I am beginning to lose track of the number of times builders have asked me if the 447 is really adequate to fly the Gyrobee. Sometimes they are worried about their weight or think they may attend a fly-in in the Himalayas - whatever! Well, I don't sell gyros for a living and I have no vested interest in what you buy, but there is one thing I can do and that is I can document how well our aircraft performs.

I can do that because we are flying with the Digipod digital instrument panel/flight recorder. In addition to providing digital readout of all essential instrument functions, the Digipod is also a digital flight recorder, storing the instrument readings every five seconds in the course of a flight.

The instrument readings that are stored include:

• Airspeed (2-150 mph)
• Altitude (in feet, either AGL or MSL, 9,990 ft. range)
• Engine Tachometer (25-9,990 rpm)
• Cylinder Head Temperature (ambient to 990^oF)
• Exhaust Gas Temperature (ambient to 2,000^oF)
• Fuel Level (% remaining)
• Rotor RPM (80-800 rpm)
• Outside Air Temperature (-30-+150^oF)
• System Voltage (0-20V DC)

To avoid unnecessary clutter, my digiscan program, which is used to view flight records, was set to display just six of the nine possible channels: airspeed (AIRSP), altitude (ALTIM), engine tach (TACH), cylinder head temperature (CHT), fuel level (FUEL), and outside air temperature (TEMP).

This particular flight lasted just over 27 minutes and Don terminated the climb at 4350 feet AGL (5240 MSL) because he was getting really cold. The ground temperature was 85 degrees but, at altitude, had dropped down to the upper 50's. Not thinking ahead, Don took off without a jacket, so the wind-chill was getting quite uncomfortable.

This was not a full-throttle climb test and the throttle was set at about 6000 rpm to provide a reasonable rate of climb without having to worry about the engine over-heating. CHT readings peaked at about 390^oF early in the flight (around 5 minutes) but then fell slowly to about 360^oF with the fall in ambient air temperature with altitude. Note the short-term variation in CHT in the range of 40-80^oF. This is mainly a consequence of small changes in the flow of the relative wind around the engine as a result of a combination of airspeed variations, aircraft attitude changes, and even slight shifts in the pilot's position in the seat!

A reasonable attempt was made to hold airspeed constant during the climb, but there is quite a bit of variability at low altitudes as a result of thermal activity. As one might expect, this variability is reduced with altitude. Because he was cold and didn't want to kill a lot of time getting back down, you can see two points in the descent (23 minutes and 24.5 minutes) where Don brought the aircraft down to a very low airspeed (about 10 mph and 5 mph respectively), which, together with a fairly low throttle setting (about 3600 rpm), got the aircraft into a rapid, almost vertical, auto-rotational descent. If you want to lose altitude fast in a gyro, this is a safe way to do it as it avoids the high airspeeds usually associated with a steep descent (note that the aircraft hit about 60 mph on final!).

The fuel probe on the tank is located toward the front and thus the indicated fuel level is attitude-sensitive. You can see some of this as the aircraft is maneuvered more aggressively near the end of the flight.

While this flight only went to slightly over 5200 feet MSL, the digital flight record provides more than enough data to predict a meaningful service ceiling for the Gyrobee with the Rotax 447.

Note that the climb rate for the latter half of the climb stays quite constant. Although there was certainly a reduction in engine and blade performance as altitude increased, this was balanced by a fall in ambient air temperatures and the reduction in gross weight as fuel was burned off. By projecting fuel usage from the relatively straight portion of the fuel curve during climb, you can pretty well determine at what point you would reach the 5% point on the fuel remaining - more than enough to get down, assuming you are flying over the airport and not out in the boondocks. This would occur about 30 minutes into the flight (see above). If the terminal climb rate is projected to the 30-minute point, the altitude would be about 6000 feet AGL or very close to 7000 feet MSL. Just for the record, normal cruise endurance for the Gyrobee is 70 minutes (no reserve), but, as you would expect, a long climb burns gas at a much faster rate!

Note that this 7000 MSL "service ceiling" is not limited by climb performance as much as it is by the 5 gallon fuel limit of Part 103. To find out how high the Gyrobee can really climb, we would need to carry more fuel and that we cannot do!

The bottom-line is that the aircraft flies just fine on the 447. There is certainly no harm in putting on a 503, but if you can get a bargain on a used 447 that you can recondition, you can save a lot of money and still have a machine that will fly very well. I would note that gyro guys tend to sell used 447's at fire-sale prices because everyone "knows" you can't fly a gyro on 40 hp!

The previous flight record was a basic altitude test. This next one is a more typical flight. In this case, a 27 mile flight from Bergeon Field to Maple Grove airport in Fowlerville, Michigan, to attend the annual three-chapter (63, 70, 77) Michigan Gyro Fly-in.

As with all our digital flight logs, you can learn a lot about the aircraft, and flying in general, with a careful examination of the data.

