AoA instrumentation

[Genghis the Engineer]

I'm kicking off a bit of a research project into the use of AoA gauges.

I'm aware of the use that the USN makes of AoA gauges, pretty much across the fleet. I also know that the Jaguar used to have a cockpit AoA indicator with selectable warning limits depending upon configuration.

I've also seen various aftermarket devices recommended particularly for kit aircraft.

I believe also that the 737 has an AoA indication with a limit which displays on the MFD once flaps are selected. Is that across all models?


However, can anybody tell me what other aeroplanes have AoA indication - in particular I'm trying to get a handle on how they are used? - is anybody but the USN, for example, flying to AoA values (perhaps for approach or climb), rather than airspeed or Mach values? I've also spotted that Boeing hold some patents on a combined AoA/Mach indicator for combined limits - did that ever get fitted onto anything?

And what do Airbus do? Any other fast jets?

All wisdom gratefully appreciated.

G

[FLEXPWR]

Hi,

Some old versions of A320 used to have AoA indicators in the cockpit, left of the captain seat. Small analog gauge/needle pointer. Did not seem to be used for any kind of purpose other than providing extra info to the flight deck.

Haven't seen any on latest A320 models.

Flex

[CONF iture]

Something you can find on the Airbus ... as an option.

http://i35.servimg.com/u/f35/11/75/17/84/aoa_0110.gif (http://www.servimg.com/image_preview.php?i=70&u=11751784)

[Cardinal]

Airbus takes the AoA information along with the rest of the air data, sends it to the Flight Augmentation Computers which then compute and display relevant information on the PFD speed tape:

Green Dot (max l/d),
Vls (lowest selectable speed w/ autopilot),
Alpha Prot (below which sidestick requests an AoA, bank limited to 45, speed brakes retract, a/p disconnects, pitch up trim inhibited),
Alpha Max (cannot be exceeded in normal law)

Additionally the Alpha Floor function utilizes AoA and other parameters to determine when to apply TOGA power to extract the aircraft from a dire low energy situation.

Airbus has recently developed the Back Up Speed Scale, which in the event of a triple air data failure replaces the airspeed tape on the PFD with an AoA scale of a conventional design. Keep it in the green range, respect the chevrons. So simple I could do it.

http://i122.photobucket.com/albums/o278/FL420/th_AirbusBackUpSpeedScaleBUSS-1.jpg

[zlin77]

Had a little time in a North American Saberliner 40 over 30 years ago, it had an AOA indicator on the glareshield, approaches were flown with reference to this.

[manitoubrian]

Pilatus PC 12 has them :ok:

[PBL]

Airbus has recently developed the Back Up Speed Scale

It is not strictly an AoA indicator. It is an indicator that your pitch+thrust combination is appropriate, or too "high", or too "low" (however you want to interpret "high" and "low"). For an explanation of what it does, see an article by Joelle Barthe, published in Safety First, 5 December 2007. I think a version has also appeared on eurocockpit.com, but I don't have a link.

PBL

[Checkboard]

I believe also that the 737 has an AoA indication with a limit which displays on the MFD once flaps are selected. Is that across all models?

I've flown 737-300s and -700s and never seen AoA displayed. In the NGs you can display a velocity vector, and the difference between that and the pitch indication is angle of attack - but it's not directly displayed.

I have flown business jets with a needle & dial display, with angle of attack as a percentage of Vs and you could bug a Vs percentage which would relate to a small "traffic light" head up display on the glare shield.

[Spooky 2]

I believe that all current Boeing production models offer the AOA as a customer option. I know that DAL and AA were the driving force behind this offering.

The B787 has a form of synthetic airspeed combined with AOA to help handle unreliable airspeed issues should they develop.

[Denti]

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.

[BOAC]

Essential on VSTOL eg Harrier (GR3/T4) where it allows you to assess the amount of engine v wing-borne lift required - the old '"8 Units, lad, 8 Units" chant.

