ccsmith51
Well Known Member
I have an RV-6 that has a heated Gretz pitot tube from a C170. It is mounted below the front of the left leading edge, with the nose of the probe in front of the leading edge a bit. You can see it in the photo below, between the prop and the cowl. Sorry I don't have a better one.
The ASI reads up to 19 MPH high, depending on speed. At stall it inidicates about 50 mph with full flaps and 55 clean, which is about right I think. But at at higher speeds it is off, and the faster I go the more it is off. I tested it by making GPS tracks on N, E, S, W headings at 1,000' and constant indicated airspeed. At 110 mph indicated the four GPS speeds averaged 98 mph, a 12 mph error. At 190 mph indicated the four GPS speeds averaged 171 mph, a 19 mph error.
So, I'm looking for some help in determining how to find the error and correct it.
From the pitot standpoint, some people have told me that I should get rid of the Gretz and replace it with the standard Van's pitot, in the standard Van's location. Others have told me that the current location is fine as it is in front of the wing and at our low speeds there is not a compression issue.
I don't want to move it if I don't have to as there would be a hole in the wing that I would have to patch.
As far as the static system goes, the ports are from the same 172 that the engine, pitot, rudder pedals, instruments, etc., came from. They are strange looking devices with a step in them, as shown here:
They are mounted in front of F606 with the high side facing forward. I am told that the proper placement is one bay back in front of F607. I sent Van's an e-mail asking if the placement made any difference. I got a prompt reply from Ken Scott saying "We've seen the ports in both locations and it seems to make almost no detectable difference." I did not mention to him that they were not standard Van's ports.
I tried an experiment. I put a piece of tape over the hole in the port on the right side, and made a spacer the same thickness as the step on the left port, with a hole in it. I taped that on and then flew and found that the same error was there.
BTW, I just installed another ASI. The plane had two, one TAS gauge that had a bad return spring and read 7 mph higher than the other, and one a standard gauge. I pulled both and replaced the TAS gauge with another TAS gauge that came out of a -4. Readings were the same as on the non-TAS gauge I removed. I put a G-meter in the extra hole.
So, asking all the Guru's out there for any suggestion on what to do to get my ASI reading correctly. I am at a loss, and don't want to do anything rash, like moving a pitot, if I don't have to.
Thanks and Happy New Year!!
The ASI reads up to 19 MPH high, depending on speed. At stall it inidicates about 50 mph with full flaps and 55 clean, which is about right I think. But at at higher speeds it is off, and the faster I go the more it is off. I tested it by making GPS tracks on N, E, S, W headings at 1,000' and constant indicated airspeed. At 110 mph indicated the four GPS speeds averaged 98 mph, a 12 mph error. At 190 mph indicated the four GPS speeds averaged 171 mph, a 19 mph error.
So, I'm looking for some help in determining how to find the error and correct it.
From the pitot standpoint, some people have told me that I should get rid of the Gretz and replace it with the standard Van's pitot, in the standard Van's location. Others have told me that the current location is fine as it is in front of the wing and at our low speeds there is not a compression issue.
I don't want to move it if I don't have to as there would be a hole in the wing that I would have to patch.
As far as the static system goes, the ports are from the same 172 that the engine, pitot, rudder pedals, instruments, etc., came from. They are strange looking devices with a step in them, as shown here:
They are mounted in front of F606 with the high side facing forward. I am told that the proper placement is one bay back in front of F607. I sent Van's an e-mail asking if the placement made any difference. I got a prompt reply from Ken Scott saying "We've seen the ports in both locations and it seems to make almost no detectable difference." I did not mention to him that they were not standard Van's ports.
I tried an experiment. I put a piece of tape over the hole in the port on the right side, and made a spacer the same thickness as the step on the left port, with a hole in it. I taped that on and then flew and found that the same error was there.
BTW, I just installed another ASI. The plane had two, one TAS gauge that had a bad return spring and read 7 mph higher than the other, and one a standard gauge. I pulled both and replaced the TAS gauge with another TAS gauge that came out of a -4. Readings were the same as on the non-TAS gauge I removed. I put a G-meter in the extra hole.
So, asking all the Guru's out there for any suggestion on what to do to get my ASI reading correctly. I am at a loss, and don't want to do anything rash, like moving a pitot, if I don't have to.
Thanks and Happy New Year!!
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