Vortex bursting

[John Farley]

Cosmo

That was a great link of yours re slender deltas over on Concorde Accident Part 2

Any chance of emailing me the origins of that stuff?

As to vortex bursting, certainly the Aero Flight boffins saw this as the limit for slowing down in the 115. We used smoke generators to visualise the vortex so that a formating Auster could call stop just before the burst got to the trailing edge. By then speeds were less than 50 kts (with alphas in the 40s). The boffins were more concerned about the dramatic changes in the control forces should the burst touch the large unpowered elevons than in lift reduction issues. They suggested it could whip the stick out of your hand.

Another issue that relates to the usability of very high Cl values is what happens when one wing does eventually give up (in whatever manner). The H126 in which we reached a Clmax of 7, at the same time in history, tipped on its back too fast to see when it eventually stalled on one side.

Regards

[cosmo kramer]

Dear John

I'm just a simple human and I have no problem admitting that I don't know much about these issues nor comprehend fully.

However, I read and listen to what other people says and take what I find most reasonable.

One of the sources I found creditable is Stanford university's Department of Aeronautics and Astronautics. The page I linked to is a study called AA 241A,B Aircraft Design: Synthesis and Analysis (http://adg.stanford.edu/aa241/). If you go to the link Course Notes (http://adg.stanford.edu/aa241/AircraftDesign.html), and look for page 7.5 in the right frame, you will find the link I refered to 7.5 CLmax for SSTs (Low Aspect Ratio Wings at High Angles of Attack) (http://adg.stanford.edu/aa241/highlift/sstclmax.html)

The course notes is a "pre-release development version of a system of programs and textbook material to be released shortly on CD" by a company called Desktop Aeronautics, Inc. (http://www.desktopaero.com/)

Of interest to the Concorde accident, I wonder at what speed and alpha the votex would burst? From the photage, show on TV again and again after the accident, the AOA didn't seem particularly high.

Edit: Ouups - didn't

[ 02 September 2001: Message edited by: cosmo kramer ]

[John Farley]

Thanks cosmo.

As to the alpha the BEA traces show it hovering about 18 to 19 until just before control was lost when it went up to 25 and platformed briefly before coming steeply down. Without knowing the sensor/recorder type it looks to me as if the short platform could be an "on the stops" indication that may be hiding a momentary higher peak value that could have been 28ish. But that would be quite academic at that point in the flight.

I believe that the Concorde was expected to have a pitch up departure mode at about 25, but do not know that for sure. Again all academic as I would opine that it did not have anything like the thrust to hold speed at such an alpha, even with four engines, at least not at takeoff weights.

The guys who fly it (and the sim chaps) would know all this stuff for sure, but one can see why they are not wishing to get involved in such a chat as this.

Regards

John

[john_tullamarine]

Thanks for the references, I missed the earlier discussions.

regards,

JT

[WOK]

JF surmises rightly that the Conc fleet isn't keen to get involved in the current threads - however, wrt his comments above I can confirm that, in the sim at least, the Conc departs at an alpha of approx 25(slightly less, as I recall) and the mode is, unsurprisingly, uncontrollable pitch up followed by roll-off.

I don't think he really needed me to tell him that, though!


(Edited for grammatical incompetence)

[ 02 September 2001: Message edited by: WOK ]

[WOK]

PS If John Farley is ever at a lose end, I'd love to hear what the HP115 was like to fly, especially with regard to ground effect on landing characteristics. ;)

[New Bloke]

For those (like me) whose interests have been grabbed by these threads
http://www.concorde-jet.com/e_inside_story65.htm

A picture of what JF was talking about

[New Bloke]

Doing a search on “HP115” produced about the most eclectic bunch of stuff I have ever got from a search engine.

Concrete Products, Handley Page 115 (what we are really interested in), Picture No hp115 from a porn site (not even good porn) even Hush Puppies have a shoe of that nomenclature. How about Hand Painted Sarongs and something called “FUZOKU/KYOSHOU” –whatever that is.

[John Farley]

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 ]

[John Farley]

New Bloke

Well found.

