Old 14th Jun 2010, 03:24
  #25 (permalink)  
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Giuones and others,

I seem to have trouble explaining myself and getting my points across. Since my career is partly based on explaining myself and getting my points across, I find this situation somewhat puzzling. I have met it before, often, on PPRuNe, which is partly why I no longer contribute on a regular basis.

I have tried before the strategy of writing long, pedantic pieces to try to get my points across. It doesn't always work, but let me try it just once here. If this doesn't work, I'll just stop, for I don't know any other ways of trying.

s bakmeijer was talking about recovery from stall in an MD-80 series jet. Now, I don't know anyone who worked on the MD-80 wing, but I do believe I can guess how they thought and worked in general terms. I do work on a regular basis with an aerodynamicist who was responsible for the wings on some other well-known commercial jets. And I am not yet sure that everyone here is thinking the issue through. So, at the risk of being pedantic, let me tell a story which I think is very likely to be true of, say, the MD-80-series wing design process.

There you are, designing a wing, looking at the wind tunnel experiments, and you notice that, in particular circumstances, when the airstream separates from the upper surface and the lift drops off (it may do so suddenly or gradually; I don't know the story for the MD-80-series), the turbulent wash blanks out the tail (or flows over the place where the tail should be; it may be that your wind tunnel model doesn't have a tail attached).

Now, you and everyone else knows that you cannot certify the aircraft if this is allowed to happen in real life, for it is a certification requirement that it not do so.

So you put in a stick pusher. When you do so, you have to choose at what point you design it to activate. Do you design it to activate at the point at which the airstream separates? Well, that would be unwise, indeed useless, because at that point, as you have seen in the wind tunnel, the tail is blanked, and that is exactly what you are trying to avoid. You design it to activate *just before* that point, so that the separation does not occur, the tail is not in danger of being blanked, and you and your DI can go home and sleep comfortably.

Mostly, lift increases up to the range at which that potentially-blanking airflow separation occurs. You are triggering the stick pusher just before it gets to that point, so you are not triggering it "at" CL_max in physical terms, but just before that point.

And maybe you tell your certification agency: we aerodynamicists define the "point of stall" to be the point at which we trigger the pusher. Because there is some leeway allowed to you to do so if you wish. The stall phenomenon is actually a range of aerodynamic behavior over a range of airspeed and you can set the point for definitional purposes more or less wherever you choose to do so in this range.

Years later, some pilots, say, MD-80 pilots, say that stick pusher activates "at the stall" and call what happens "stall recovery", because that is what everyone else in their community says. So be it. But if you want to talk about aerodynamic phenomena, as I often do, this phraseology does not accurately represent what is going on.

Guiones, I know that flight tests were done and sims data was validated for this whole endeavor. But I believe that it misdescribes what was done to say that the airplane was stalled and recovered and the sims were modified to reflect that behavior. I think you will find that the airplane was taken up to the point at which unpleasant handling characteristics started to occur (maybe up to the engineering-department-defined "point of stall" - which may or may not be at the point of maximum lift, depending on your airplane - and maybe to just before, but not over that point, since going over it is possibly dangerous and people who do these tests generally do not want to kill themselves, and their bosses don't want them to do it either). And that simulator behavior was validated up to that point. But sim behavior may not be trusted after this break, for the simple reason that nobody has flown the airplane after this break to know what it really does then.

And you can ask an aerodynamicist responsible for a specific wing, about behavior post-stall: "what happens here?" And, depending on what you are asking about, the answer may very well be "I don't think anybody knows". I know this because it happens to me a lot.

Can we agree now? I am not going to belabor it further, because if this way of putting things does not suffice to explain where I am coming from, I don't know what I can say that will.

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