Summary

On June 17, 2005, the prototype CarterCopter Technology Demonstrator (CCTD) achieved a Mu of 1 at a flight speed of 170 mph and a rotor rpm of 107. The flight was stable and extremely smooth, and the pilots reported there were no vibrational indicators that they were even in a rotary-wing aircraft. The following graphs were produced from data taken during that flight. The graphs compare the Lift to Drag ratio (L/D) vs. airspeed of the Carter demonstrator to the most efficient helicopters and several of the most efficient and popular fixed-wing kit aircraft.

Raw flight test data was provided to the Army. The Army performed its own data analysis and comparison to other rotorcraft, and it is from that analysis that much of the rotorcraft data presented here was taken. The Georgia Institute of Technology provided the S-92 performance curve, and Carter provided the CCTD Projected Performance curve.

At first glance the Carter L/D curves might appear to be poor, but at high speed cruise, the L/D value is similar to fixed wing aircraft and much better than pure helicopters. With the addition of a rotor tilting mast, the CarterCopter could greatly improve drag at low speeds, producing an L/D curve similar to that of the fixed-wing aircraft. The tilting mast, as explained in more detail later in the report, would allow the wing to operate at its best L/D angle of attack from about 30 mph through cruise, rather than in the stall to deep stall that was the case with the CCTD during these tests. Not only will this change the wing drag significantly, but it will also greatly reduce the air separation drag over the rear aft section of the fuselage. The drag due to air separation is so great that at the point where the air separates, the fuselage/wing could be terminated with a flat plate and not significantly change the drag.

CCTD Performance Compared to Other Aircraft

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• The CCTD Projected Performance curves were calculated based on extrapolating the measured CCTD L/D and L/DE curves out to 200 mph, determining the drag coefficients required to achieve that performance, and then using those coefficients throughout the speed range, as would be possible with a tilting mast like that planned for future Carter aircraft.
• Low-speed L/D and L/DE were adversely affected by the huge drag caused by deep wing stall & fuselage flow separation, which will be corrected in future aircraft with a tilting mast.
• L/D values for fixed-wing aircraft derived from data in flight test reports from the EAA Comparative Aircraft Flight Efficiency Foundation
• Airspeeds of Rotorcraft and Fixed-Wing Aircraft Corrected to 4000' Density Altitude
• CCTD flight data is plotted by rotor rpm (ranges of 25 rpm)

Method of Taking Measurements

The CCTD is equipped with many sensors to monitor flight data and aircraft parameters. These sensors feed data into a small network of computers, which monitor the parameters, provide automatic control for some systems of the aircraft, display certain parameters in the cockpit, and transmit parameters via microwave to a receiving radio on the ground. The receiver sends the data to a PC computer, running a special program to display the parameters in real-time on an electronic strip chart, as well as writing the data to the hard disk for review at a later date.

Thrust was measured by having the propeller thrust bearing push against a doughnut shaped Teflon piston machined to have exactly 10 square inches of cross section and the use of an instrument quality pressure transducer to read the pressure. Climb rate and altitude were taken from calibrated instruments provided by the U.S. Army. True airspeed was measured with a propeller type anemometer designed by Carter and fabricated using CNC tooling. Torque was measured by mounting the engine on a bracket with a pivot on one end and a hydraulic cylinder with a pressure transducer on the other. Knowing the distance from the pivot to the cylinder, the torque was determined by measuring the pressure in the cylinder. RPM was measured with a magnetic digital pulse generator (4 pulses per revolution), and, used in conjunction with the torque reading, was used to calculate horsepower.

Calculations of Aircraft Performance

Lift to Drag ratio was calculated, using Thrust, Airspeed, Climb Rate, and a measured Test Weight of 3800 lbs which included 30 gallons of fuel. Lift was assumed to be equal to the weight of the aircraft. Drag was determined by looking at the power produced by the propeller (thrust x airspeed), and calculating how much was going into the rate of climb (or being provided by a descent), and how much was going into countering drag.

Because of the difference in operation between airplanes and helicopters, there is no readily available Lift to Drag data for helicopters. Discussions with a former Bell engineer revealed that one standard method of calculating L/D in the helicopter industry is to use the horsepower required, and to calculate an assumed drag from that as though all of the horsepower were going into thrust to counter the drag. Although this does not give the true L/D, it does provide a useful figure to compare the performance of different rotorcraft, known as the Effective Lift to Drag, L/DE. To compare the performance of helicopters against the CCTD, Carter used horsepower vs. airspeed data provided by the Georgia Institute of Technology. The effective drag was calculated simply as:

To reduce the data scatter and still provide a meaningful number of data points, the data was filtered so that only data points from relatively smooth, unaccelerated flight were used. Carter looked at the period 3 seconds prior to the selected data point, with the limitations being that airspeed could not be changing more than one mile per hour per second, that ROC could not be changing more than 33 fpm/sec, that rotor rpm could not be changing more than 1 rpm per second, and that the rate of climb or sink rate at that instant could not be greater than +/-110 fpm.

