AoA instrumentation

[PBL]

Originally Posted by Wizofoz

[PBL: One of the "variables" which I suggest you regard is speed. Stall AoA at, say, 0.8M is rather different from stall AoA at, say, 170 kts CAS, in almost any airplane capable of that performance variation.]
Can you back that statement up?

Just for an indication, stall at 0.8M on contemporary commercial jets will happen at AoA of less than about 10°; on most significantly lower. Exactly where "stall" is defined, though, is another matter. It will mostly be defined by uncomfortable buffet (see the certification criteria) rather than by the point at which C_L starts to drop off radically. There can indeed be a degree or two between the two. In other words, at "defined stall" C_L may still be increasing.

In contrast, stall at 170kts on contemporary commercial jets will be somewhere in the higher 'teens of degrees.

Originally Posted by Wizofoz

It was always my understanding that critical AofA was a fixed value for a given airframe.

Yes, well, you are not alone.

PBL

[Wizofoz]

So again we are down to terminology.

I've never seen onset of buffet defined as "Certified Stall". It is an indication of APPROACH to stall, but still, the only definition of stall I know of (and certainly the one John Farley and others here where discussing) was ClMax.

Are you saying that AofA for ClMax changes with airspeed, and can you cite an explanation as to why?

ETA:- Actually, thinking about it, I think you are correct the Vs, as used for calculating other Vspeeds (Vref, Vcl Etc.) is onset of buffet.

Would still be interested in a cite as to why Critical AofA varies with Airspeed.

[PBL]

Originally Posted by Wizofoz

So again we are down to terminology.

Again we are down to misreading.

Originally Posted by Wizofoz

I've never seen onset of buffet defined as "Certified Stall".

Neither have I.

Originally Posted by Wizofoz

....the only definition of stall I know of ...... was ClMax.

I suggest you check the certification criteria. There is a difference between C_Max and the largest usable C_L, which is defined mostly by the buffet boundary (about 25% lower than that at the buffet boundary, I believe). For example, Shevell Figure 227 shows the buffet boundary on a graph plotting C_L against Mach number.

I'll let others speak for themselves, if they care to do so.

PBL

[Wizofoz]

PBL,

You may have missed my edit, but yes, I understand you are correct about "Usable Cl" as a certification criteria.

My question was, do you have a cite to an explanation as to why AofA for Cl max varies with Airspeed?

[John Farley]

Wiz

As PBL says it is Mach number that reduces the AoA for separation of flow on top surface/stall/reduction in Cl max/buffet (or whatever characteristic you think of as marking the "stall")

At GA light aircraft speeds forget mach effects but with a high performance aircraft expect a reduction in the AoA at which stall effects are detected above about 0.3M. By the time you are at .8/.9 the max AoA you can sensibly use will probably be down by 2 or even 3 degrees depending on the aerofoil.

JF

[PBL]

Wiz,

Try Shevell Chapter 8, Section Airfoil Pressure Distributions, pp125-131 of the 1983 edition (the one I have).

WTH, I could do with a break, so here's typing it in.

The pressure coefficient Cp is the difference between local pressure at a specific point on the airfoil, and the freestream pressure, divided by the freestream dynamic pressure (thus normalised). At a given point on the chord of a given wing section (shape), there is going to be a presscoeff Cpu on the upper surface and one Cpl on the lower. If you integrate the difference between them over the chord, and devide by the chord length, you get C_L of the section.

The Cp's are actually a function of the ratio between freestream velocity and local velocity at the point, to be exact (1 - ratio^2).

As the Mach number increases beyond the value 0.3 given by John Farley, the Cp's begin to increase significantly even at the same AoA; that is, the pressure difference between local and freestream increases faster than the freestream dynamic pressure. As far as I know that is numbers, not theory, but Prandtl and Glauert gave an approximate expression in their 1930's texts. Shevell says it is "quite good up to Mach numbers of 0.7 to 0.8.

Deriving C_L_Max for an airfoil is something done in the wind tunnel, because all those different sections are going to have all their different C_L_Max's, though in principle you could weight the experimental results for each section with the appropriate geometry if you had enough spare time ...... and then check your calculations against the tunnel data for the entire wing. This helps explain why C_L_usable is likely lower than C_L_Max, namely all those sections which are beyond max (max'd out?) are rattling you around enough to discourage you from increasing AoA further.

PBL

[fauconpoilu]

PBL : I'm sure you're right about compressibility effects. However in the circumstances John Farley was mentioning, the context is landing / low pressure altitudes. In that case, speed becomes a factor linked to the load factor for the onset of stall. AT least that's what I would say

[Wizofoz]

JF and PBL,

Many thanks!!

