Draker
Well Known Member
RV-7A, IO-360-M1B + Hartzell CS Prop
I've done about 4 hours of sawtooth climbs and idle-power glides in an attempt to determine the important V speeds.
I'll start with my test method.
For the climbs, I used this method: Target a particular density altitude (in this case, 4,500). Start about 1,000-1,500 feet below target altitude. EFIS does the density altitude calculation for me so I know what indicated altitude to start at. Set autopilot to IAS mode and establish test airspeed. Apply full power, highest RPM, and full rich mixture, and "help" the autopilot with a little control stick force. I found my AP servo could hold within ± 3kts, and with some "nudging" I could help the servos hold ± 1kts. Level off about 1,000 above target altitude. Let things cool off a bit and then do a glide test.
At home, I find a 30-60 second sample of data from the logs with uniform airspeed and with 4,500 as the midpoint altitude, subtract starting density altitude from ending density altitude, and divide by how long it took to fly through that block. I'm ignoring the VSI data, although I found the average indicated VS for each climb to be pretty close to the end-start/time calculation.
Same method for the glides. Pick a pressure altitude, start a bit above that, set AP to IAS mode, power to idle, prop pulled fully out. Then divide altitude lost by time, at my computer at home to find sink rates.
For the best angle calculations (Vx and best glide), I converted IAS to TAS (since I believe vertical speed is a true airspeed number) and used inverse tangent to find the angles.
Climb Results
Vy is tough to call. Configured at 1600 pounds weight, 4500ft density altitude, my rate of climb curve is essentially flat, about 1475f/m at all airspeeds between 73-93 KIAS. This might be expected with a CS prop. Tests were done full rich though. I should probably be leaning for best power at that density altitude.
Vx is basically "as slow as I'm willing to climb." The airplane will climb at pretty much any reasonable angle you command it to fly.
Neither Vy or Vx seem to be very relevant to actual flying, since CHT is ultimately the limiting factor, and prevents me from sustaining these climb airspeeds.
Glide Results
Best idle-power glide is also flat: 10:1 ratio at all glide airspeeds between 75 and 95 KIAS. Tests were at 1600 pounds and pressure altitude of 3500ft.
Best idle-power sink seems to be around 70 KIAS (788f/m), but I want to do a few more runs to confirm it.
So far, kind of disappointing to not be able to plot those nice, obvious airspeed-vertical speed curves that the flight testing docs show. I think I simply need to do more testing and gather more data. Or change something about my test method.
I've done about 4 hours of sawtooth climbs and idle-power glides in an attempt to determine the important V speeds.
I'll start with my test method.
For the climbs, I used this method: Target a particular density altitude (in this case, 4,500). Start about 1,000-1,500 feet below target altitude. EFIS does the density altitude calculation for me so I know what indicated altitude to start at. Set autopilot to IAS mode and establish test airspeed. Apply full power, highest RPM, and full rich mixture, and "help" the autopilot with a little control stick force. I found my AP servo could hold within ± 3kts, and with some "nudging" I could help the servos hold ± 1kts. Level off about 1,000 above target altitude. Let things cool off a bit and then do a glide test.
At home, I find a 30-60 second sample of data from the logs with uniform airspeed and with 4,500 as the midpoint altitude, subtract starting density altitude from ending density altitude, and divide by how long it took to fly through that block. I'm ignoring the VSI data, although I found the average indicated VS for each climb to be pretty close to the end-start/time calculation.
Same method for the glides. Pick a pressure altitude, start a bit above that, set AP to IAS mode, power to idle, prop pulled fully out. Then divide altitude lost by time, at my computer at home to find sink rates.
For the best angle calculations (Vx and best glide), I converted IAS to TAS (since I believe vertical speed is a true airspeed number) and used inverse tangent to find the angles.
Climb Results
Vy is tough to call. Configured at 1600 pounds weight, 4500ft density altitude, my rate of climb curve is essentially flat, about 1475f/m at all airspeeds between 73-93 KIAS. This might be expected with a CS prop. Tests were done full rich though. I should probably be leaning for best power at that density altitude.
Vx is basically "as slow as I'm willing to climb." The airplane will climb at pretty much any reasonable angle you command it to fly.
Neither Vy or Vx seem to be very relevant to actual flying, since CHT is ultimately the limiting factor, and prevents me from sustaining these climb airspeeds.
Glide Results
Best idle-power glide is also flat: 10:1 ratio at all glide airspeeds between 75 and 95 KIAS. Tests were at 1600 pounds and pressure altitude of 3500ft.
Best idle-power sink seems to be around 70 KIAS (788f/m), but I want to do a few more runs to confirm it.
So far, kind of disappointing to not be able to plot those nice, obvious airspeed-vertical speed curves that the flight testing docs show. I think I simply need to do more testing and gather more data. Or change something about my test method.
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