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Torque and Velocity vs. MRE - Helicopter Airfiles

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Percent Torque at Takeoff and Cruise Velocity as Functions of Main Rotor Effectiveness Numbers One and Two for the Bell 206B in Flight Simulator 2002

By
Matthew A. Isham
19 September 2004


INTRODUCTION

There are a number of third-party aircraft for Flight Simulator packaged as military and high-performance helicopters available on the Web. With high-performance comes the need for greater power and speed in addition to responsive control. Aerodynamics and other flight characteristics of these models are based largely upon those provided for the default Bell 206B Jetranger III. To enrich our experience with Flight Simulator we might, therefore, take a closer look at some of the ways the various aerodynamics and other parameters listed and available in the Bell 206B’s .air and aircraft.cfg files affect its performance.

One measure of aerodynamic performance in the simulated Bell 206B is percent torque required at takeoff for a given aircraft weight. A measure of speed is cruise velocity in straight and level flight for a given aircraft weight. Jordan Moore of Hovercontrol (www.hovercontrol.com) has shown that greater speed can be achieved by reducing the frontal surface area, listed as an entry in the .air file, and by reducing the parasite drag scalar, as an entry available in the aircraft.cfg file.

At least two other parameters affect performance in such ways. These are designated Main Rotor Effectiveness #1 and Main Rotor Effectiveness 2 (hereafter MRE #1 and MRE #2 respectively) in the .air file. They are found under record #1402-72, and 1402-80. Steve Baugh did the foundational work on these and other parameters, recording his observations in his groundbreaking “1400-1404 Technical Notes.” The default value for MRE #1 is 0.75, suggesting that it is a scaling factor which ranges between zero and one. The default value for MRE #2 is 2.5, suggesting that it can take other non-negative real number values. This report presents the results of varying the values of MRE #1 and MRE #2, measuring the results as percent torque required at takeoff, and cruise velocity for each pair of values selected. Results in this report are preliminary in that only one or two data have been collected at each set of values. However, reasonable confidence is given to the assumption that further data will not change the results substantially, but only smooth the curves.

METHOD

Since two parameters are identified as variables where results are to be presented in graphical form, one axis was designated MRE #1, another MRE #2, with the results plotted on a third axis. On one axis, MRE #1 was assigned values of 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, and 0.95 in ascending order. Likewise, MRE #2 was assigned values of 1.4, 1.45, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 in ascending order on another axis. Data at each set of values were recorded in the respective cells. Anecdotally, values less than about 0.5 for MRE #1, and 1.4 for MRE #2 did not produce results in that the aircraft did not take off though the collective was raised to its maximum. For MRE #2, values greater than about 5.0 produced appreciable pitch instability. Microsoft Excel 2002 (10.6501.6714) SP3 was used to plot the results. The .air file was edited using William Roth’s “AirEd” version 1.0.5.2.

The simulated aircraft used was the default Bell 206B Jetranger III for FS 2002. Aircraft empty weight was 1760 lb. At each set of values the test began at the simulated Edwards AFB, at an indicated outside air temperature (IOAT) of 50*F and a barometric pressure of 29.92 in. Hg, and consisted of two parts. The first part was measurement of percent torque required to take off. The second part was measurement of velocity in straight and level flight.

To measure percent torque at takeoff the collective was raised smoothly and continuously, at a rate of about one percent torque per second, until the aircraft just began to take off. The collective was then lowered to its minimum and the aircraft allowed to settle. The test flight commenced directly upon this procedure for measurement of percent torque. Navigation frequencies were set and the test flight departed Edwards AFB on a route to Palmdale, CA. Test altitude was 3000 ft. ASL. Altitude was achieved at a vertical speed of about 500 ft./min. Measurement of velocity was recorded as soon as straight and level flight was achieved with the collective set to 85% torque with the aircraft at 3000 ft. +/- 100 ft.

At typical settings for display quality, aircraft performance depends substantially on the capacity of the PC on which the software is loaded. The computer with which this experiment was performed is an E-machines T2796 with a 2.7 GHz main processor. Installed are 2 X 512 megabytes of DDR and an ATI Radeon 9200 PCI graphics card with 128 MB DDR. The software platform is Windows XP Home Edition Service Pack 2. The frame rate achieved for this experiment was between 28 and 30.

