KEEPING IN TRIM WITH FS2  PART III by Ben Campbell  ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| In the last two articles I described the balancing forces and moments acting on an aircraft in flight. By changing the nose up/down attitude, airspeed can be controlled and, by using varying power settings, we could control the rate of climb and decent of our aircraft. This time we'll go a little further to see the effect of retracting the undercarriage (wheels), and discover why reduction of aerodynamic "Drag" is some important. So, lets get down to business. I'll assume that you can now get safely off the deck and establish a steady climb at 70 Kts (say at 2000 RPM). This time we'll change our view so we can see see exactly what happens to our aircraft. After you've climbed to about 500 ft, tap the "s" key - this will give you a "Spot Plane" view. Next use your mouse to click on the "View" Menu and select "Set Spot Plane". Notice that the default altitude of the Spot Plane is 20 ft (ie the Spot Plane will fly 20 ft above the aircraft we are flying). Click the "down arrow" box twice to bring the "Spot Plane" level with our aircraft and then click the Close window gadget. At this point you can also try using the F10 & F9 keys to zoom in & back towards our aircraft. Now try slowly increasing and reducing power and watch the reaction of our aircraft. Finally take the power back to 2OOO RPM and if we're correctly trimmed for 70 Kt, we should be climbing at about 250 ft/min. Next tap the 'U" key. Notice how the wheels disappear from our aircraft and also the movement on the undercarriage indicator on the instrument panel. Now check the airspeed. Initially it increased (the aircraft accelerated because the Drag has been reduced significantly and the available Thrust far exceeds the forces that are acting in the opposite direction (primarily Drag). Allow things to settle down and we'll see that our aircraft settles back at 70 Kts but with a far greater Rate of Climb. (Thus the energy formerly being consumed by the Drag of the Undercarriage is now being used for generating additional Lift.) Without touching the throttle try lowering the undercarriage by tapping the "U" key once more. Notice how the airspeed "washes off" (our aircraft decelerates), then the nose lowers and finally we settle back to our original set up. There's plenty of food for thought in what we are observing, but I'll get you to try one more thing before we analyse all that has occurred. With the undercarriage down and flying at 70 Kts adjust the power so that the aircraft is neither climbing nor descending. Next, retract the undercarriage and then apply progressive forward trim so that the aircraft continues to neither climb nor decend. (This little exercise will really keep you on your toes.) It's a bit like trying to keep a baloon underwater - just as you get things steady the nose starts to rise again. This somewhat frustrating state of affairs continues to occur for quite some time. Finally there comes a point where stability is achieved and you'll notice that the aircraft is doing about 130 Kt.  What can we deduce from all this? First and foremost is the significant increase in Airspeed that is achieved for the same power setting. (With the Undercarriage lowered we couldn't get much past 100Kt even at full Power). Obviously this has significant implications in terms of economy, increased performance and energy saving. (The same goes for your family car too.) Any reduction in Aerodynamic Drag means either greater speed for the same applied power or, less Power (& fuel) to maintain a given speed. Well all this might seem pretty obvious, but what about the way the aircraft kept going into a climb until 130 Kt had been achieved? What was happening to cause this? In my first article (MD27) I described the four forces acting on an aircraft in flight (Lift, Weight, Thrust & Drag) and how these Forces balance out. I also mentioned Bending Moments or Couples (which result from Forces acting through different lines of action) and how, in straight & level flight, these Bending Moments are balanced. (Refer FS2 Fig 1). Note that the magnitude of each force is indicated by the length of the Force Line. As mentioned in my earlier article, the forces of Lift & Weight are of far greater magnitude than Thrust & Drag. If we had sufficient Thrust, say in the case of an FA-18, we could climb vertically because the magnitude of the Thrust can equal or exceed the Weight. Such is not the case in a humble Cessna 182 hence the need for judicious Energy management. So what happens when we give our Cessna 182 full throttle? Well firstly Thrust is increased. This causes a nett imbalance of forces and our aircraft accelerates. Drag is proportional to Velocity squared, so as our Velocity increases Drag also increases - at an exponential rate. The result is an ever increasing Bending Moment, tending to lift the Aircraft nose up. And how do we control this upwards Bending Moment to hold our aircraft straight & level? Simple really, by using the Elevators on the tail to produce an opposing bending moment. Maintaining a desired attitude is referred as "Trimming" the aircraft.. and we need to apply progressive "Nose Down" Trim until the Drag Force finally reaches its maximum .. at max airspeed. At maximum airspeed, equilibrium between Thrust & Drag is reached and the bending moment resulting from Thrust & Drag will then remain constant. Hence the opposing bending moments provided by the action of Lift & Weight and Elevator Trim will no longer require adjustment.  Raising and lowering the Undercarriage Having inwardly digested the above, we can now consider the action of raising & lowering the undercarriage. Yes you guessed it.. another Bending Moment that tends to make our Cessna "Nose Down" when the undercarriage is lowered and "Nose Up" when the gear is retracted. By dint of good design, the aerodynamics of our Cessna are such that on retracting the gear the "Nose Up" results in additional Rate of Climb - and the same airspeed is maintained at this increased Climb Rate. All by itself our Cessna finds a new equilibrium. Conversely on lowering the landing gear our aircraft lowers its nose to an equilibrium point - again at roughly the same airspeed as before. It's beautiful to behold ..and we can only marvel at this wonder of Applied Mechanics where critical design parameters are calculated and configured to make our aircraft behave in a safe and forgiving manner. It also gives one a new found respect for pioneer avaitors who had to contend with less forgiving machines that were prone to result in an early demise from a moment's inattention at the wrong time. Well that's about it for this session. In my next article we'll look at the effect of airspeed in achieving our best Glide Ratio?? This could be critical if we have an engine failure and need to pull out every trick in the book to make it to a safe landing place. Until then happy landings!  ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 |||