[epsum]

Learjet 60s has 2 of them, just above the PFD

[John Farley]

Genghis

One of the very great advantages of having AoA available in a Harrier was that by using it (and NOTHING else) you could optimise any slow approach even if you did not know your actual weight, the ambient conditions or the delta of your particular engine versus the standard donk.

It went like this:
a. Fly conventionally with gear and flap down
b. Decide what RPM/JPT you wanted to use for this approach
c. Set that RPM/JPT
d. Control flight path with stick
e. Pull nozzle lever back to hover position
f. Speed reduced smartly and so required an ever increasing AoA to maintain desired flight path.
g. Once AoA reached the optimum of 8 note IAS and use nozzle lever like a throttle to maintain this IAS

You are now flying a fully optimised approach despite all the variables present on any given day. Amazing eh?

A few comments re some of the above:

Re a. Anything between 250 and 180 kt would do.

Re b. The selection of RPM/JPT would be based on how vital it was to achieve max performance on that approach. Clearly the greater the power the less wing lift needed and the lower the stabilised IAS but the greater the engine life counts used. If you used full throttle it also meant that you had no power available to do an overshoot and the use of nozzle lever to tickle speed (and lift) up would involve a fair height loss. Better to leave a margin from full throttle of say 3% which was about 1000lb of thrust.

Re f. Ideally you would start moving the nozzle lever forward to stabilise the increasing AoA as it approached the optimum of 8. The wing stalled at rather more than 12 in these circumstances so you could use more than 8 if you were with it. Avoid more than 12 at touchdown or you could scrape the tail bumper.

The above ’fixed throttle and varying nozzle’ technique was my favourite but rejected by the RAF who taught to leave the nozzle at 60/65 and vary the throttle to control speed/AoA. This certainly gave a crisper glide path response but you never knew the counts you would use or what your power/overshoot margin was at any time without much monitoring of the engine.

JF

[Wizofoz]

I think what GTE was refering to as the AofA display on the 737 is, in fact the Pitch Limit Indicator (PLI), which displays the pitch attitude co-incidental with onset of stick-shaker.

I think every EFIS equipped Boeing has these.

[Denti]

The pitch limit display (eyebrows) with flaps other than up is of course standard. But there is a customer option (god knows, boeing has even more than porsche) for a true AoA display within the PFD. I believe there is a picture somewhere on Chris Bradys webpage.

[fauconpoilu]

On the type of Bizjet we fly on, there's no value of AoA per se (Falcon50).
However, the AoA information is used for stall margin indications on the PFD, changing with the A/C configuration (slats/flaps/airbrakes/landing gear).
It is also used for triggering stall warning / protection (automatic extension of slats under a certain speed).

As John Farley said, stall happens at a fixed AoA value, for a given configuration, regardless of variables.

[Spooky 2]

Denti, I think thats what I said back on past 9, or at least that's what I meant to say.:}

Your right on regarding the Boeing options list. Only the checkbook has limits to the possibilities.

[ImbracableCrunk]

AoA is available on 737. I've seen it primarily in the HUD displays, but it can be on the PFD or both. And this is not the PLI.

[PBL]

As John Farley said, stall happens at a fixed AoA value, for a given configuration, regardless of variables.

Well, I took my magnifying glass to what John Farley said, because it would have surprised me had he said that. John said "in these circumstances". That is not by any means the same as "regardless of variables".

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.

I did find a source for what appears to be the paper I referenced on BUSS, by Joelle Barthe (http://aviationtroubleshooting.********.com/2009/06/af447-unreliable-speed-by-joelle-barthe.html). It doesn't contain much information about the details of the system. Installing it requires, on some aircraft, replacing the ADIRUs with a model in which the AoA sensorics passes through the IRs.

PBL

[Wizofoz]

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? Are you talking about compressibility factors?

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

[PBL]

[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.

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]

So again we are down to terminology.

Again we are down to misreading.

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

Neither have I.

....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 :D

[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 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.

