Useful But Simple Modifications To Aircraft.Cfg
Entry posted by xxmikexx
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I told webmaster Nels Anderson yesterday that I would like to write a series of short How To ... articles regarding making simple changes to Aircraft.Cfg for the purpose of overcoming common nuisances. He likes the idea because he would like to see more of the technical kind of How To ... articles.
But how did I come to be able to make such changes? After all, the common perception is that when it comes to FDE, only experts can make such changes because "Everything interacts with everything else."
Yes and no.
Yes, to a certain extent everything does interact with everything else, see below. But no, for many common problems and annoyances you don't have to be an FDE expert. All you really need is an understanding of basic arerodynamics, which will be covered by the first article in the series.
In advance of the article I'll give a short summary and a simple example ...
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You will be way ahead of the game if you understand that, in straight and level flight at constant airspeed, lift must precisely equal the weight of the aircraft, and engine thrust must precisely equal the sum of all the drag forces acting on the aircraft.
If lift didn't equal weight the aircraft would either climb or descend. If thrust didn't equal total drag the aircraft would either speed up or slow down. Common sense, nothing more. No requirement for wearing a beanie with a twirly thing on top.
Now let's talk about drag. It isn't immediately obvious, but when you think about it there must be basically two kinds of drag actiing on the aircraft. The first kind is obvious -- so-called "form drag".
Form drag results simply from the aircraft moving forward through the air. The aircraft has to force the air aside as it flies. The higher the airspeed the more air needs to be shoved aside per unit time -- and therefore the greater the retarding force. Thus form drag clearly must depend on airspeed -- very high at Mach 3, nearly zero at taxi speeds. Again, common sense.
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The other kind of drag is "induced drag". As you already know, to generate lift the wing must maintain a non-zero and positive angle of attack. Remember sticking your hand out the car window when you were a kid? Edge on and there was relatively little drag. But when you tilted your hand it would climb -- and be forced back.
The rearward acting force is induced drag. It comes about because the airfoil has a positive angle of attack when it is generating lift -- and this positive angle of attack causes the lift vector to be tilted away from the vertical. The result is that the vertical component generates the lift while the rearward component generates the induced drag. You can't have the one without the other.
Now ... If you think about it, to maintain constant lift a fast-flying aircraft will need a shallower angle of attack than it would at low airspeed. As the angle of attack becomes lower with increasing airspeed, the reardward component of the lift vector becomes smaller. That's right -- as airspeed increases, form drag decreases, and conversely.
And now for the part that is a bit less obvious but just as easy to understand ...
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Clearly the total drag on the aircraft must be the sum of form drag and induced drag. Again, common sense. But what isn't immediately obvious is that if you were to make a graph of total drag versus airspeed you would find it to be shaped like a U or a bathtub -- it would have a minimum value at some particular airspeed.
And now you will just have to take my word on something. (If you don't want to take my word on it you will have to study fluid dynamics.)
At faster than that minimum airspeed, the form drag component of total drag will increase somewhat rapidly at the same time that induced drag is gradually decreasing. At slower than that minimum airspeed, induced drag will increase somewhat rapidly while form drag slowly decreases. Therfore total drag will be higher on BOTH sides of the minimum drag airspeed, and this is why the graph of total drag versus airspeed is bathtub-shaped.
And that's almost everything we need to know about aerodynamics to be able to solve problems like the following ...
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When I inherited the Erick Cantu 727-200 airframe with the Charles Fox FDE I found that the aircraft, when slowed at cruising altitude from .81M to .60M in preparation for descent, was taking about 10 nm to slow. I happen to know that real 727s are "slippery" and require more nearly 20 nm to slow than 10 nm. So the question on the floor becomes, How do you make the aircraft take more distance to slow down?
Well, in principle you could artificially increase engine flight idle thrust, somewhat lessening the effect of total drag on the slowdown process. There is a parameter in aircraft.cfg that would allow you to do exactly this. Trouble is, it would increase thrust in all other situations too. ("Everything interacts with everything else.")
So we're not going to do that. Instead we're going to think about the problem ... ... ??? ... ... ... ?!? ... ... !!!.
Go to the head of the class if you said that we need to decrease the form drag. This will allow us to leave the engine thrust parameter alone, and it will have little effect on the low-speed handling characteristics of the aircraft, where the drag forces are dominated by induced drag rather than form drag.
So if we make a modest reduction in form drag, something like five or ten percent, we should find the aircraft slowdown distance increasing ... And so it does.
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But now we have created another common sense problem. The lower form drag means that at cruising airspeed the aircraft will require less thrust to maintain airspeed. Less thrust means lower fuel consumption, and it also means an artificially lower reading of the fuel flow gauge.
But those are the only important interactions. It isn't EVERYthing that is interacting with EVERYthing here, it's form drag interacting with thrust and therfore fuel consumption.
The situation is actually very simple when you think about it, and all that we needed was an understanding of basic aerodynamics in order to see which aircraft.cfg parameters need to be modified ...
form drag
engine thrust
fuel flow
So that's what these articles will be about. Not about how to achieve the utmost in realism in all parameter behaviors in all flight regimes, but instead HOW TO MAKE THE SIMULATED AIRCRAFT DO WHAT YOU WANT IT TO DO.
Once more, common sense prevails. It's not black magic and we don't have to be beanie-wearing propellerheads to make the right things happen.
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Gee ... I guess I just wrote that first article, didn't I? :D That's fine, when I want to release it I'll know exactly where to find it, and in the meantime almost nobody will have read it, except perhaps for my good friend skylab.
Edited by xxmikexx
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