[GrayHawk]

I'm posting this as a new thread so as to not hijack the current thread about possible statib error in an RV-7.

I bought a flying RV-6A several years back. I've always been somewhat concerned that the static port was not in the suggested Van's location (which has substantial data by now that says 'good location').

This plane has the inexpensive dual tube (one is pitot, other is static) arrangement mounted on the left wing.

Does anyone have any real use data that would say this is static pressure neutral, high or low?

Thanks,
GH

 

[Kevin Horton]

This plane has the inexpensive dual tube (one is pitot, other is static) arrangement mounted on the left wing.

Does anyone have any real use data that would say this is static pressure neutral, high or low?

Wings, by design, cause the pressure below them to be higher than ambient pressure, and the pressure above them to be lower than ambient pressure. This is how they make all that lift we need to fly.

But, there is quite a it of pressure variation with location. At any given condition, some locations below the wing will have pressure that are close to ambient pressure, but the locations that are so lucky change with angle of attack. It is difficult to find a location near the wing that has pressure close to ambient pressure over the whole speed range of interest.

It is quite likely that this static port has errors. The only way to know one way or the other is to do some testing. The errors will be whatever they are no matter what opinions you get on this forum.

Rough approach - You can get a very rough idea of the accuracy by stopping in the middle of your runway and adjusting the baro setting on the altimeter so the altimeter reads field elevation (or zero, if you are close enough to sea level to allow that). Then, take off and do a very low pass down the runway and note the altimeter reading. Compare the altimeter reading to what it said when you were stopped on the runway. If the height reported by your Mark I eyeball agrees with difference between altimeter reading in flight vs what it said on the runway, then the static system is reasonably accurate at that airspeed. Repeat at other airspeeds, as the error in the static system will vary with airspeed.

Accurate approach - More accurate static system error testing will first require ground testing to determine ASI instrument error. Then you will do a bunch of testing at various speeds gathering GPS data to calculate TAS, and calculate the CAS. You will compare the calculated CAS to the IAS corrected for ASI error. If your static system has no error, the CAS you calculated from TAS will equal IAS corrected for ASI instrument error.

If you want to do some more exact flight testing, contact me and I'll talk you through it, including providing spreadsheets to do the various calculations.

 

[GrayHawk]

Thanks for your response Kevin.

I'm currently doing some fairly extensive work on the plane (instrument panel upgrade, new cowling, new fairings) so testing is precluded right now. But I will test thoroughly later. My thought right now is to just go ahead and rework the pitot/static systems to match what most of the RV-6s have & get rid of the current dual tube setup. The new instruments will need to be carefully tested anyways.

GH

 

[L.Adamson]

Thanks for your response Kevin.

I'm currently doing some fairly extensive work on the plane (instrument panel upgrade, new cowling, new fairings) so testing is precluded right now. But I will test thoroughly later. My thought right now is to just go ahead and rework the pitot/static systems to match what most of the RV-6s have & get rid of the current dual tube setup. The new instruments will need to be carefully tested anyways.

GH

I do have.....what now has become a ridiculously expensive, heated pitot/static tube that hangs about six inches under the wing. When I purchased it, around the year 1999, it was cheap. But it does provide quite an accurate reading as compared to flying with other aircraft. I also have the Van's statics, and someday may complete the plumbing with a tee for the sake of comparison & redundancy. It's already plumbed to the panel area.

L.Adamson --- RV6A

 

[GrayHawk]

I do have.....what now has become a ridiculously expensive, heated pitot/static tube that hangs about six inches under the wing. When I purchased it, around the year 1999, it was cheap. But it does provide quite an accurate reading as compared to flying with other aircraft. I also have the Van's statics, and someday may complete the plumbing with a tee for the sake of comparison & redundancy. It's already plumbed to the panel area.

L.Adamson --- RV6A

I'm sure the one I have now is at least 6" below the wing.