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

[Machinbird]

PBL,
A few coments on the subject:
The LSOs have an extremely accurate means of determining AOA of an approaching aircraft. It is familiarly known as the Mk 1 Mod 0 eyeball. The LSOs stand abeam the touchdown area on the carrier and observe aircraft attitude using key landmarks on each aircraft's structure. Things like how much of the rudder is visible or the position of the horizontal stabilizer relative to the wing are used. The aircraft also illuminates its approach light system which mirrors the pilot's AOA indications. Green light=slow, Amber=onspeed, Red=fast. There is/was a doppler system aboard ship for measuring closing velocity. In my day, an enlisted man pointed it at the aircraft and a readout was available at the LSO platform, on the PLAT (TV) and in other locations around the ship. But the key indicator of approaching aircraft AOA was the visual "picture".

One of the key reasons for the USN method is the strange concept:} that an aircraft should be fully trimmed up to maintain stable flight. Once an aircraft is trimmed up to approach speed, even an approach speed well on the backside of the power curve, the aircraft will try to hold that speed without much intervention. Now if you add sufficient power to hold a 3.5 or 4 degree glideslope, you will have a stabilized glidepath. If the pilot sees that the aircraft is trending down on the glidepath, he adds sufficient power to reverse the trend for a few seconds and then resets the original power plus a small amount (I used to be able to do this by ear). The pilot could then fly an almost completely heads up approach scanning the 3 key factors-meatball, lineup, AOA without fear of falling out of the sky approaching the spud locker. His stick inputs are done with three fingers and are directed at damping the phugoid and controlling lineup.

Navy carrier aircraft are speed stable on approach. The F-4 for example, although having an artificial feel control system, used a bellows balanced against downsprings for pitch feel control. The bellows took a pitot air signal and the force it generated was used in a balance beam system to control airspeed. As you trimmed the aircraft, you adjusted the fulcrum of the balance beam.
Personally I don't see any other way to think about the power-airspeed-climb/descent relationship. If aircraft were not naturally speed stable (at least as they are now designed) then perhaps power would equal speed and the attitude/ nose position would equal climb, but to me this is bass ackwards.
In airline type operations where you aren't really flying the aircraft (the autopilot is) then of course using more power will make you go faster, but that isn't the way the aircraft actually natively acts.

[PBL]

Machinbird,

you say that because of the speed stability of certain aircraft, things work best with using pitch to affect airspeed and power to affect flight path. Well, yes, that is also the way I learnt to think in my instrument training in the U.S.; as JF says, it is a culture thing.

Here is the other point of view, the aerodynamic one

Pitch affects AoA most directly, AoA means coefficient of lift, and coefficient of lift means lift which means, balancing against weight, flight path.

Pitch also affects thrust vector, and flattening that is also going to take something out of total lift.

Speed stable or not, aerodynamics says you're going down, and directly.

Isn't that just as true? Indeed, truer! Fly flatter with the same thrust, you're going down, instantaneously.

Why is that aspect somehow suppressed?

There has to be some reason why the one way of looking things is more compelling to a large proportion of pilots. I don't know what it is at this point, likely because I have not thought it through enough. I am not yet convinced that speed stability is the explanation.

PBL

[Machinbird]

PBL
Seems that the difference is flying in stabilized flight versus unstabilized flight.
If I wanted to intercept something 45 degrees above me, I'd plug in burners and haul the nose up to put it where I wanted it-but the aircraft's kinetic energy would be changing (decreasing) as the potential energy would be increasing. That is the "aerodynamic approach to flying. When you are trading potential and kinetic energy back and forth, why bother to trim?

On approach to the ship at 1.1 Vs, if you see you are going to be a little short and inch the nose up, you had better match that with a power change or in two seconds you will begin to devellop a dangerous rate of sink due to airspeed loss. Typically there is only 11-15 feet of tail hook to ramp clearance for on glide slope operations so unstabilized flight on approach to the boat is a no-no.

And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.

[italia458]

Regarding these quotes...


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.......I agree. I'm a glider pilot myself and if you want to speed up you pitch down, to slow down you pitch up. Every glider pilot should understand that.

