Take On Helicopters Page 4

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Flight Model When I first learned that Take On Helicopters was going to be a high-fidelity helicopter simulation, I jumped up and down and giggled like a little school girl. Unfortunately, I was at work and my coworkers still give me funny looks from time to time. As a hardcore simulation fan, my favorite sims include “switchology” titles like DCS: Black SharkOrbiter, and Falcon BMS, all with full realism difficulty settings. I couldn’t wait to add another high-fidelity flight sim to my library. While I admit my anticipation was a bit out of proportion when the pre-release came out, I was crushed by what felt like a very sloppy helicopter flight model. I initially thought I must be playing with the wrong difficulty settings or an incorrect controller setup, but no arrangement of options fixed the behavior I was experiencing. Disappointed and dejected, I relegated my copy of Take On Helicopters to the back of the bookshelf and somehow managed to get on with life. As I mentioned earlier, the announcement of Take On Hinds peaked my attention and when I heard there was a flight model patch in the works, I just had to give it another shot. Although I’m glad I did, the physics package (even in Beta patch 89299) isn’t quite the complete fix I was hoping for. Any quick search for helicopter dynamics will produce a number of sites, like this one, that list basic phenomena that all helicopter pilots or enthusiasts will need to be concerned with. These lists don’t compare fully with Bohemia Interactive’s technology checklist of items included in the sim, but let’s take a look at them.

Autorotation When a helicopter loses engine power, autorotation enables the helo to glide down to a safe landing. This is accomplished by setting a descent rate such that air rises up through the rotor disk, maintaining a safe rotor RPM. Near the ground, the rotor blades are flared, trading RPM for vertical lift, softening the landing. In Take On Helicopters, while it is possible to maintain a glide in a power-off condition and use the collective to flare near the ground, the entire feeling is just off. Changes in cyclic have an exaggerated effect on the descent rate and the rotor disk inertia feels way to heavy for the light class of helicopters. Add in the completely unexpected roll torque coupling with the collective inputs and you have a very unrealistic autorotation. Translational Lift When a helicopter is hovering in stable, stationary air, the rotor disk is operating in its own vortex, reducing lift effectiveness. As the helicopter picks up horizontal airspeed, the rotor disk begins to operate in “clean” air, greatly increasing effectiveness (as much as 40%). In Take On Helicopters, there appears to be a little change in rotor effectiveness between hover and forward flight, but not as much effect as I would expect, especially in the Light helicopter. Transverse Flow Effect (a.k.a., Not Translational Lift) At low airspeeds, only a portion of the rotor disk is biting into clean air while the rest of the disk is working in its own wash. Commonly associated with translational lift, these are, in fact, two different phenomena. In Take On Helicopters, the helicopter shudders from time to time, apparently modeling this effect, but it seems to come and go at the strangest times. Not on the BIS checklist. Retreating Blade Stall Retreating blade stall is when the helicopter’s forward airspeed is so great that the rotor blade headed backwards (i.e., “retreating”) actually has a net airspeed less than its stall speed. The effect results in sudden and significant loss of lift on one side of the rotor disk, resulting in a strong rolling tendency — capable of flipping light helos on their side before the pilot even knows what’s going on. In Take On Helicopters, I was not able to induce this dynamic, even at very high speeds in all helicopter categories, even though the checklist says this is included. Vortex Ring State (a.k.a., Settling With Power) When a helicopter descends too quickly with too little forward… or backward… or sideways… airspeed, the rotor disk is dragged down into its own downwash, seriously degrading the performance. This results in sustained descent rates that cannot be powered out of without other corrective actions. Increasing collective just exacerbates the problem, causing this to be one of the more fatal helicopter dynamics issues in real life. In Take On Helicopters, this critical helicopter safety issue is completely missing from the dynamics model. Dissymmetry of Lift In forward flight, the rotor blades on the half of the rotor disk going forward are generating more lift than those on the half of the rotor disk going backwards. This difference in the production of lift causes a tilting moment on the helicopter. In Take On Helicopters, dissymmetry of lift appears to be incorrectly modeled to the collective input instead of the cyclic. Not on the BIS checklist. Loss of Anti-Torque Effectiveness Just like their big brothers, tail rotors need clean and stable air to produce torque. At low airspeed with wind, especially around wind-deflecting obstacles like buildings, it is possible to enter a condition where the tail rotor experiences the horizontal equivalent of the vortex ring state and becomes ineffective, allowing the helicopter to swing wildly to one side. It’s even possible for the spin rate to reach a point where its no longer possible to recover. In Take On Helicopters, this effect does not appear to be modeled. Not on the BIS checklist. Ground Effect Rotary-winged aircraft experience ground-effect just like their fixed-wing brethren, resulting in more effective lift production near the ground than at higher altitudes. This is due to the suppression of wing tip (or rotor tip) vortices and a reduction in the associated induced drag. Effectively, this means the closer to the ground, the less collective is required to maintain a hover. In Take On Helicopters, ground effect went from really bad in 1.00 to an almost negative effect, where the closer you get to the ground, the faster it sucks you in. There appears to be larger random forces acting on the helicopter while in ground effect that may be an attempt to model turbulence. Whatever the rationale, helos in ground effect (IGE) don’t fly as expected. Not on the BIS checklist. Torque Newton’s third law tells us that when we’re up the air and trying to spin a disk of giant rotor blades through the air above our heads, the reaction force is going to try to spin us around in the opposite direction. Without this blasted law, helicopter designers could get rid of the cumbersome, ungainly, dangerous, and engine-power sapping tail rotors from all helicopters, so I urge you to write your congressmen / congresswomen and senators / senatorettes demanding that they take action! In the meantime, real-life pilots have to balance anti-torque inputs with changes in power or collective setting to maintain a steady heading. In Take On Helicopters, torque effects due to changes in collective inputs seem to be modeled, but are a bit off. Without real helicopter analogues to compare with it’s difficult to quantify the difference, but torque effects seem largely minimized. Not on the BIS checklist. Low RPM Rotor Stall When the rotor RPM drops, the airspeed of the rotor blades drops and, just like any lift-producing airfoil, if the airspeed is too low the blades will stall. In a fixed wing, you push the elevator down, drop the nose, and pick up airspeed. In a helicopter, the rotor blades are your control surfaces, and once they stall there’s no way to recover, so the helicopter will plummet all the way to the ground. Since we are immortal in flight sims, I attempted to stall my rotors out every which way I could, and failed miserably. Even when I shut the engine off and held full collective, I was able to maintain flight control. The lack of this dynamic and the omission of manual throttle controls remove one of the more significant challenges facing helicopter pilots. Not on the BIS checklist.

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