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Determining airspeed without pitot tubes

Started by Richard McDonald Woods, Tue, 4 Aug 2009 13:57

Tor

#20
Hello Opher
I find your views interesting, although I may not agree. Anyway, as I was curious to read more that you wrote, I checked some of your previous posts. Regarding military vs. civilian trained pilots, in my opinion you don't need a license for a motorcycle if you are going to drive a car. Or a race license to drive on the road. Flying an airliner it about not bringing yourself into a position that requires daily experience with unusual attitudes. Lot of fighter pilots has died over the years and are as such not people incapable of making mistakes. Also you mention the MD-11 accidents as an example of civilian pilot error, where I would say it's a poor design to have a wing break before a landing gear collapse. I would compare this to the F100 Super Sabre, with it's "sabre dance" that cost dosens deaths of fighter pilots. The design of an aircraft should take into account that what will happen if the pilot indeed makes (or unforseen changes in the wind causes) an error. For the MD-11 i think it's unacceptable the recent landing accidents we have seen, and I can't recall another aircraft type replies to a hard landing by killing it's crew and passengers.

Below quote is from: http://aerowinx.com/forum/topic.php?post=612#post612
Quote from: opherbenThe Airbus system design (e.g. flight control system) to me is not suitable for human piloting. First, FBW with a sidestick with limited travel doesn't give the pilot sufficient cues as to control surface status. Can't comment about control feel and feedback since I haven't flown it. Secondly,  the control laws and their failure states and fallbacks are far too complex, and deviate from a natural design.  One could possibly mitigate some of this by extensive and frequent simulator training, but it will remain a major problem. The Boeing concept is the right one.

As an aircraft I too perfer Boing (as an "office" the Airbus flight deck sure looks nice though with living room carpet and everything. If I ever get tired of flying I may want to switch types for the better comfort :-). However, the sidestick in it function is no different from an F-16. Strangely, I have never hear any pilot saying that they wouldn't want to fly an F-16 or "that the pilot is not really in control" or as you say about the Airbus that "is not suitable for human piloting". Incidentially I jumpseated on an Airbus 321 last year, and the first officer came from the airforce having flown F-16. He said that the sidestick felt just the same with the Airbus being slightly more sensitive (at low deflecions of the stick of course). Regarding the control law I agree that it seems way to complex. Another pilot I know that now flies the Boing used to fly Airbus and had once experienced the Airbus switching laws in flight due to a minor malfunction (can't remember if it was direct or alternate or which malfunction). We discusses this after the AF incident and his comment was that it was very different to fly after it switched law and that he wouldn't have wanted to be in their situation having to cope with more severe malfunctions and possibly severe turbulence at the same time. I guess the F-16s alternate law is the ejection seat. Which by the way probably have saved many military pilots from entering fatality statistics.

Regarding feedback, I have the hobby of flying radio controlled model airplanes, where technically you have no feedback what so ever. However, your brain will adjust to getting feedback from other sources. When I fly model airplanes I can certainly "feel" everything the model is doing. This ofcourse be eyesight combined with the corrections I have to make and the needed amount of deflection I have to do to get a certain response, sloppy controls due to low speed are e.g. easily indentifiable. Add to that that the pilots of the Airbus also have their seat of the pants feeling.

If you are not familliar with RC model airplances and how they can be flow today, here is a little video I made a couple of years ago. This type of aerobatic is flown alot of the time with the wing stalled, and it is also easily identifiable when the wing is flying when it's transitioning into a stall and vice versa.
http://www.precision.dk/video/HyperionYak54-180.wmv (right click "Download linked file as" it's 32mb)
P.s. the aspect ratio should be 16:9, it worked when I encoded it 2 years ago, but some players shows it as 4:3 today for unknow reasons. If you use VLC you can select the aspect ratio from the video menu.

