Covenant
As I said on the other site, you might find this interesting. I did.
There are several Concorde features relevant to slow speed flight, I have only included those relevant to the speed of the accident flight.
Firstly autostabilisers, which are there to improve handling qualities generally:
1 There is an automatic trimmer based on angle of attack. This compensates for the forward CoP movement, due to increasing alpha, that is a characteristic of the design. It works by automatically applying a nose down pitch trim, in both manual and automatic flight. Operates between 10.5 - 19.5 degrees alpha.
2 There is another automatic trimmer based on speed. This compensates for the rearward CoP movement during acceleration by signalling an up elevon demand, in both manual and automatic flight. Operates between 200 - 600 kts IAS.
Secondly high alpha protection systems:
1 Above 13.5 degrees alpha, a “Super Stab System”, produces a down elevon signal proportional to the alpha, pitch rate and deceleration.
2 Above 14.5 degrees alpha the autopilot trim function is inhibited
3 Above 17.5 degrees alpha the autopilot is automatically disengaged.
4 At 16.5 degrees alpha, detected by any ADC, both control columns will shake. They will also shake at any time that the stick wobbler operates.
5 At 19 degrees alpha, both control columns will "pulsate" against any manual nose up control force, in an unmistakable warning. This is the stick wobbler.
6 There are pitch rate detectors which can advance the Stick Wobbler warning to 16 degrees alpha under high pitch up rate conditions.
So what does all this mean?
In your post you commented:
My point is that for a delta wing, I suspect there is an optimal angle of attack, probably below 20 degrees, which is not heralded by buffet or stick-shaker and beyond which you should not go without masses of available power to pull you out of trouble.
The systems info above shows that the designers had your concerns well in mind, indeed wanted to restrict alpha during routine operations to well below your 20.
If we think of the aircraft at say 19 alpha, as shown on the BEA traces, then:
1 Incidence Trim and Super Stab would have been working to lower the nose - or, if opposed by pilot action, increasing the stick force necessary to maintain that angle of attack.
2 Autopilot Trim would have been inhibited and any engaged Autopilot would have been disengaged.
3 The Stick Shaker and Stick Wobbler would have been operating continuously.
In piloting terms, because the stick shaker and stick wobbler trigger incidences are set so (relatively) low on the Concorde CL/alpha curve, it seems reasonable to suggest that a pilot should, in extremis, properly adopt what might be called subsonic flying techniques, and operate on/just below the stick shaker, in order to avoid ground contact.
Certainly, the BEA traces show that Marty held 19 alpha - as much as he could without loosing speed. Indeed as I said on another post he seemed to finesse about a two knot increase. This was all the while at a radio height of 200 feet or a tad less. Stop for a moment and think what that would look like. Could you really think of giving half that away given the power lines and buildings about? No, IMHO he was doing brilliantly at this point. Then he lost more power and frankly became a passenger.
My bottom line?
Lift was not the problem. The aircraft had too much drag for the thrust available.
Incidentally the manufacturer’s notes have this to say about lift:
"This slender delta wing is still lifting strongly at 25 degrees alpha...but the angles are far too great to be of practical use because of the extremely high thrust requirements for level flight, and the impracticability of having passenger cabins at such attitudes."
I have been led to believe that during early test flying, the greater problem was thought to be roll and yaw control at very high alpha, with the aircraft having a tendency to sideslip as the limits were reached. I can only say that was very much the forecast case following the HP115 experience.
So far as Vzrc goes, the perhaps the following data may be of interest. It is for the aircraft at MTOW (185,070 kg) at LHR, on an ISA day with zero wind:
V1: 164 kts
Vzrc-3e-Gear Down: c190 kts (so not routinely calculated)
VR: 193 kts
V2: 215 kts
Vzrc-2e-Gear Up: 260 kts (Calculated for every departure)
Vzrc-2e-Gear Down: 316 kts (Above the sea level Vmo of 300 kts!)
My final comment: Just look at the jump in Vzrc when you go from three engines to two engines. About 70 kts. If the gear is stuck down that becomes an awesome 126 kts.
Regards
edited for terrible English and my usual UBB pigs.
[ 03 September 2001: Message edited by: John Farley ]