RESULTS AND DISCUSSION

Figures 1 and 2 show contour plots of percent torque at takeoff and velocity in straight and level flight, respectively. The plots are similar in appearance, as one might expect. Both show a plateau in their lower left corners. In Figure 1, this plateau depicts axis values at which the aircraft did not take off though the collective was raised to maximum. Similarly, the plateau in Figure 2 depicts those values at which velocity is zero. The contour lines in both figures are, for the most part, negative in slope and bowed toward the lower left corner. This indicates that MRE #1 and MRE #2 are inversely proportional to one another. As one increases, the other decreases to achieve either the same torque at takeoff, or the same velocity. That the contour lines are nearly vertical is significant. This indicates that MRE #1 contributes less, overall, to both torque at takeoff and velocity in straight and level flight than does MRE #2. A lesser change in MRE #2 effects a greater change in resulting torque or velocity. That the contour lines are somewhat bowed toward the lower left corner indicates that the relative contribution of MRE #1 increases slightly at its lower values regardless of the value of MRE #2.

The plateau in Figure 2 appears to be surrounded by a much steeper cliff, as it were, than the plateau in Figure 1. While all other contour intervals are relatively broad, the several very near the default settings for MRE #1 and MRE #2 are narrow and closely packed. This is an artifact of the plotting software. The scale chosen to show a sufficient number of contour intervals forces the software to discriminate unnecessarily among speeds not achieved experimentally. Whereas the software divides the range of velocities from zero to 135 KIAS into equal parts, no velocity less than 81 KIAS, with the exception of zero KIAS, was achieved. As these closely spaced contours can be considered part of the zero-velocity plateau, they indicate MRE #1’s proportionally greater contribution to forward velocity at lower values of MRE #2. This would indicate that, in this region, it is translational lift, in the main, which allows the aircraft to achieve altitude and forward velocity.


Figure 1. Percent torque required at takeoff. Lower left region depicts axis values at which the aircraft did not take off.


Figure 2. Forward velocity in straight and level flight.



CONCLUSIONS

The aerodynamics of the Bell 206B result from a complex interaction of parameters and their values in the .air file and aircraft.cfg file. This report further clarifies the effects of Main Rotor Effectiveness #1 and Main Rotor Effectiveness #2 on two measures of performance over a range of values. It lends itself to the conclusion that, while both MRE #1 and MRE #2 are necessary components of the .air file, MRE #2 is a major contributor to lift and forward velocity.

While MRE #2 is certain to contribute to forward motion, its majority contribution to lift suggests that it is an adjustment for output from the collective. Anecdotally, the set of values in the upper left corner of the plot (MRE #1=.95 and MRE #2=1.4) while allowing the aircraft to just lift off, did not lend itself to a gain in altitude, at 85% torque, without the benefit of translational lift. Conversely, the set of values in the lower right (MRE #1=.5 and MRE #2=5.0) allowed the aircraft to climb in hover to over 6000 ft. It would have climbed higher in hover had the experimenter not terminated the flight. This is consistent with Baugh’s observations regarding MRE #2. In his experience, higher values of MRE #2 cause the aircraft to leap from the ground and climb. Moreover, increases in velocity at higher values of MRE #2 come by way of exaggerated forward cyclic to maintain level flight resulting in appreciable pitch instability, likely due to drag on top surfaces of the aircraft. That MRE #1 contributes in only small part to lift, but where it contributes, it contributes to horizontal motion suggests that it is an adjustment for output from the cyclic. At low values of MRE #2, MRE #1 appears to contribute at least equally to velocity. Baugh also observed that MRE #1 contributes to speed of the aircraft. Intuitively, this might be expected of adjustment to output from the cyclic. Since these parameters are included under the general category of “Helicopter Main Rotor” it is possible that they refer to configuration of the more specific swashplate assembly.

Although adjustments to collective and cyclic outputs do not influence forward velocity to the extent that Moore’s approach does, they contribute significantly to it. Since the aerodynamics of the simulated Bell 206B result from several, if not many, interrelated parameters, it is possible that combining several approaches to aircraft performance can produce a more enjoyable experience for the flight simulation community.






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