I agree that in the majority of flight "situations" and the majority of aircraft, the concept that speed is controlled by AoA (pitch) and climb/descent controlled by thrust (throttle), will provide the best control and handling of the aircraft. Only in some flight situations is it reversed; one I can think of is slow flight recovery... If you break it down into two separate actions you'll see that if you first pitch forward, you will lose altitude. So by adding thrust first you will prevent altitude loss, and then pitching forward will build airspeed. That is what I teach in those situations. Both actions should happen very close together.

Advocates of pitching down first then adding throttle use the argument that when you pitch forward you are decreasing the AoA (correct), and therefore you decrease induced drag and allow the aircraft to accelerate by adding throttle. To do it this way, you will lose altitude the moment you pitch forward. In slow flight you have not stalled and so stall recovery is not beneficial.

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.This is something like the recovery from slow flight. You're on approach and 10 kts slow and you don't want to lose altitude on your recovery. Applying full throttle in this case would be the thing to do.

This is leading to my last point. Although these are treated as two separate inputs that affect two separate outputs, if you change one you have to change the other! If you are at your correct speed on your approach and are descending too fast, if you add throttle to compensate, you will have to pitch up slightly to maintain your speed. (Most aircraft have a couple that will pitch the nose up when thrust is increased so you might not need to adjust pitch much.) Pitch is the fastest/most efficient way to control airspeed in most flight situations. Throttle is the fastest/most efficient way to control altitude in most flight situations. However, pitch still can affect altitude and throttle still can affect speed.

Going back to the glider pilot stuff.... it illustrates it very well because your "thrust" is controlled by moving the stick forward or back and speed is controlled the same way. If you pitch down you increase speed and increase rate of descent. If you pitch up you decrease speed and increase rate of climb. They are tied together so a good glider pilot has a very good understanding of conservation of energy and how to most efficiently use the energy.

On approach to the ship at 1.1 Vs, if you see you are going to be a little short and inch the nose up, you had better match that with a power change or in two seconds you will begin to devellop a dangerous rate of sink due to airspeed loss. Typically there is only 11-15 feet of tail hook to ramp clearance for on glide slope operations so unstabilized flight on approach to the boat is a no-no.

And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.I agree with this.

[PBL]

Machinbird,

Seems that the difference is flying in stabilized flight versus unstabilized flight

We are talking here about longitudinal static stability, that is, what happens in the aircraft x-z plane, which in the case of approach and landing is also the earth x-z plane.

I have no experience with, and thus little intuition about, military kit. My interest in pursuing this question is as follows. We're discussing "pitch is primary for AS; thrust is primary for VS" versus "pitch is primary for VS, thrust is primary for VS". Let me introduce the acronyms p/as,t/vs versus p/vs,t/as for these “rules”. My question is whether one can make objective sense of the attribute "primary for".

I can see three possible reasons.

First is cultural: one learns to drive on the left; one learns to drive on the right. There are habits and reactions appropriate to how one learnt, the habits and reactions of the "other-siders" are at first "foreign", but one can learn. JF suggested the p/as,t/vs versus p/vs,t/as question is cultural.

Second is handling qualities. The airplane reacts "more appropriately", whatever we can construe that to mean, if a pilot uses, say, p/as,t/vs. Or the other way around.

Third is aerodynamics.

Pitch control and thrust control influence a vector in the x-z plane, namely the velocity of the aircraft. This velocity has a magnitude, airspeed, AS, and its projection onto the vertical (earth-z axis) is vertical speed, VS. AS and VS are not independent unless the wing is flying at AoA for zero lift (usually slightly negative). Let's assume low-speed (incompressible) flow.

AoA = Pitch + FPA. Since we are talking about pitch and FPA, It's more convenient for me to use aerodynamic normal and axial force (the aerodynamic forces on the wing normal to and parallel to the chord) than about lift and drag.

The forces are (1) thrust, (2) normal force, a function of dynamic pressure and AoA, proportional to dynamic pressure and (in the range we care about) AoA; (3) weight; (4) axial force, also a function of dynamic pressure and AoA proportional to dynamic pressure, but roughly to the square of AoA. There is also a moment, say about the quarter chord as usual, proportional to dynamic pressure and AoA. The dynamic pressure is proportional to the square of AS.