Zinger

Hi Tor,
I'm on the way to a chata (cottage) for the weekend and will reply later. Last RCs I built were a large Extra with a superb Hungarian seal-less engine, a composite Excel helicopter w/gasoline engine, controlled by 8-channel Futaba.
Regards, Zinger

Zinger

#22
Quote from: TorHello Opher
I find your views interesting, although I may not agree. Anyway, as I was curious to read more that you wrote, I checked some of your previous posts. Regarding military vs. civilian trained pilots, in my opinion you don't need a license for a motorcycle if you are going to drive a car. Or a race license to drive on the road. Flying an airliner it about not bringing yourself into a position that requires daily experience with unusual attitudes. Lot of fighter pilots has died over the years and are as such not people incapable of making mistakes.

a. If you examine accident investigations related, let's take one issue as example- airspeed indication error in automated transports, you will evidently conclude that something is wrong. During the investigation of a B757 which lost control one minute after takeoff and plunged to the ocean due to a single source airspeed indication error, 3 more crews were tasked with a similar simulator flight. All ended up losing control and crashing shortly thereafter. Draw your own conclusions. 18 out of 19 B747-400 pilots with all levels of experience did not react to below glidepath on final approach or PFD annunciation with gross errors, during a NASA/ Boeing/ FAA and academy research to identify automation problematics. To me all is written on the wall clearly. It has nothing to do with military versus civil, combat versus transport, it has to do only with understanding aircraft behavior and being able to fly out of trouble with this ability. You want an analogy from driving? master driving on ice, snow and water when you live where I do, train to avoid out of control oncoming traffic, learn to get back on the road after having deviated off. Anyone here got this training to qualify for a license?. Every winter here, 2-3 busses full of passengers lose control, many travellers die. Just yesterday, national mourning for 20 lost lives ended. That was alcohol, not ice. Any authority requires a device which stops the vehicle in the presence of alcohol fumes at the driver position?

Also you mention the MD-11 accidents as an example of civilian pilot error, where I would say it's a poor design to have a wing break before a landing gear collapse. I would compare this to the F100 Super Sabre, with it's "sabre dance" that cost dosens deaths of fighter pilots. The design of an aircraft should take into account that what will happen if the pilot indeed makes (or unforseen changes in the wind causes) an error. For the MD-11 i think it's unacceptable the recent landing accidents we have seen, and I can't recall another aircraft type replies to a hard landing by killing it's crew and passengers.

b. The MD-11 has unlike most transports, neutral longitudinal static stability, designed for improved fuel economy. It poses challenges requiring a different flying technique from stable aircraft. It became a problem for the two crews mentioned while landing in weather. Again both issues surface, especially for pilots of trusted automatics who only take over control and fly manually on occasion.

Below quote is from: http://aerowinx.com/forum/topic.php?post=612#post612
Quote from: opherbenThe Airbus system design (e.g. flight control system) to me is not suitable for human piloting. First, FBW with a sidestick with limited travel doesn't give the pilot sufficient cues as to control surface status. Can't comment about control feel and feedback since I haven't flown it. Secondly,  the control laws and their failure states and fallbacks are far too complex, and deviate from a natural design.  One could possibly mitigate some of this by extensive and frequent simulator training, but it will remain a major problem. The Boeing concept is the right one.

As an aircraft I too perfer Boing (as an "office" the Airbus flight deck sure looks nice though with living room carpet and everything. If I ever get tired of flying I may want to switch types for the better comfort :-). However, the sidestick in it function is no different from an F-16. Strangely, I have never hear any pilot saying that they wouldn't want to fly an F-16 or "that the pilot is not really in control" or as you say about the Airbus that "is not suitable for human piloting". Incidentially I jumpseated on an Airbus 321 last year, and the first officer came from the airforce having flown F-16. He said that the sidestick felt just the same with the Airbus being slightly more sensitive (at low deflecions of the stick of course). Regarding the control law I agree that it seems way to complex. Another pilot I know that now flies the Boing used to fly Airbus and had once experienced the Airbus switching laws in flight due to a minor malfunction (can't remember if it was direct or alternate or which malfunction). We discusses this after the AF incident and his comment was that it was very different to fly after it switched law and that he wouldn't have wanted to be in their situation having to cope with more severe malfunctions and possibly severe turbulence at the same time. I guess the F-16s alternate law is the ejection seat. Which by the way probably have saved many military pilots from entering fatality statistics.