However the HP115 picture at your link has a rubbish caption. The pic shows the position of the LE smoke canister and the visualisation of the centre of the vortex quite well but as to where the vortex bursts it’s rubbish. Alpha in this pic looks 15 – 20 tops, and the burst point is some way behind the aircraft and well out of shot.

WOK

The handing of the 115 during landing was an absolute doddle. The usual comment when somebody came back from their first trip was "why don't they make them all like that?"

The reason so many different types of pilots thought it lovely was undoubtably in part due to the fact that it had to be landed so fast (90 kts) to give any sort of go around capability because the drag was starting to build and the poor little Viper could not cope. Since that speed was over twice as fast as it could be flown (lift wise) the response to a little back stick was an immediate flare with none of the "I'm driving the back end down, and actually thumping the gear on harder" effect.

If you landed with six deg of drift in a cross wind it just smoothly puled the nose round of its own accord. With higher drift angles some attempt at easing off or even kicking off was easy to do.

During the landing run in strong cross winds it could develop a negative weathercock. This was put down to the high rolling moment due to yaw pressing the downwind gear onto the ground harder than the other one so dragging the nose out of wind. The 115 was not alone with this, several other high rolling moment due to sideslip aeroplanes that were in the same hangar would do ditto, the H126 and the FD2 being a couple. But I emphasise this was in no way an issue - just the only thing to talk about so far as landing the 115 was concerned.

Regards

[Covenant]

John Farley

Thanks for all that information. very interesting and very informative.

As an addendum to your comment about New Bloke's photo link: it might not show vortex bursting, but it does nicely show the leading edge vortex detaching from the leading edge pretty close to the wing root. Maybe this was what they were trying to say, but ended up using the wrong terminology?

Obviously not as dramatic an effect on lift either, but it would still reduce the vortex lift below the theoretical maximum. I wonder if this behaviour was considered a problem in the HP 115 design and whether it was improved upon when it came to the design for concorde's wings?

[John Farley]

Covenant

For what it is worth, and we may be bordering on semantics here, I don’t think the picture necessarily shows the separation of the vortex from the top of the wing as I suspect the smoke is just visualising the centre of a quite large diameter and complex vortex.

Again, at the risk of going over old ground too much, the BEA traces show that once the accident aircraft departed from control (and the alpha shot up) the extra lift that was released took it up to 800 feet.

I guess all that says is that the slender planforms do generate enormous lift at high alphas but that it is quite unusable (and so academic) due to stability problems and the associated enormous induced drag. This is really why I dislike the use of the word “stall” when talking of this accident because that word makes most people think of a shortage of lift.

Thanks for your private message BTW

Regards

[Covenant]

John Farley

You may be right. The picture does not necessarily confirm that the vortex has detached from the top of the wing and become a free vortex. Unless I am much mistaken, though, it has drifted inboard significantly from the leading edge, beginning near the wing root, which I believe would start to produce stagnation zones and reverse flow areas - part of, as you say, a large and complex vortex. Not a major problem as far as lift is concerned, but definitely, I would have thought, degrading its Cl below the theoretical maximum.

I do agree with your analysis of the final dramatic altitude gain of the accident flight and very much with your dislike of the term "stall" which I suppose is what prompted my original post to the R&N forum.

Regards,

[Edited for typo]

[ 04 September 2001: Message edited by: Covenant ]

[gaunty]

John Farley, Cosmo, Covenant and others
Thank you for the fascinating and hugely educational thread.

As a boy in dunnunda (now aged 55) I was only able to watch and read from afar about the goings on and wait for the agonsingly long time it took for our ABC to televise the BBC recording of Farnborough. I finally made it to some real ones much later.
Until about 6 moves ago I had every "Observers Book of Aircraft" up to when I think they stopped them or we could no longer get them here. They were a much anticipated birthday present.
I recall vividly watching unblinking throughout them all including the types discussed above. And we didn't have VCRs either.

To be sitting in my office at home calmly/actually excitedly watching the guys who designed and flew them chat away, is the real miracle of the internet.

[Genghis the Engineer]

JF and others...