Start from equilibrium (stabilised). If you are on approach, FPA is 3° say. Say you want to speed up.

Equilibrium parallel to the chord: Thrust= AxialForce + Weight x sin(Pitch). Thrust is probably less than 0.5 MaxThrust. Let's do it for AoA = 10° and for AoA = 15°; so Pitch = 7°, resp. 12°.

So, let's do it for a Cherokee. Say 0.5 x MaxThrust = AxialForce + 2500 lbs x 0.12. If you put in full thrust, which you get quickly, then the force accelerating you along the chordline is 0.5 x MaxThrust (namely, the reserve thrust).

Now say, instead of putting in thrust, you reduce AoA. Then thrust remains at 0.5 x MaxThrust, but AxialForce is reduced and the contribution of weight = 2500lbs x 0.12 is reduced to 2500 x 0.(something less). So you get a net force in the direction of aircraft-positive-x, but it is rather less than 0.5 x MaxThrust.

Conclusion: if you look at the magnitudes, and want to speed up, then putting in thrust is the most effective way, aerodynamically.

This is for a Cherokee, but I bet the same reasoning works for many airplanes, including your Navy combat aircraft on approach.

So it doesn't look as if the aerodynamical viewpoint (your suggestion it was entailed by speed stability, then the second suggestion that it was entailed by stabilised versus unstabilised flight) will give us the p/as,t/vs conclusion.

Here is another way of looking at it aerodynamically.

If you want to go vertical, you have to use thrust for AS. If you want to go horizontal, you also have to use thrust for AS (otherwise drag will slow you down to zero AS, and we are back to the vertical :-). What happens in between pitch=90° and pitch=0°? Whatever it is, it is going to be continuous (all the physics in sight is continuous functions), so, for some region around pitch=90°, AS is mostly going to be affected by thrust. Similarly, for some ways around pitch=0°, AS is mostly going to be affected by thrust.

You want to say that somewhere in between 0° and 90°, it is the case that p/as,t/vs is "best". If the reason is to be aerodynamic, that must mean (continuous physics) that there is some point between 0° and 90° when t/as changes to p/as, and then some higher point at which it changes back. Can we say anything qualitative and useful about those points? I doubt it. But it does mean that, for any aircraft whose thrust capability is at least equal to its weight, there are going to be those two “switchover” points at which thrust being “primary for” AS change to pitch being “primary for” AS, and then back. It cannot be the case that pitch is “primary for” AS over the entire pitch range 0° to 90°.

So I am going for either pure cultural, or handling qualities. But then, if handling qualities, how would that be explained?

PBL

[HazelNuts39]

The pilot uses elevator to control pitch, and power lever for thrust. The latter control loop is usually less responsive than the former, particularly for jet engines at low power.

regards,
HN39

[PBL]

The pilot uses elevator to control pitch, and power lever for thrust.

Let's hope so!

The latter control loop is less responsive than the former, particularly for jet engines at low power

I imagine that is dependent upon the aircraft. Is a heavy transport aircraft more "responsive" in pitch than its engines are to commanded thrust?

Besides, if thrust has higher latency, it's going to have that whether you are using it to control airspeed, or using it to control rate of descent.

PBL

[EMIT]

A good article about the Boeing vision on this can be found in the Boeing Aero magazine, issue 12.
It explains well why the world of large airliners does not use AoA as indicator on approach, such as fighter jets do. Shape of lift curve has to do with it, civvies with long slender wings, fighters with short, stubby wings being the big difference.

Still, having AoA as backup instrument for case of pitot static problems would be a nice thing to have.

In terms of equipment, most fighter jets will have AoA indication available to the pilot. As does the US Navy, all F-16's are flown on approach using AoA instead of speed for approach. Limit for landing an F-16 is not (low) speed, but body angle on touchdown (you will scrape airplane parts if angle is too large).

[PBL]

Link is Boeing Aero 12: Angle of Attack (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

PBL