What do test pilots do?

[pdr]

Well obviously they test fly aeroplanes - everyone knows that! But why? what are they trying to find out? 

Common sense says they are looking to discover how the aeroplane performs, but sadly "common sense" is rarely both. The purpose of flight testing is not to discover how an aeroplane behaves & performs - not for the last 40-odd years, at least. Aeroplane performance and behaviour is established in computer simulations so that test pilots are not required to explicitly fly into unknown situations.

A computer model is used to predict the performance over ranges of speed, weight, altitude etc etc. Engineers take these predictions and define a set of test points which can be measured. A test point may be a specific speed, attitude and altitude or pulling a specific G at a particular weight & configuration (centre of gravity location, wheels up or down, flaps at a setting, carrying/not carrying external fuel tanks etc). These test points are built into a flight test plan and the test pilot then creates that situation so that it can verify that the simulation's predictions match the observed behaviour of the aeroplane. The initial test points will be well within the "safe" region of known behaviour, and then progressively and methodically moved to the extremes. If any test shows behaviour that's different to the prediction the test programme is stopped until a reason can be found - either an error in the test procedure or an inaccuracy in the simulation model. The model is deemed "good" as long as its predictions match the observed behaviour of an aeroplane. If observed behaviour deviates then the model is "bad" and the aeroplanes are grounded until it's corrected. 


The "Flight envelope clearance" of an aeroplane is then based on the analytical predictions of the model, not the empirical observations of test pilots. And the purpose of flight testing is to verify/validate the model, not the aeroplane. As well as being more effective, this approach has made the Profession of "Test Pilot" much safer...

NALOPKT(&EFGAS)


PDR 

[crissdee]

Thanks Pete, that was properly interesting!

[jenny]

It was indeed interesting.

[tetsabb]

On a related topic, we watched a programme on BA2069, a flight from Gatwick to Nairobi in 2000, when a passenger entered the cockpit and tried to take control.

en.m.wikipedia.org/wiki/British_Airways_Flight_2069

The aircraft went up at an alarming rate, stalled, then headed groundwards at speed. Control was only retaken at under 10,000 feet.
Amazing it did not break up under tge stresses

[barbados]

That sounds like an interesting programme Tetsabb, what platform was it on?

Also pdr I’ve heard tell that oil rigs are designed to withstand a 100 year wave, how extreme do the designers go when designing an aircrat?

[suze]

The documentary that tetsabb watched was on Channel 5 on Friday evening. I did note its presence, although we didn't watch it.

It is available on My 5, Channel 5's version of iPlayer.

[tetsabb]

Jul 21, 2024 10:43:21 GMT barbados said:

That sounds like an interesting programme Tetsabb, what platform was it on?

Also pdr I’ve heard tell that oil rigs are designed to withstand a 100 year wave, how extreme do the designers go when designing an aircrat?

As suze says, it was C5 on Friday, probably viewable on their replay service. First of 4 or 5 . 'Terror at 30,000 feet' . Next one is this coming Friday at 2100. A380 which lost an engine to a fire.

[alexanderhoward]

Back in the day though... In the film First of the Few, David Niven plays the test pilot for Supermarine, and RJ Mitchell's righthand man, from the Schneider Trophy days to the first Spitfire. He has that breezy, laughing-in-the-face-of-chance-and-death attitude we all imagine of test pilots. Early on he is the first man to experience a high-G blackout, during a high-speed turn in a race.

The real Supermarine test pilots were cut from the same cloth. One of the test pilots barrel-rolled a Lancaster bomber; without the aid of computer analysis of the forces and maximum structural loads. The Lancaster must have been a more sturdy beast than it looks. Leonard Cheshire used them for precision dive-bombing.

[pdr]

Jul 31, 2024 10:58:17 GMT alexanderhoward said:

Back in the day though... In the film First of the Few, David Niven plays the test pilot for Supermarine, and RJ Mitchell's righthand man, from the Schneider Trophy days to the first Spitfire. He has that breezy, laughing-in-the-face-of-chance-and-death attitude we all imagine of test pilots. Early on he is the first man to experience a high-G blackout, during a high-speed turn in a race.

Sadly not even vaguely true. The issues around high-G red-out and black-out where known experiences way back into the first world war - as soon as manoeuvrable "fighters" replaced the previous "aerial gun platforms" with aircraft like the Fokker EIII & DR1 and the DH2, Sopwith Pup etc it was a known constraint. Pilot training texts reference anti-G techniques (clenching the stomach muscles, raising the feet etc) as far back as the late 1920s. Soon after the introduction of the new high performance monoplanes in the mid 30s the RAe medical research team conceived what became known as the "Bathfire" system when the pilot's lower body was surrounded with water to provide external pressure that opposed the movement of blood to the lower body. This increased typical G-tolerance from around +5G to well over +9G. Initial experiments were done with an actual waterbath in a centrifuge, but pressure in liquids is only dependent on depth so the practical implementation was a set of external double-skinned trousers with a layer of water between the skins - the original "G-suits". These weren't used in combat in WW2 because that kind of manoeuvring lost too much energy and left the aeroplane vulnerable. But they were used in all the early jets. There were also experiments done with "prone pilot" configurations where the pilots lie on their chests to minimise the height difference between brain and feet, but they never produced workable solutions and were abandoned - although they left a legacy in that modern fighters have the seat incline backwards to raise the feet a bit for this reason.

