters the panel with your thumb and forefinger of your free hand, and then roll very slowly to wings level without allowing the slightest foreward or aft movement of the control yoke, and you'll soon be deep into the stall buffet or fully stalled due to the effects we've been discussing, so long as you don't allow your fingers to slide by even a centimeter or two. Use a faster roll rate and you'll also get to see the nose climb like crazy and the flight path curve upward but that's a different effect, a non-equilibrium energy management sort of thing where kinetic energy is converted to altitude because you are still pulling G's (haven't bled off enough airspeed, and haven't reduced the angle-of-attack) when you arrive at wings-level. This will actually defeat the point of the experiment because any upward curvature of the flight path in the pitch axis will cause the stall angle-of-attack to occur with a further-aft stick or yoke position, so using the very slow roll rate is definitely the best to see the difference in stick or yoke position at stall angle-of-attack in the different situations. Though if you get into a series of wings-level pitch oscillations with the yoke in a fixed position you'll see that the stall horn tends to sound at the tops of the oscillations and not at the bottoms, as described in the earlier post. Or use a sharpie pen to make an ink dot on the control yoke torque tube at the point where it enters the panel at the angle-of-attack where the stall horn sounds (or where the stall buffet starts if you prefer), both in a 60-degree bank with a nearly constant airspeed (2-G) and in a wings-level attitude with a nearly constant airspeed (1-G). The two dots (2-G vs. 1-G) will be in different places due to the different curvatures in the flight path in the pitch axis.

Remember--"Faith is an island in the setting sun...but proof is the bottom line for everyone."

PPS Sorry to keep adding to this post but wanted to point out that the fact that the control stick or yoke position for stall, and also for best lift coefficient, varies according to things like flap setting, power setting (except when there is no propeller on the nose and the engines are neither above nor below the CG), balance (fore or aft loading), and amount of curvature of the flight path in the pitch axis (closely related to G-loading) is one of the main reasons why it's not usually not very practical to improve safety (exception: Aircoupe?) simply by putting a stop on the elevator to prevent it from reaching the position for the stall angle-of-attack. For example in flapless sailplanes where many of these effects disappear but the curvature in the flight path and relative wind (airflow) becomes particularly important due to the low airspeed and low radius of curvature, if the elevator reached the stop before the wing reached the stall angle-of-attack in 1-G wings-level flight, it would not be possible to do an efficient, steeply banked thermalling turn.

Fly safe everyone... Steve

posted 09-01-2005 10:45 AM         

Though as far as Mr Denker is concerned, he should first understand what a tailplane REALLY does before launching into these great theories of varying stick positions vs AoA. If The CG of an aeroplane was behind the main wing's centre of lift (at any AoA) as he suggests, and the horizontal tail did have to therefore produce lift in standard level flight, then we would all have to pull back on the stick to maintain straight and level inverted flight... Not forward stick, as is the case.
Look at some aeroplanes with fixed horizontal tails (as opposed to all-moving, for obvious reasons) and take a real close look at the angle of incidence, they're either just on neutral or slightly negative (even if only a few minutes - that's all it takes). This is very obvious on airliners. Alternatively, conduct another experiment - Mark accurately where your yoke is when in level flight, then once on the ground, use your vice grips to reproduce this position, walk around to the tail and consider the angle of incidence of the mean chord - see what you get. Admitedly I haven't tried this myself just yet(just thought of it then! ) but my bet is you'll find you will have a slightly negative angle. I will try this myself when I next fly, although I can't use your vice grip method, I have a stick - not a yoke, so I'll have to come up with another system.

posted 09-01-2005 11:11 AM         

Another good way to tell that all aeroplanes (except for canardless delta winged aeroplanes - different flying concept) are nose heavy, is to note where the main wheels are placed on tricycle type undercarriage. They are usually not very far aft of the wing's center of lift range** , if they had a CG further back than that, they would all be parked nose up in the air.

** By "centre of lift range", I mean the maximum aft (low AoA) and maximum forward (high AoA) possible positions of the centre of lift along the mean chord of the wing

[This message has been edited by InvertedSpin (edited 09-01-2005).]

posted 09-01-2005 01:13 PM            

Your comments about the direction of forces in inverted flight almost had me convinced that lifting tails would behave strangely in the negative-G regime but I think you overlooked something. As I understand it a basic requirement for stability is that the rear surface (normally the tail) flies at a lower angle-of-attack than the front surface (normally the wing). In a lifting tail arrangement imagine that the wing is set at 10 degrees of incidence and the tail is set at 5 degrees of incidence, all with respect to the fuselage (totally hypothetical numbers) and the plane flies at trim with fuselage at 0 "angle-of-attack" in relation to the airflow. Assume all airfoils are symmetrical for simplicity. Assume an all-flying horizontal tail for simplicity. 5 degrees difference between wing and tail. Now invert it. The fuse needs to point 20 degrees skyward to put the wing at the same 10-degree lifting angle-of-attack. The leading edge of the tail is now pointing 20 degrees skyward - 5 degrees earthward = 15 degrees skyward which is too much. The leading edge of the tail needs to be rotated 10 degrees earthward to put it at the same 5-degree skyward angle-of-attack in relation to the airflow. This is a nose-down (stick-forward) pitch input from the pilot's point of view.

