Sunday, October 16, 2011

Falcon 4.0 BMS 4 - Refueling - Memo from the Flight Control Computer: All Viper Flight Control Surfaces Are Belong to Us

I refueled the Falcon ... Once ... In 2005.

I remember it as one of the most frustrating experiences in simulated flight.

As if the whole ordeal of getting fuel dashing at 300 knots, 20,000 feet above the ground in a simulation where you get no sense of speed or acceleration was not hard enough, I also remember getting almost obsessive with getting it done. So back then I decided not to do it again.

Now its 2011 and the BMS mod has this great flight model (no more flying on rails) that makes the sim a joy. It's like I want to try everything again. The "commit" button was there in the mission menu ... Calling me. Click. Damn! A week went by just practicing (and obsessing about) refueling.

After a few quick and dirty trials, I got expelled from mid-air refueling school. Have to slow down to the tanker's airspeed, I heard. Sounds reasonable.

So OK, I'm flying level into the tanker and I need to slow down. How do you control the airspeed of a Viper? Throttle only, baby!

I know you are going to blast me about this, but the way I adjust my airspeed in other aircraft  is by adjusting the angle of attack with the elevator and then adjusting the throttle for level flight. It can be done differently: adjust throttle and then apply elevator to raise or lower the nose for level flight. In any case, a regular aircraft with a well-trimmed elevator for a given angle of attack will fly straight and level if your throttle is set correctly. Moving the throttle from the position that works for level flight will result in a climb or a descent.

In regular aircraft, adjusting the flight control surfaces for flying straight and level at a given airspeed is a throttle and elevator affair.

But, oh well, the Viper is no regular aircraft. It has its center of gravity off the center of lift, so it's intrinsically unstable. Which is a great thing for dodging missiles and getting into the 6 of a MIG. But it is so unstable that it needs a flight control computer to keep the aircraft from flying like a kite without a tail. So when you apply pressure to the the stick, the flight computer interprets what you want to do and adjusts the surfaces of the Viper so it ends with a flight regime that is mostly what you shook into the joystick. Yes, the computer knows better.

And hell, this flight control computer is damn good at keeping the Viper flying towards wherever you point the flight path marker. Every time you let the stick go, the flight computer leaves the aircraft trimmed for that last input you made. Furthermore, if the airspeed changes, it will modify that trim all by itself and without your input, to keep the aircraft moving in the flight path you aimed for with the last joystick input.

So, trying to trim the Viper in the conventional way you do in a regular aircraft is an impossible premise. Flight control surfaces change without your input. Once you get into a given flight regime in the Viper and you want to transiently change it for a few seconds, any joystick input will erase the flight control computer's memory for that previous state.

Only thing the flight control computer has no saying on? You guessed, the throttle!

So here is a little experiment from the whole week of fun flying that I had. I start flying straight and level at 300 knots.

300 knots, straight and level. The horizon is the big horizontal yellow line. Note the flight path marker (circular symbol with three lines, resembling the shape of an aircraft seen from behind) and the gun cross (the cross just below the 5 degree pitch ladder). The angular difference between the flight path marker and the gun cross is the angle of attack. In this case is around 2.9 degrees.
Then I move the throttle forward and I increase the airspeed to 400 knots. No joystick input.

400 knots now. Note how the angle of attack (angular difference between the flight path marker and the gun cross) has decreased. 
At 400 knots the aircraft started a very shallow climb. In this case the rate of climb was around 500 fpm. The flight control computer apparently failed to keep the aircraft flying level. But, hey! How many other aircraft you know where you can increase your airspeed 100 knots and not starting a wild climb?

Then I moved the throttle back and aimed for 300 knots again. Absolutely no joystick input.

Back to 300 knots. 
Still climbing at 500 fpm, the same climb rate I had when I was at 400 knots.

Next, I move the throttle back and aim for 250 knots. Again, no joystick input.

250 knots.
Now with the airspeed reduced from 300 to 250 knots, the Viper didn't start a descent. Indeed, it was still moderately climbing. The rate of climb was 100 fpm, almost nothing.

All the above experiments done without any joystick input for the purposes of exploring the system. Off course during a mid air refueling you can adjust your climb or descent with the joystick.

In closing, much air has gone through the virtual wings of my Viper this week and I am slowly making peace with the flight control computer.


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