[Genghis the Engineer]

JF - thanks for the description of the Harrier approach; apart from the obvious omission of the nozzle lever, is that broadly the same, so far as you know for a naval jet such as the F-14 or T45?

CB & PBL, unusual but I'm pretty certain that certification stall on the Jaguar was marked by wing rock (the consequences at higher AoA had been demonstrated best not encountered: divergent inertia coupling if memory serves).

G

[John Farley]

Genghis

No. The USN are taught to control speed (AoA) with the stick and hold the glide path using power. Its a culture thing. It helps that they don't flare of course!

An enlightened US boffin has tried to get them to control speed with power and flight path with stick (like their autoland laws are rigged) but without success. When I did my first T-45 approach to a Pax dummy deck using stick for flight path and throttle for speed it was not as good as I wanted. The USN guy with me said "NO you must use blah blah blah" "OK" I said and did another (without changing my technique) "There" - he said - "that's the way" I did not bother to tell him how I was doing it as I knew there was no point.

Shades of BLEU who tried to get the RAF to fly their aircraft with throttle for speed in the early 60s (after flying both techniques and showing conclusively that the auto pilot did a getter job if given throttle for speed) Same result until the late 60s early 70s when the RAF gave in.

UK GA still do speed on stick of course (or at least preach that). Me I don't like the idea of a control technique that CANNOT be used close to the ground when landing! When I was being taught to be an instructor in the RAF at CFS my instructor insisted I patter the official line. One day when I was playing at being a thick student I put us 10 kts slow over the lead in lights and said "you have control Sir please show me again how you lower the nose to make the speed increase" Of course he slammed the throttle forward, refused to speak to me again and I was given a new instructor.

J

[BOAC]

Ah! John, but had he been a bit sharper and seen his awkward student coming, he would have pattered that as a superhuman, water-walking and god-like creature he had merely applied (full) throttle to anticipate the sink caused by lowering the nose to regain the speed and "You have control again"

[John Farley]

BOAC

True O King!

J

[Machinbird]

No. The USN are taught to control speed (AoA) with the stick and hold the glide path using power. Its a culture thing. It helps that they don't flare of course!

Lighten up guys. You are getting out of your league. USN aircraft are flown much closer to stall than about anything else out there that is fixed wing. Playing with the nose can have undesirable effects like dropping the hook closer to the ramp on a carrier approach. Dropping the nose in close can cause a bolter. Landing a bit fast can beat up the arresting gear and cause scary stuff to happen.
With an on speed (AOA) approach you are controlling the aircraft by making very small nose movements and adjusting thrust continuously but in small increments. It must be a stabilized approach. The LSO is there to oversee the process.
For JF, there is no autoland process on a carrier. You fly it all the way to touchdown as if the deck isn't there. The Navy type struts are designed to absorb that vertical component of velocity and this reduces the load on the arresting gear. I wouldn't have trusted my Airspeed on approach to do anything else than tell me my AOA was operating in the ball park. As fuel weight decreased, the approach speed decreased comparably and I didn't have to look it up in a chart which was good because I had my hands full flying the aircraft.

[John Farley]

Machinbird

'Fraid it is getting harder to keep up these days. Perhaps no autoland on your unit but USN Pax trials go way back. Try this for open source reading.

Test results of F/A-18 autoland trials for aircraft carrier ...
by G Johnson - 2001 - Cited by 2 - Related articles
Test Results of F/A-lS Autoland Trials for Aircraft. Carrier Operations. Greg Johnson ... Abstract-Raytheon and the US. Navy conducted Aircraft ...
ieeexplore.ieee.org/iel5/7416/20152/00931358.pdf?arnumber... - Similar

Like I said everybody who has programmed an autopilot to do coupled approaches (from the early BLEU work in the late 50s) to today gets better results using power to control speed. Ship borne aircraft are actually easier when it comes to autolands because (as you point out) they do not need to flare which really simplifies things compared with the control laws for an aircraft that MUST flare.
JF

But please don't take my word for it just read what is out there or talk to Pax boffins.

[Machinbird]

JF, perhaps a difference in terminology. To me autoland implies a flared landing. ACLS and now probably JPALS runs an aircraft down a stabilized glidepath until it flies into the boat. There are likely some last second adjustments to correct for expected ship position now (the ship is anything but a stationary object). I've flown ACLS equipped aircraft in the late '60s but they didn't have full approvals for use at the boat then. We did use the approach power compensator (autothrottles) which used AOA as its input with fair success. Then you could fly with the stick only while monitoring throttle motion, but there could be problems in stabilizing the approach that way in night, black hole conditions.

