I am not an aeronautical engineer, I just fantasize about being one. (Yes, that is weird.) For most airplanes we can get by with a very limited understanding of aerodynamics ("houses bigger, houses smaller") but this airplane is different. The airplane will do things you aren't expecting if you get it into a part of the envelope it shouldn't be. So you need to have a better understanding of your stick and rudder.
Everything here is from the references shown below, with a few comments in an alternate color.
We learn early on that airspeed doesn't fly airplanes, angle of attack does. We also learn that the angle of attack is the angle between the chord line of the wing and the relative wind through which it is flying. Okay, close enough. More about AOA: Aero / Angle of Attack.
Gulfstream makes a big deal to differentiate "normalized angle of attack" against "angle of attack" and the engineer in me appreciates it. But the pilot in me thinks this is much ado about nothing. Is "Normalized" any different than what we think of as AOA? Not really. To be technically accurate, AOA is expressed in degrees but that isn't really useful to us since the stall angle of attack changes with wing configuration. We think of AOA as from 0 to 1, where 1 is the maximum lift we can get out of the wing. That's normalized AOA in a nutshell. It is curious that none of our instruments say "NAOA" but the manuals do:
Photo: Normalized AOA Readout on PFD, Symmetry Guide, §2B-04-00, p. 38
Click photo for a larger image
Now wait a minute. If 1.0 AOA is the maximum lift we can get out of the wing, and anything higher than that is the — cue dramatic music — aerodynamic stall, how can we have an AOA of 1.10? Well first off, there is a mistake in the Symmetry Guide illustration. It says "0.00 to 1.10 degrees" and that is wrong. Normalized AOA is a ratio and doesn't have any units of all. The actual stall angle of attack is likely to be something well above that. But I digress, what is this about a Normalized AOA of 1.10?
The critical thing to understand is that most wings do not stop producing lift when they get to the point where they are producing maximum lift. In some ways calling this point the "stall angle of attack" does a disservice, because while the wing is beginning to stall, it is still producing lift. This effect is more pronounced with swept wing airfoils. So while the amount of lift produced goes down, the AOA can indeed exceed the stall angle of attack and the NAOA can exceed 1.00.
[G500 Ground and Flight Operations, p. 95]
I'm not sure about this as written. Perhaps: "The fraction (0 to 1) of usable positive lift, AOA, and available g are essentially the same." This becomes a useful tool. The lower your AOA is, the more energy you have available to maneuver. If you are flying at 0.33 AOA, for example, you are using 1/3 of your energy (in terms of lift) to fly but still have 2/3 left over (in terms of available g) to pull back on the stick or turn. If you are flying slower, say at 0.67 AOA, more of your energy is taken up with the task of just keeping in the air so you have less energy available to maneuver.
In my opinion, all of this is true but not particularly relevant. The reason we have NAOA, indeed every airplane I've ever flown with displayed AOA has been this way, is that it is incredibly useful to know what your ratio of angle of attack over maximum available angle of attack is. Giving pilots the AOA in degrees over the camber of the wing is unlikely to be as readily understood.
These AOA values are in units of degrees and you end up with a ratio that has no units.
Hysteresis is a simply lag in the system.
I took the airplane around our local pattern with a video camera on the instrument panel just to have a log of what a normal airplane should look like, in anticipation of one day having the airplane not behave normally. The airplane was light: just 7,000 lbs of gas, two pilots and a jump seat photographer. But it was a gusty day and our VREF additive was 13 knots, which I carried until the flare. A side benefit is that I got a good view of the indicated NAOA. Here is what I found:
|Begin takeoff roll||Down||20||40 (on the "peg")||0.00|
|Gear fully retracted after takeoff||Up||20||185||0.28|
|Flaps fully retracted after takeoff||Up||0||200||0.35|
|30° bank turn, level flight||Up||0||200||0.54|
|10° flaps, crosswind, level flight||Up||10||180||0.40|
|20° flaps, final, level flight||Up||20||160||0.45|
|On glide path||Down||39||135 (VREF+13 (for gusts)||0.55 - 0.65 (gusts)|
|Begin flare at 25'||Down||39||135||0.52|
Here is the video:
The High Incidence Protection Function (HIPF) matters to us because it will overrule our actions at times that don't seem to make intuitive sense. It isn't explained at all in the usual places and is only documented incidentally. So what follows are collected notes from various sources, as well as the few places it is actually mentioned in our manuals.