Calculations of Projected Performance Without Flow Separation

It was determined that wing stall and flow separation were adversely affecting aircraft performance at low speeds, as will be discussed below. Carter had already planned for a tilting mast to be used on future aircraft, which at low speeds would allow the wing to be held at its optimum angle of attack, keeping it out of a stall. To see how the CCTD would have performed with the inclusion of such a device, a projected performance was calculated for the CCTD. The inputs to the performance calculation were extrapolated to match a projected L/D of 7.0 at 200 mph. This appears to be a conservative point to use to baseline the performance projection, as the slope of the measured data does not show any tendency towards decreasing at 170 mph, indicating that the L/D would continue to increase, at least for a little while, with a further increase in airspeed.

The "helicopter method" of calculating L/DE discussed above was also applied to the Projected Performance calculation of the CCTD. As described above, the inputs to the spreadsheet were extrapolated to match the projected L/DE value of the CCTD at 200 mph. The exact same values for the inputs were used for the "helicopter method" of calculating L/DE as were used in the conventional method. The spreadsheet calculated the horsepower required at the different airspeeds, and then, based off of the propeller efficiencies calculated for different airspeeds, the required engine horsepower was calculated. From this horsepower, L/DE was calculated with the "helicopter method" as described above.

Discussion

Once the L/D (and L/DE) of the CCTD was calculated and plotted, it exhibited an atypical curve. L/D increased as airspeed increased, from 100 mph up to the maximum speed of 170 mph achieved during this flight-test. The slope of the L/D curve at the point of max speed achieved does not show any tendency towards decreasing, indicating that the L/D should continue to increase, at least for a little while, with a further increase in airspeed. This L/D curve is very different from both fixed and rotary-wing aircraft, also shown on the graph. There are basically two reasons for this difference. First, the rotor rpm was decreasing with increasing airspeed as can be seen in the graph above. This principle of decreased drag with slowed rotor rpm is the main reason for investigating the slowed-rotor/compound concept, so this effect was expected.

The main reason for the improved L/D (reduced drag) with increased airspeed was due to reduced flow separation on the wings and over the aft portion of the fuselage. In order for the rotor to provide the required lift at the lower airspeeds, its aft tilt angle forced the fuselage to be tilted up until the rotor lift essentially passed through the aircraft CG. This aircraft pitch up attitude caused the wing to operate in a deep stall, very high drag condition at the lower speeds, and not to come out of the stall until the aircraft reached higher airspeeds. As mentioned earlier, the L/D was still increasing (drag decreasing) as the speed reached 170 mph. At 170 mph the average wing lift coefficient (CL) was about .83, assuming the rotor was producing 300 lbs of lift, and should have been operating near its best L/D. However, the drag still decreased as airspeed increased due to better airflow on the fuselage. Video of tuft tests run on earlier flights show that even near 170 mph, the airflow was still separated at the wing root and over the rear aft portion of the fuselage; however it was clear that the separated area over the wing and aft section of the fuselage decreased as the speed increased. Because the fuselage width starts to decrease at the same location that the wing thickness starts to decrease, this caused the air flowing over both the wing and aft section of the fuselage to separate when they might not have, otherwise. Separated airflow produces very high drag, so for the drag at 170 mph to decrease as speed increased, means that there was still significant drag due to separated airflow and that it was reducing faster than the profile drag on the rest of the aircraft was increasing.

Conclusions

The calculated cruise L/D of the CCTD demonstrates the promise of this aircraft design. The drag problems observed during the testing can be solved in future aircraft with 1) the addition of a tilting mast, which will allow the fuselage and wing angle of attack to be independent of the rotor angle; i.e. from about 30 mph up until the airspeed where the rotor lift is substantially reduced and the rotor is producing approximately 20 % of total lift (tilting the tall mast can keep the wing operating at its best L/D angle of attack through the low and mid speed ranges) and 2) with the use of computational fluid dynamics analysis software, which Carter currently possesses, which will allow the design of the wing/fuselage interface and the aft portion of the fuselage such that separation is minimized on future next generation designs. (Note: This type of software was not available to Carter when the CCTD was originally designed.)

Despite these minor correctable deficiencies which significantly reduced the L/D of the CCTD at lower speeds, it can be seen from the graph that the aircraft has a similar L/D at cruise speed when compared to the most efficient popular kit fixed-wing general aviation aircraft and much better than any other VTOL aircraft. The data indicates that the L/D of the CCTD at 175 mph should at least be approximately 7.5, and since the separated drag problem can be eliminated on future Slowed Rotor designs, their L/D curves at the lower aispeeds will be much better than that demonstrated on the first Carter prototype. Even though airplanes may achieve a higher peak L/D ratio, their efficiency at cruise, where most flight time will be spent and which has the largest impact on fuel and range, will be comparable to that of the CCTD. And the CCTD is a Slowed-Rotor Compound rotorcraft, the basic concept of which is capable of a vertical takeoff and landing. The efficiency of the CCTD in low vibration, stable high speed cruise is far greater than that of any other helicopter design, and deserves further study.