Blade Spin-up

The 447 was started about 20 seconds into the record, initially peaking at 2700 RPM. There is then a slight drop as the electric VDU has brought the blade speed up and I begin to taxi toward the active, putting more speed on the blades. Note that both tach and airpeed readings start low but build together as the rotor is able to absorb more wind. Note that there is a slight drop in engine RPM at 3.8 minutes. This marks the point where I had to make a moderately sharp 180 degree turn onto the active runway and I wanted a bit less throttle during the turn. At that point the tach rises smoothly to a maximum of slightly over 6300 RPM as I smoothly accelerate for the actual takeoff run. The gyro lifts off at 4.6 minutes, with a slight bobble in the airspeed between 30 and 35 mph. A perfect rotation would be at 35 mph, the best climb airspeed for the Gyrobee, but in the real world it is all done by feel, not while staring at the instrument readouts!

Rate of Climb

The initial sustained rate-of-climb to 450 feet is about 350 fpm. This looks low when compared to the Rotorcraft directory entries, where everyone's machines can climb at 1000 fpm, but most of those figures are marketing hype. This is a real-world climb-out with temperatures in the mid-80's, humidity in the low 90's, 220 pounds of pilot in the seat, and a full fuel load. Corrected for standard conditions it would look better, but this is the real world! Actually, it is a very comfortable rate-of-climb when you consider that the aircraft is flying along at 35-40 mph. We have seen the Gyrobee climb at 1000 fpm, but that was in the dead of winter with an ambient temperature of 0^oF! The other way to generate an impressive climb is to do what you see at the fly-ins. Make a high-speed descending pass down the runway and then haul back on the stick to convert that extra momentum into an impressive climb. If you watch carefully however, all but the most powerful machines run out of steam after a few hundred feet or less of altitude gain. Ignoring the marketing hype, most single-seat machines climb at under 500 fpm and often require 50-60 mph airspeed to achieve even that. Droning along at 35-40 mph, the Gyrobee gets up there with the rest of the pack but is doing at with only 40 hp! Note that the CHT peaks at about 370^oF so you are not pushing the engine!

The bottom-line is that I have specified the Gyrobee's maximum rate of climb at 750 fpm - what you would see at full-throttle with a 170 pound pilot, half a load of fuel, and standard temperature. You will also see a note telling you what you can expect under more typical summer flying conditions!

Cruise Airspeed

Where most manufacturers get their cruise airspeed is a mystery. In the case of the Gyrobee, the best-climb airspeed is 35 mph, which is essentially the point where the aircraft has the greatest surplus of power for climb. If the goal was endurance, you would fly at 35 mph since that would require the least throttle and hence the lowest fuel burn to maintain level flight. To fly either faster or slower will require more throttle and hence a higher fuel burn per minute.

With a typical fixed-wing, cruise speed is what you would expect in level flight at 75% power, but I doubt that any gyros are rated in that manner. Practical cruise speed in the Gyrobee is a trade-off between airspeed and fuel consumption. For a trip like this one, I try for about 50 mph, which requires just above 5900 on the tach for level flight. Note the constant tach reading in cruise, with the CHT stabilized at about 360^oF. Airspeed varies a bit as a result of mid-day thermal activity, but is generally varying around the 50 mph target.

This particular day was characterized by significant overcast and relatively poor visibility (marginal VFR) and you might not expect thermals. However, it was a hot day and every place that features a slight thinning of the cloud cover was generating thermal lift. Just a note - these thermals have less impact on the gyro at 50 mph then they would at a slower speed. Note that this results in only minor altitude variation throughout most of the flight. A cruise altitude between 600 and 700 feet was chosen this day because of a steadily increasing wind gradient at greater altitudes. Normally I would shoot for 1000-1200 feet for a cross-country flight, but the cross-wind from the south (I was flying ENE) was pretty gusty as you got above 800 feet!

The interrelated nature of throttle, airspeed, and climb are neatly shown between 28 and 38 into the flight. Up to this time, the machine was trimmed for level flight at 50-55 mph indicated airspeed. By this point I was passing Webberville, Michigan and approaching Fowlerville, my destination. Maple Grove airport is hard to spot under the best of conditions and this day the visibility was really rotten. I elected to slow down a bit, dropping the airspeed to the 40-45 mph range in order to best sort out my flawless(?) navigation. Since the speed reduction moves the gyro closer to the optimum 35 mph and I hadn't touched the throttle, you would expect the gyro to begin climbing as the airspeed was reduced. That is exactly what it did, peaking out at about 1000 feet AGL as I entered the pattern at Maple Grove!

Flight Duration/Range

Published range figures for most gyros are also a bit flakey, because they lack quite a few specifics. The range figures I have posted for the Gyrobee are not guesses or estimates but hard figures based on 5 gallons of available fuel. The flight was flown with a target airspeed of 50 mph and took 34 minutes from takeoff to landing. The wind (from the south) was obviously not helping, since the calculated ground speed is 47.6 mph for the 27 mile flight. This figure agrees well with the average airspeed for the flight, which includes a slower airspeed for the initial climb-out plus allowances for messing around in the pattern at either end of the trip. On landing, the fuel level was 52%, so the no-wind range of 50 miles (duration of 60 minutes) is a fair estimate of the aircraft's capabilities that would leave a very modest fuel reserve.

Many of the published performance numbers for various gyros are well-intentioned estimates. Others range from copying everyone else's numbers to simple smoke and mirrors. I trust I have demonstrated that we know how the Gyrobee performs and that those numbers can provide a realistic basis for your own planning!