[John Farley]

Machinbird

Understood. It did occur to me that the generic term autoland might have been the reason for our confusion.

Incidentally the UK back in 16 May 2005 did its first full auto recovery to a ship from a long way away. But that was cheating 'cos it was a Harrier and did a VL (In my book a VL is the only way to go - especially at night or with huge amounts on ship motion)

JF

[Mad (Flt) Scientist]

I'll throw this one into the discussion as to whether stall AoA is speed dependent.

I know of several aircraft where the stall AOA of the wing, even at low speeds (not even in the "normal" transonic region) is significantly affected by the variation in freestream Mach number - even at speeds as low as M0.2 freestream.

The key is that the acceleration of the flow round the rather sharp leading edge of a high performance aerofoil can generate local Mach numbers well in excess of the freestream values. So what appears to be an insignificant change in the subsonic freestream value can end up being a rather significant change in the local mach number.

And, yes, this even applies to the low speed landing configuration ...

[waren9]

Further to the OP's question and one of the first reply's:

On the Airbus PFD you can display a "flight path vector" which will show the aircraft trajectory relative to the horizon. The difference between this and the aircraft pitch attitude (if you know the angle of incidence) will give you a rough idea. Not intended as an AoA indicator though.

I used to fly 300 series Fairchild Swearingen Metro's and from memory they had a AoA guage in them.

[SlowAndSilly]

Yup, customer option on the 737 for the AoA display (round dial on the PFD). However i'm not aware of any european airline using that. Quite questionable anyway as all unreliable airspeed problems in our fleet in the last 2 years or so were directly linked to malfunctioning AoA sensors, not to static port problems. Since wrong AoA inputs affect both airspeed and altitude values displayed to the pilot it is not a trivial thing.

I know SAS uses the AoA on their NGs. I would love to have them on our NGs

I've also seen an analouge AoA in a C172. Looked a bit like the airbus one, with a green arc in it!

[PBL]

John Farley and Machinbird have been discussing

Originally Posted by John Farley

The USN are taught to control speed (AoA) with the stick and hold the glide path using power. Its a culture thing...................
An enlightened US boffin has tried to get them to control speed with power and flight path with stick (like their autoland laws are rigged) but without success.

I thought I would substantiate how powerful this culture is by quoting what might well be a source for it, Hurt's Aerodynamics for Naval Aviators (NAVWEPS 00-80T-80, 1959, revised 1965) which I pulled off the shelf in my other office yesterday for other reasons. It gives the rationale.

Originally Posted by Hurt, AfNA, p27

Note that for the conditions of steady flight, each airspeed requires a specific angle of attack and lift coefficient. This fact provides a fundamental concept of flying technique: Angle of attack is the primary control of airspeed in steady flight. In the same sense, the throttle controls the output of the powerplant and allows the pilot to control rate of climb and descent at various airspeeds.

The real believers of these concepts are professional instrument pilots, LSO's, and glider pilots.......[examples follow]

Here is my take on his examples.

A glider pilot in still air only has gravity as an external force which can be used to change kinetic energy, thus any change in equilibrium values of flight parameters must be accomplished by changing the components of gravity with respect to the aircraft axes, i.e., you gotta pitch down to speed up. As speed changes, so do AoA and lift coefficient because these are all coupled. But it does not follow from this that AoA is somehow primus inter pares.

An LSO has instruments to judge flight path and AoA of an approaching aircraft, indeed can do AoA better than the pilot (according to Hurt). I presume instruments to do this are more robust, and maybe even more sensitive, than instruments to judge rate of closure (i.e., from which follows airspeed when wind and ship velocities are factored in), which I imagine would be done by Doppler radar. Or it might be simply more complicated than they wished to factor in wind speed and ship speed in a sufficiently robust manner.

The example of a "professional instrument pilot" is simply an assertion that that's what they do. I am not sure of any pedagogical value to this.

My take on the reason for "speed on stick" for lower-performance aircraft is that it is the fastest means of control of AS. If I want to get 20 kts more AS ASAP on a Cherokee, I put the nose down rather than the throttle forward.

Since USNAviators are approaching while quite a ways behind the power curve, the same reason might well taken to be valid there. If you are well behind the power curve, flattening out may well be a quicker way to accelerate than putting in power.

Am I right about this?

Also, in a "normal" airplane, if I may use this term around Harrier pilots, the only way you can vector thrust is through pitch. So if you want to accelerate as fast as possible in a particular direction you have to put the nose there. So it may not really be about AoA, but more about altering the thrust vector. Whereas Harrier pilots have more flexibility in this regard.

PBL