hevansrv7a
Well Known Member
I am posting this merely to share information that may benefit others.
1. I wanted to re-calibrate my TAS on my GRT after tweaking the static as described by Paul Lipps (tape behind the hole). I found I could fly into the wind such that the difference in magnetic direction between the GPS and the magnetic heading was 2 degrees or less. The cosine of 2 degrees is .999391. For the trig challenged this means that the average of the upwind and downwind ground speeds will be as close or closer than 99.94% of TAS. Close enough for me. I usually want 3-way runs, but this allowed re-calibrating the GRT during the flight and then double checking it. The 3-way needs the NTPS spreadsheet. Nothing new here as Paul has said this more than once in this forum.
2. The tape behind the static looks to have worked. My GPS altitude and my baro altitude were no more than 30' apart, using various speeds and altitudes up to 200 mph and 10,000'. Owing to the different concepts, they don't need to agree perfectly. Thanks, Paul. I used "hockey tape" which is a thick, clear plastic tape commonly available in Canada. It stayed on well.
3. My correction at 120 KIAS is essentially zero. But my correction at 155 KIAS at 8000' DA (173-174 KTAS) is still about -4 kts. I think I need to look in a new direction for this. At my next IFR certification, I will have the ASI checked in a larger range of speeds.
4. Now for a more controversial subject - power percentage calculations. The altitude corrections are my focus in this item. First, I made a startling discovery recently that probably many others already knew: The pressure altitude changes at a different rate than the air density as you go up. This is, I think, the result of the interaction of the PVT law of gasses and the adiabatic lapse rate built into the standard atmosphere model. Translation: The same MP at different pressure altitudes will give you different air density and thus different maximum ability to consume fuel. At 8000' the difference from sea level is about 5.82% more density. Most formulas and power charts suggest that at standard temperature, you gain about 1 or 1.1% of your HP per thousand feet of altitude. So I've just discovered where 2/3 of that comes from. The usual explanation is lowered pumping and exhaust back pressure losses. I suspect they are less important than we were thinking. Comments from engine experts welcome!
1. I wanted to re-calibrate my TAS on my GRT after tweaking the static as described by Paul Lipps (tape behind the hole). I found I could fly into the wind such that the difference in magnetic direction between the GPS and the magnetic heading was 2 degrees or less. The cosine of 2 degrees is .999391. For the trig challenged this means that the average of the upwind and downwind ground speeds will be as close or closer than 99.94% of TAS. Close enough for me. I usually want 3-way runs, but this allowed re-calibrating the GRT during the flight and then double checking it. The 3-way needs the NTPS spreadsheet. Nothing new here as Paul has said this more than once in this forum.
2. The tape behind the static looks to have worked. My GPS altitude and my baro altitude were no more than 30' apart, using various speeds and altitudes up to 200 mph and 10,000'. Owing to the different concepts, they don't need to agree perfectly. Thanks, Paul. I used "hockey tape" which is a thick, clear plastic tape commonly available in Canada. It stayed on well.
3. My correction at 120 KIAS is essentially zero. But my correction at 155 KIAS at 8000' DA (173-174 KTAS) is still about -4 kts. I think I need to look in a new direction for this. At my next IFR certification, I will have the ASI checked in a larger range of speeds.
4. Now for a more controversial subject - power percentage calculations. The altitude corrections are my focus in this item. First, I made a startling discovery recently that probably many others already knew: The pressure altitude changes at a different rate than the air density as you go up. This is, I think, the result of the interaction of the PVT law of gasses and the adiabatic lapse rate built into the standard atmosphere model. Translation: The same MP at different pressure altitudes will give you different air density and thus different maximum ability to consume fuel. At 8000' the difference from sea level is about 5.82% more density. Most formulas and power charts suggest that at standard temperature, you gain about 1 or 1.1% of your HP per thousand feet of altitude. So I've just discovered where 2/3 of that comes from. The usual explanation is lowered pumping and exhaust back pressure losses. I suspect they are less important than we were thinking. Comments from engine experts welcome!
Last edited:
