Ed_Wischmeyer
Well Known Member
Has anybody tried Nigel Speedy's technigue for determining Vx and Vy from aircraft acceleration? (Kitplanes, Nov 2018)
I had skimmed the article last night, couldn't find where I put it this morning, so went out and flew some acceleration runs anyway. Yes, the results were pretty bad and my altitude hold attempts with the autopilot and some button pushing errors didn't help. Next time, for example, I'll be content with wings level and not try to use the autopilot to hold heading, because in the slowdown before the acceleration runs, I always got a little off heading.
Some questions came up:
* Given that all the action happens below 125 knots in his RV-8, why record data out to max speed? Is that for a better curve fit?
* Speaking of which, the article merely says, fit a fourth order polynomial. Okay, how?
Today's "plan" was to get Vx and Vy data with the fixed pitch prop and see how that compared with Vx and Vy after I put on the constant speed prop, which starts tomorrow. The plane is a little low on gas, so I might try the acceleration runs again tomorrow morning before taking the plane apart.
I did see only a little increase in power with speed (fixed pitch prop), but that's part of real world data for Vy. I also plotted TAS/IAS and the data was quite noisy, with a range of 1.035 to 1.055, all at more or less the same altitude and quick fluctuations. Outside air temp was not recorded, but the airmass appeared uniform. And the difference between baro altitude and pressure altitude changed by as much as 30 feet in just a few seconds. ????
But with the constant speed prop, I've got to think that my best strategy will be to let the autopilot do the flying in smooth air at various airspeeds and just check to see which speeds give the best rate of climb. And knowing rate of climb and true airspeed, it's easy enough to calculate climb gradient. Max climb gradient occurs at Vx, but remember that for flying Vx you want to use indicated airspeed. Maybe I'll try that tomorrow when I'm getting gas, but I'll have to reset minimum speed on the autopilot to 50 kt or so to capture Vx, I suspect.
So I'm doing well, aside from piloting skills, mathematics, and basic understanding of the technique...
I had skimmed the article last night, couldn't find where I put it this morning, so went out and flew some acceleration runs anyway. Yes, the results were pretty bad and my altitude hold attempts with the autopilot and some button pushing errors didn't help. Next time, for example, I'll be content with wings level and not try to use the autopilot to hold heading, because in the slowdown before the acceleration runs, I always got a little off heading.
Some questions came up:
* Given that all the action happens below 125 knots in his RV-8, why record data out to max speed? Is that for a better curve fit?
* Speaking of which, the article merely says, fit a fourth order polynomial. Okay, how?
Today's "plan" was to get Vx and Vy data with the fixed pitch prop and see how that compared with Vx and Vy after I put on the constant speed prop, which starts tomorrow. The plane is a little low on gas, so I might try the acceleration runs again tomorrow morning before taking the plane apart.
I did see only a little increase in power with speed (fixed pitch prop), but that's part of real world data for Vy. I also plotted TAS/IAS and the data was quite noisy, with a range of 1.035 to 1.055, all at more or less the same altitude and quick fluctuations. Outside air temp was not recorded, but the airmass appeared uniform. And the difference between baro altitude and pressure altitude changed by as much as 30 feet in just a few seconds. ????
But with the constant speed prop, I've got to think that my best strategy will be to let the autopilot do the flying in smooth air at various airspeeds and just check to see which speeds give the best rate of climb. And knowing rate of climb and true airspeed, it's easy enough to calculate climb gradient. Max climb gradient occurs at Vx, but remember that for flying Vx you want to use indicated airspeed. Maybe I'll try that tomorrow when I'm getting gas, but I'll have to reset minimum speed on the autopilot to 50 kt or so to capture Vx, I suspect.
So I'm doing well, aside from piloting skills, mathematics, and basic understanding of the technique...