c. The F-16 is an unstable aircraft to the point that when only the last of 4 channel autoflight remains functional, the pilot is directed to eject. In unstable aircraft the control surface position is meaningless, that point was about highly stable transport aircraft. I have a few hundred hours using such fllight control in a stable aircraft, I found it rather convenient until you encounter aircraft control problems. Let me briefly direct you to CFIT stats in F-16s. My AF after a few years modified the system such that the pilot has override of autoflight at will, exactly due to these CFITs. One of them, prior to the mod, was an outstanding squadron commander in his 3rd sqaudron command. In his last 10 seconds, autoflight took him for a ride. In his previous aircraft, F-4E, he wouldn't have hit the ground. BTW I knew Harry Hillaker personally and think the F-16 is the best tactical aircraft ever made. I was also in close personal contact with AF CO, when he prepared for an earlier assignment as the first IAI/IAF Lavi fighter developmernt program manager. I showed him how and why F-16XL instead of Lavi would be more cost effective. Following  PM Begin's go ahead, Lapidot was no longer in position to not start the program, which was terminated years before series production, and after US taxpayer $2B sunk.
Nice weekend all!


Regarding feedback, I have the hobby of flying radio controlled model airplanes, where technically you have no feedback what so ever. However, your brain will adjust to getting feedback from other sources. When I fly model airplanes I can certainly "feel" everything the model is doing. This ofcourse be eyesight combined with the corrections I have to make and the needed amount of deflection I have to do to get a certain response, sloppy controls due to low speed are e.g. easily indentifiable. Add to that that the pilots of the Airbus also have their seat of the pants feeling.

If you are not familliar with RC model airplances and how they can be flow today, here is a little video I made a couple of years ago. This type of aerobatic is flown alot of the time with the wing stalled, and it is also easily identifiable when the wing is flying when it's transitioning into a stall and vice versa.
http://www.precision.dk/video/HyperionYak54-180.wmv (right click "Download linked file as" it's 32mb)
P.s. the aspect ratio should be 16:9, it worked when I encoded it 2 years ago, but some players shows it as 4:3 today for unknow reasons. If you use VLC you can select the aspect ratio from the video menu.
Regards, Zinger

74pilot

#23
Yes, I would use any source available to confirm where a possible failure is originating.

The first thing you would do is check pitch and thrust. You normally have an idea of what your thrust should be, and if you fly a normal pitch angle, usually 1-2 degrees nose up on a 747 on cruise, then you should normally not be over speeding or stalling, unless some factors like wind shear, severe structural damage or likewise is happening. If you are flying too slow then you can end up on the back side of the power curve obviously, and in this case, I would check the ground speed.

The paradox can be that you have a stall warning and over speed warning at the same time. Which of them is lying? If you have any idea what direction you are going (standby compass), any idea what the wind is (your flight plan), and what approximate flight level you are at, you can use these sources to double check. If the aircraft is flying straight down then of course your ground speed would be quite low, because you are not covering much ground distance.

At no wind, ground speed on a 747 should be in the order of 500 knots. If it is 600 knots or more, you would have a high tailwind, if it is less than 400, a good headwind. If you are anywhere close to your optimum level, your mach should be pretty close to the normal range. Each .1 Mach is 60 knots around 25000' or so (where the local speed of sound is 600 knots). At MSL it is around 65 knots, while 57 knots at altitude (because of a lower temperature). For practical purposes, use 60 knots for each .1 Mach. This means .85 Mach is around 510 knots (using 60/.1), whereas it is slightly less, depending on the actual temperature. At -56 OAT, the LSS is 573 knots, so 60 is pretty close.

So if you are in a 747 on cruise and you get serious and multiple pitot/static failures, then fly your attitude, set (or leave) the thrust in a normal range, and cross check with the ground speed, keeping the wind in mind. I would use similar techniques on other aircraft types, but I have no experience with Airbus aircraft.

And if I really don't know, in a 747 I would rather have a (slight) over speed than a stall. Meaning, if you fly your pitch and increase the thrust, you are not going to break up in-flight, just like that. Yes, if you pitch down AND increase the thrust, otherwise no. And the drag will increase quite drastically when increasing the speed, so you should stabilize the speed at some non-destructive speed. But if you stall at altitude, then things start happening, especially having flight instrument failures ...

If you are thinking of the AF Airbus accident in particular, then I think that some probably contributing factors are that they are flying at night, and probably IMC in severe turbulence. If it gets really turbulent, it gets a lot harder to read flight instruments, so whether they show wrong values or not is irrelevant, if you cannot read them