GH

 

[elippse]

Wings, by design, cause the pressure below them to be higher than ambient pressure, and the pressure above them to be lower than ambient pressure. QUOTE]

Actually, that's not entirely true with most modern airfoils, which have below static pressure on the bottom surface, too, but not as low as what is on the top. This high pressure on the wing bottom only applies if the wing is flat on the bottom. When you really get down to the actual physics of lift and where it takes place on a wing, it is due, of course, to pressure difference. But if the wing is hollow, it is the pressure difference between the air inside the wing pushing up on the top surface and pushing down on the bottom surface. The force from the surface is transferred from the skin to the ribs and from the ribs to the spars. That's why the fabric came off Wittman's wing which led to his death. It's also the reason why a canopy wants to rise up; it's the air inside the cabin that pushes up on it. It is something that is hard to wrap your mind around that the air inside a wing, being carried along with the plane, actually causes the lift force, especially since we've always been told the forces are all outside of the wing, but just think about it for a while. I'm always accused of heretical ideas! In the macro, it's the pressure differential between the top and bottom, but in the micro that doesn't really show what's happening! Suction is not a real force; it's just a subjective reaction as is hot or cold. There is a really nice coaxial pitot-static tube that is used on Pitts biplanes that is available from ACS for about $120 that mounts on the leading edge of the wing, where it is much less affected by airflow deviations.

 

[Kevin Horton]

Wings, by design, cause the pressure below them to be higher than ambient pressure, and the pressure above them to be lower than ambient pressure. QUOTE]

Actually, that's not entirely true with most modern airfoils, which have below static pressure on the bottom surface, too, but not as low as what is on the top. This high pressure on the wing bottom only applies if the wing is flat on the bottom.

I agree that my original statement shouldn't have been worded to make it apply to all airfoils. I suspect my statement does apply to the NACA 23000 series airfoil as the original poster would have on his RV-6A. Unfortunately I can't run XFoil here at work to check.

 

[Bill Dicus]

pitot static

The Pitts tube has multiple static ports spaced evenly around the tube's (fairly large) circumference. It seems to work well upright or inverted and at extreme angles of attack.

 

[elippse]

Airfoil upper and lower pressures

I agree that my original statement shouldn't have been worded to make it apply to all airfoils. I suspect my statement does apply to the NACA 23000 series airfoil as the original poster would have on his RV-6A. Unfortunately I can't run XFoil here at work to check.

Hi, Kevin! Based on your other postings that show that you have way more than just a passing knowledge of aerodynamics, I'm sure that your statement was just a generalized look at airfoil pressure distribution that has been preached for ages by our flight instructors as common knowledge, sort of like when you hear the word suction being applied. I know when I first heard the concept of the air pushing down on the top of a liquid and forcing it up a straw in grade-school physics, I thought that the nun had definitely lost it. After all, hadn't she ever heard of suction? She was one of those old people over 30 who was in danger of becoming senile. When I read one of these lift descriptions that describe how the air is pushing up on the bottom of the wing, I want to immediately jump in and try to set things straight! But come to think of it, 1/3 of the lift actually does come from the air pushing up on the bottom, with the other 2/3 due to Coanda effect on the top! Boy! I'll bet that one will get the buzz going!

 

[GrayHawk]

Actually, that's not entirely true with most modern airfoils, which have below static pressure on the bottom surface, too, but not as low as what is on the top. This high pressure on the wing bottom only applies if the wing is flat on the bottom.

This would be the worst case (below ambient static) as during approach, your indicated air speed would be higher tha true air speed, lending to possibility of early stall. Not Good!

GH

 

[Kevin Horton]

I agree that my original statement shouldn't have been worded to make it apply to all airfoils. I suspect my statement does apply to the NACA 23000 series airfoil as the original poster would have on his RV-6A. Unfortunately I can't run XFoil here at work to check.

It certainly has been a lot of years since I've looked at pressure distributions around airfoils. I fired up XFoil tonight, and looked at a NACA 23013 airfoil (close to the 23013.5 that the short wing RVs have) at a typical cruise lift coefficient of about 0.2 and typical Reynolds and Mach numbers. Contrary to my earlier statements, the pressure on most of the lower surface is actually a bit less than ambient pressure, just as Paul suggested.

The pressure near the wing skin in the fore and aft location about where a static port on a pitot-static tube would be is quite a bit lower than ambient pressure. If the static port was located there, right at the wing skin, the error in IAS in cruise would be on the order of 15 kt, and the altitude error would be on the order of 290 ft at a typical cruise condition. The errors would get less as the static port was moved away from the wing skin, as the pressures would be closer to ambient pressure. I have no idea how far away from the wing skin the static port would need to be to get acceptable errors.

The pressure distribution changes significantly as the angle of attack is increased, which suggests that the optimum location for an under-wing static port would vary with angle of attack too.

I think the plan to switch to the typical RV static port is a good move, as it is likely to yield lower errors than an under-wing static port.

 

[GrayHawk]

Thank you all for your time & analysis.

I'll go ahead with the plan to move the static to Van's suggested RV-6 location & type port.

GH