We could go through the same analysis in the case where the wing was mounted at zero incidence on the fuselage and the tail was mounted at 5 degrees negative incidence. Everything would be the same--in upright flight at a 10-degree angle-of-attack we still have a lifting tail (5 degrees angle-of-attack) and in inverted flight where the wing and fuse are at a 10-degree skyward angle-of-attack, the leading edge of the tail would have to be moved earthward 10 degrees to a position of 5 degrees earthward incidence in relation to the fuse, so that the tail flew at skyward angle-of-attack of 5 degrees in relation to the airflow. This is a nose-down (stick-forward) pitch input.

By the way some of my rc glider friends like to balance their models (why, I'm not quite sure!) to be so pitch-neutral that no forward stick is required in inverted flight. P.S. I'm now modifying the rest of this content because I had some more insights as to how this could work. Again we'll take the simpifed case of symmetrical airfoils and an all-flying horizontal stab. If the aircraft is in balance with the stab at zero incidence with respect to the wing, no trim change will be needed in inverted vs upright flight. The airplane can only be in balance with the stab at zero incidence with respect to the wing if one of the following is true: a) CG slightly ahead of wing, wing's downwash causes tail to lift in opposite direction of wing; b) CG right at wing's "center of pressure", wing's downwash causes tail to experience a zero angle-of-attack and create no lift; c) CG slightly aft of wing's "center of pressure", tail is slightly lifting in same direction as wing, but due to downwash, is experiencing a lower angle-of-attack than the wing (I suspect that this is what is really happening); or d) CG slightly aft of wing's "center of pressure", tail is slightly lifting in same direction as wing, downwash effects are negigible, wing and tail are at same angle-of-attack (this aircraft will have no inherent pitch stability whatsoever and would probably be unflyable). Bear in mind that by moving the CG way aft these pilots feel they create handling and lift-sensing advantages that may offset any theoretical performance disadvantages of a lifting tail, but that's a whole different can of worms that I won't open any further right now--except to note that I don't trim my rc gliders in this manner-- Bear in mind also that these models have very little stability in most of these configurations (especially if downwash effects are so small that the wing and tail really do fly at nearly the same angle-of-attack) and that an on-board pilot would probably find the stick-force-per-G characteristics to be rather disconcerting, let alone the relationship between control stick position and angle-of-attack.

I know I haven't considered complicating factors like non-symmetrical airfoils but I think those are some of the basic relationships. It looks like the relative incidence between the wing and the tail (including elevator position as part of "incidence",) not the position of the CG, is what determines the pitch stability characteristics, though for any given aircraft with given airfoil shapes, tail moment arms, etc these two things are very highly inter-related. Very generally speaking, as long as the tail flies at a less positive or more negative angle-of-attack than the wing--and now by "positive" we mean whatever direction the wing's lift force is acting, be it +G or -G -- we should have normal pitch stability and a defineable relationship between control stick position and angle-of-attack (subject to modification by all the other effects we've been discussing throughout this thread, and in some cases where the tail does not generated a strong downforce, subject to some peculiarity when the direction of the wing's lift force changes, as described above.) That's a mouthful--thanx for the food for thought!

PS to dig into the complexities of cambered airfoils, downwash effects, etc "Model Aircraft Dynamics" by Martin Simons is a good place to start. In general, with a cambered wing airfoil, if the tail is set up to create an upforce at some angle-of-attack, then at some point as the pilot chooses progressively steeper dive angles, the tail (all-moving stab or tail+elevator)will have to be moved to a position where it can create a downforce. In this case, it's not completely clear to me what factors govern whether or not there would be some wierd reversal in stick-position-per-angle-of-attack, or in stick-force-per-angle-of-attack, as the pilot chooses progressively steeper dive angles. As the pilot chooses progressively steeper dive angles, the fact that the tail surface's trailing edge must be progressively raised in relation the direction of the airflow, doesn't necessarily mean that it must be progressively raised in relation to the fuselage, because the airflow direction is not constant. But that's way more than 'nough said already.

Steve

posted 09-02-2005 10:10 PM         

None the less, I have come up with a device that will allow me to conduct similar AoA vs Stick position experiments to yours. I'm currently putting together a test card that will include all possible flight regimes, with power on, with power off, positive and negative. I can't do a flight test with flaps though, I haven't flown a plane with flaps for a couple of years now, and I don't really have access to one at this point.

I've approached an friend of mine with our subject of discussion, who is an extremely competent aeronuatical engineer with many decades of experience designing/building/test flying various designs of aeroplanes. He is digging up some old graphs, equations and thesis papers from his days working with NASA on this very topic in the 80's (believe ot or not!). With his permission, I will share these with you when they become available - Should be very interesting!

For the record, he doesn't fully agree with either of us... So we will learn something for sure.

Stay tuned!