Under contaminated conditions, the HIPF reduces the angle required to get a Pitch Limit Indicator, stick shaker, and other stall prevention measures.
The only real documentation is spread through the AFM:
[AFM, §05-01-01] VSR, REFERENCE STALL SPEED: for the G500, VSR is selected to be slightly higher than the speed at which aerodynamic stall would otherwise occur in 1-g level flight. The HIPF (High Incidence Protection Function) of the G500 limits the angle of attack that can be achieved with full aft stick such that the minimum steady speed is not less than VSR.
[AFM, §05-01-40] Variations of reference stall speeds, VSR, with weight and altitude for speed brakes retracted are shown for all flap positions. The reference stall speeds presented were developed in accordance with 1G stall speed criteria. Because of variation in the stall speeds with the operating mode of the Wing Anti-ice (WAI) system, various charts are presented.
Photo: HIPF Stall Speed Schedule Based on Flap and WAI Status, AFM, §05-01-00, figure 11
Click photo for a larger image
The AFM does not provide reference stall charts for what it calls "Pre-activation Ice Shape" so we are left to guess the stall speed goes up and assume the low speed awareness cues will give us an idea.
[AFM, §05-08-10] Buffet boundary data are presented as a function of speed, weight, and load factor. Buffet Boundary. At low speeds, the buffet boundary is restricted by the maximum achievable angle of attack at full-aft stick due to the High Incidence Protection Function (HIPF). At higher speeds, natural buffet (high speed buffet) or VMO / MMO is limiting.
The AFM gives one chart for 0° flaps that shows buffet boundary increases about 0.02 Mach (approximately 15 knots) at low altitudes with the WAI turned off.
Note that the HIPF assumes the wing is contaminated whenever the wing anti-ice is off and the flaps are up. That means in this condition the airplane will think it is nearing the stall sooner than is actually true and that can result in the airplane doing things you don't want it to. The Zero or Partial Flaps procedure restricts you to no lower than 200 KCAS "until ready to configure" even with the WAI on. We are taught that this is because of the HIPF but that isn't written down anywhere. The pilot who flew the zero flap tests told me the winglets tend to flutter below 200 knots until the flaps are extended and the flutter can be confused for signs of a stall. One of these days I'll watch the yellow and red low speed awareness bands near 200 KCAS and extend the flaps. Until then, I am using 200 KCAS as my minimum no flaps speed.
The yellow band of the Low Speed Awareness (LSA) Thermometer is your friend. The top of this band indicates how slow you can go while still safely maneuvering the airplane. It is about 5 knots below VREF, so you ought to pad the top of that yellow band accordingly. Under normal conditions, the Auto-Throttles will take over if you get to 1 knot above the LSA airspeed.
Photo: Low speed awareness thermometer, G500 Symmetry Guide, §2B-04-00, p. 42
Click photo for a larger image
[G500 Ground and Flight Operations, pp. 97 - 100]
This will impact your approach and landing if you are landing Zero or Partial Flaps or have issues with the wing anti-ice system.
[G500 Ground and Flight Operations, p. 101] a. At first indication of impending stall or stick shaker:
The airplane doesn't want you to exceed VMO / MMO and will pull the throttles back or pull the nose up to keep you within the speed envelope.
[G500 Ground and Flight Operations, pp. 107-108]
These three rules are really two rules and they come from Gulfstream. The cardinal rule in these things is to avoid rapid rudder reversals. Why? See American Airlines 587 for a case study in what can go wrong.
[G500 Ground and Flight Operations, p. 109]
Gulfstream GVII-G500 Ground and Flight Operations, released as part of the GVII-G500 AOM, §02-01-10
Gulfstream Symmetry Flight Deck for the G500 Aircraft Pilot's Guide, Honeywell Pub. No. D201110000019--003a, October 29/18
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