I've previously met (and sadly helped AAIB investigate) the phenomenon of very high lateral stability (Lv) at high AoA with a delta.

On another track, I recall that the Jaguar (which has a very low B:A, like Concorde) had a real problem with intertia coupling at high AoA causing a divergent roll:yaw oscillation (not a spin). Two gentlemen of my (and I suspect JF's) acquaintance had the dubious pleasure of jumping out of a T2 in the mid 70s for this reason.

But, the excessive Lv and inertia coupling seem to be mutually exclusive - high Lv should damp rolling oscillations, but a large SUSTAINED roll due to beta could be a problem. It sounds to me that this is a phenomenon that should be / have been seen on Concorde and the HP115. Am I right? If I am, then presumably the high alpha protection system is to prevent getting into an unsustainable high drag regime, and ultimately a large pitch-down which is likely to upset the Pax and require a fair number of thousands of feet to recover - but probably isn't related to lateral excursions?

Just idle thoughts really as I try to understand the subject.

G

[WOK]

Thank you, John Farley, for your comments ref the HP115.

Just a little correction to your post regarding lift margins, if I be so bold?

You note that lift was abundant since the a/c shot up to 800' during the departure. That seems extremely unlikely to me - the total kinetic and thrust energy just isn't there to overcome the exponentially increasing drag by that stage, certainly during windshear recoveries on the conc there is as much if not more milking of the total energy required as a conventional widebody, and this on four engines at contingency thrust and at alpha's of 16-18.

I suspect what you have looked at is the rad alt indications caused by the increasing bank angle.

[John Farley]

Hi Genghis

But, the excessive Lv and inertia coupling seem to be mutually exclusive - high Lv should damp rolling oscillations, but a large SUSTAINED roll due to beta could be a problem. It sounds to me that this is a phenomenon that should be / have been seen on Concorde and the HP115. Am I right? If I am, then presumably the high alpha protection system is to prevent getting into an unsustainable high drag regime, and ultimately a large pitch-down which is likely to upset the Pax and require a fair number of thousands of feet to recover - but probably isn't related to lateral excursions?

If you trimmed the HP115 into a S&L high alpha condition (say 80 kts) hands off, it was interesting to then do a little (say left) rudder kick, stay hands and feet off and watch. The initial kick might produce a bank to the left of say 20 deg, at which point the roll rate would be stopped by the transfer of alpha into beta from the left which generated a large roll rate to the right (your point above) The subsequent roll to the right, however, would be a bit bigger than the initial one and so off it went in a divergent dutch roll. The thing stabilized at order of +/- 60 to 70 bank (depending on the initial speed). Trying to stop this with instinctive movements of the stick was a far from certain exercise as it was hard to get the phasing right. So what to do? Answer, stuff the stick forward, dump the alpha and the oscillation stopped at once.

Flown as a run down a display line at 1000ft most ordinary people thought the pilot was demonstrating some sort of incredible aileron response (Jack Henderson did this first at SBAC 1961), whereas the aircraft was hands off.

Clearly such a characteristic is a bind, but I suspect it turned out not to be problem for Concorde because that aircraft’s alpha had to be restricted for a variety of reasons to the range where the DR was less divergent.

There has been an awful lot of analysis over on the other thread of how the accident aircraft finished up rolling/stalling/spinning etc etc. In my view that is all meaningless. When a slender delta is at high alpha virtually any beta will cause it to depart in roll. End of broadcast.

As to the second part of your quote above do you mean pitch-down? I thought the traces showed that it pitched up, rolled over and effectively fell through? All academic – what matters is that it departed at high alpha just as predicted. The game was over then.

Regards

[John Farley]

WOK

Thanks! You may have a point about the rad alt... but the red (baro) and blue (radio) traces are very similar. Perhaps the baro had a big PE effect develop during departure?

Regards

[WOK]

That would be my presumption.

As you note, aside from detail, there is little to discuss about what happened after departure.

Someone mentioned stick-wobbler earlier - this will only operate if the a/c is not in trim in pitch.A rapid increase in alpha to the point where pitch up is uncontainable may occur without the wobbler going off.