Today we don't use the bathfire system; instead we have a set of inflatable long-johns (called "G-pants" which are pressurised by an electronic control system when needed, but the principle is the same and typical ser4vice fast-jet pilots are supposed to take 9G continuously and up to 12G for 3-4 seconds provided the rate of application of G (dG/dt) isn't "instantaneous". One of the issues with the early high-agility fighters (F16C onwards) was that they could apply G so quickly the pilot would black out without knowing it had happened. There are scary cockpit videos showing pilots instantly losing consciousness and awaking a few seconds later completely unaware that they'd blacked out.

Note that all of this only works for positive G, because the problem is to do with starving the brain of oxygen as blood moves to the lower body. Negative G causes grey-out as too much blood goes into the cranium and pressurises the brain. This is a more insidious problem for lots of reasons, and to date has no "cure", so aircraft remain limited to anything more than about -4G for anything but a few seconds.

The real Supermarine test pilots were cut from the same cloth. One of the test pilots barrel-rolled a Lancaster bomber; without the aid of computer analysis of the forces and maximum structural loads.

It was done only once, by Alex Henshaw, to settle a bet. It severely damaged the aeroplane and nearly had him jailed for action likely to give assistance to the enemy in time of war. He was the sort of gung-ho test pilot who was of little value because he would never do the test he was being asked to do (because "he knew better") requiring development flight tests to be repeated by other pilots - so he was relegated to production test flying (just testing that a new aeroplane is fully working and will trim for normal flight). Another such was Chuck Yaegar - a man who used political "clout" to protect himself from the consequence of his actions. When he was on the NF104 team evaluating control systems to be used in the (subsequently cancelled) X-20 "Space plane" all his flight tests were useless because he refused to fly the test profiles needed. On one of these flights he lost control of the aeroplane in the descent and had to eject, injuring himself and destroying a valuable test aircraft. But I digress.

The Lancaster must have been a more sturdy beast than it looks. Leonard Cheshire used them for precision dive-bombing.

Not quite. When he was CO of 617sqn he was dis-satisfied by the accuracy of the target marking provided by the Pathfinder force when they were using their precisaionj "earthquake" tallboy and Grandslam weapons. He always carried some target marker flares in his bomb bay so when he noted that two pathfinder marking drops had placed flares in the wrong place he tried marking it himself. To get the precision he had to run in at low level, but then found that the marker flare skittered along the ground for nearly half a mile. So he did a second run at much higher level, pushed his lancaster into a vertical dive and then release his flare before pulling out - like dive bombing but with a flare. This placed the marker right on the target and so the rest of the force were able to bomb accurately. He did this just once. When they got back he demanded (and after much argument got) a Mosquito - a much smaller and more agile aircraft - and from that point onwards did all his own target marking. A few months later he found the Mosquito to be a but cumbersome for some the the extra-precision targets they were being assigned, so he went the next step and got himself a P51 Mustang fighter-bomber to to the job.

But Lancs were never used for dive-bombing - there was just this one occasion where one pilot used a dive-bomb profile for target marking.

PDR

[pdr]

Jul 21, 2024 10:43:21 GMT barbados said:

Also pdr I’ve heard tell that oil rigs are designed to withstand a 100 year wave, how extreme do the designers go when designing an aircrat?

Depends on the aeroplane and its intended role, and which aspect of the design. The core spec will define a "flight envelope" which might be (say) -1.5G to +2G for a civil aeroplane and then the structural design will add a margin for the ultimate design "strengths" that's typically 50% and rarely as much as 100% because it adds too much weight. Speeds will be driven by the role requirements, but airliners will be designed toi the "sweet spot" of their engine performance. With older turbojets this was typically Mach 0.86-0.87, but with modern turbofans this has dropped to around Mach 0.82. So yes, VC-10s were faster across the atlantic than a modern A330/B767 or similar. No subsonic airliner will ever be intended for much above Mach 0.90 because of the Critical Mach Number issue. But that's a different topic.

PDR

[tetsabb]

I read somewhere that Neil Armstrong and many others of the Apollo crews saw their missions very much as test flights. Just a bit different.

[crissdee]

I'm not sure if I ever posted this on the old forum, a quick search turned up nothing, but it may have been pruned.

My late father once told me a story of his time in the RAF. He was at RAF Scampton, where could be found a number of Lancasters. He told me that one day, a group of American pilots who were there on some kind of exchange, were mocking the abilities of the RAF pilots. They were then taken up in a Lanc by one of those they had mocked, who proceeded to fly around the perimeter of the airfield sideways, tracing the line of the fence with a wingtip. This sounded very impressive, but could it have been done?

[pdr]

Jul 31, 2024 21:15:04 GMT crissdee said:

I'm not sure if I ever posted this on the old forum, a quick search turned up nothing, but it may have been pruned.

My late father once told me a story of his time in the RAF. He was at RAF Scampton, where could be found a number of Lancasters. He told me that one day, a group of American pilots who were there on some kind of exchange, were mocking the abilities of the RAF pilots. They were then taken up in a Lanc by one of those they had mocked, who proceeded to fly around the perimeter of the airfield sideways, tracing the line of the fence with a wingtip. This sounded very impressive, but could it have been done?

I doubt it. The interior of a Lancaster is very cramped and they have no spaces for "passengers", and while they were stable & predictable at low level that sort of thing sounds apocryphal.


PDR