Static ports located on Pitot/AOA probe

[cujet]

Good Afternoon all,

I modified a straight Dynon Pitot/AOA probe to include static information. I did this by drilling a ring of 12 small holes, at 30 degree intervals, about 30mm distance back from the tip.

I then removed the silicone sealant from the inside of the tube.

I installed another 3/16 inch aluminum tube in the pitot assy and sealed it with PRC-890 B2, by hand application (I crammed it in there) .

After the sealant cured, I added 100mm to the length of the pitot tube by inserting a machined section into the aft end, with more PRC sealant. Inside this, I poured PRC 890 A2 (thinner sealant) .

I pressure tested the pitot, static and AOA and all were intact.

It is installed on an Extra 300L, experimental exhibition, with a Skyview setup. The reason for this: There are no static ports on the Extra 300L, as they were part of the original pitot tube.

So far, the results are quite promising. Airspeed, static and AOA information are working quite well. I have not had the opportunity to test this against another "Extra", but I suspect the information is at least as accurate as stock.

We carefully monitored the altitude information on T/O (many times) and there was no change as the aircraft accelerated. We pulled up to 20 feet and flew the length of the runway accelerating to 140Kts. The altitude did not vary, other than to say we were 20-30 feet higher. So far, so good.

Now, at 4500 feet, we used Dynon GPS altitude as our altimeter. There was about an 80 to 100 foot discrepancy between GPS and absolute (remember S. Florida is at sea level) . We slowed to 90Kts and increased speed to 160Kts. While remaining at 4500 feet GPS ind alt. We did see a drop in indicated altitude of about 50-60 feet, when our speed changed from 90 to 160Kts.

However, I swear we descended a touch... And I believe GPS altitude is not perfectly accurate (but not sure on that one)

What do you guys think?

 

[DBRV10]

Fly a GPS box amd post the results as follows;

Fly the aircraft in smooth air, and hold a constant airspeed. Take note of the Dynon EFIS TAS. This is the TAS you want for each leg of the GPS test.

Now fly 4 legs in TRK mode (0/90/180/270) on your Autopliot because it is easier, to hold the track, and note down the Ground Speed when your speeds have stabilised and the TAS has settled at the original noted TAS as above.

Post us the results, or do your own calc's if you want.

Then you will know how good it is. It sounds promising.

 

[cujet]

No autopilot yet, so that's out.

I'll fly the GPS box or triangle sometime soon. Looking for a day with light winds...

Of course, the airspeed information is heavily biased towards pitot pressure. Slight differences in static information will make very small changes in indicated airspeed. In this case, I'd guess any airspeed variation would be beyond my ability to determine with accuracy.

 

[lolachampcar]

I've done a similar installation on an MX2 although my holes are not as well drilled as Chris'.

I flew the 8 separate headings and found IAS reading too high (by about 17 knots) which started me looking at pressure altitude versus IAS.  The attached plot depicts several different static configurations.

My question is - How much does the Skyview software alter pressure altitude (if any) as a function of IAS?

 

[lolachampcar]

I started looking into how static error might affect IAS and I must be doing something wrong....
Per AN 01-10-94, IAS = sqrt(In H20 * 1526.5)   or  sqrt(In Mercury * 20752.92)
pressure charts show approximately 0.107 inches of mercury per 100 foot of altitude around sea level which agrees with the old rule of thumb 1000 ft is 1 inch of baro.

if my math is correct, a 100 foot difference in pressure altitude (static error) would cause a 47 kt IAS error. 

This seems a bit much.  Am I missing something here?

 

[lolachampcar]

I tried three layers of DR30 shrink tubing on the pitot just before the static port and at 0.368 ft/kt I seem to have bracketed 0 ft/kt of static port change.

 

[cujet]

If, the 3 layers are too much, and 2, not enough, what is the "perfect dimension" of the "ring"?

In the case of our Extra 300L, the pitot tube has an extension on it and is farther from the wing leading edge than the MX-2 with similar setup.

Yet, I believe we are seeing similar issues. A recent level, 80 to 160Kt flight shows the altitude driving lower as the airspeed increases.

I intended on placing a ring aft of the static holes in an attempt at managing static/airspeed ratio. I may have to re-think that one.

Bill, have you done similar flights in your stock Extra? Does it do the same thing?

 

[cujet]

It might help to have a few pics to look at:

 

[lolachampcar]

So this is where I ended up on static port performance. There is about ten feet of PAlt movement from picking up to pulling off the end of the runway. This has been repeatable over several days/flights. GS now brackets TAS in the air when flying different headings.

Dynon support; I'm still interested in your thoughts on static error's affect on IAS.

 

[cujet]

Here is a pic of the pitot tube used in the above graph.

Interestingly enough, it's not that different from the original one on our Extra 300L.

I spoke with Dynon customer service about this, and the distinct impression I came away with is "why?". I'd like to explain my position. It's my opinion that static ports on experimental aircraft are often quite poor. Many are improperly located, improperly configured, inaccurate and needlessly complex. In fact, modern RVSM requirements (for those who don't know, RVSM is reduced separation in high alt flight) have driven equipment to be much more accurate. The best systems use static ports located on the Pitot tubes. Coupled with sensors in the pitot tubes. In other words, no pitot or static lines are involved!

In addition, locating the static ports on a forward mounted pitot tube keeps it out of disturbed air in nearly any possible mode of flight. And, they are easy to "tune" in that location.

So, come on Dynon, get with the program!

 

[dynonsupport]

if my math is correct, a 100 foot difference in pressure altitude (static error) would cause a 47 kt IAS error. 

It would at 0 IAS, but not at flying speed.

First, 100' of altitude change is ~0.06 PSI at 0' MSL, ~0.04 PSI at 10K' MSL, and ~0.03 PSI at 20K'. So even here it matters at what altitude you are.

Second, IAS is not linear. 0.06 PSI absolute is 50.4 Knots, but if you are going 100 knots already, changing your pressure by 0.06 PSI only changes airspeed by about 1.2 knots. If you are doing 200 knots, 0.06 PSI is less than 1/2 knot. So, if you really did have 100' of static error at 50 knots, you would have a huge error, but you would also have a static port that was actually a pitot port and would be thousands of feet off at 150 knots.

Now you see why our airspeed indicators don't show below 20 knots. 20 knots is 0.01 PSI and 25 knots is 0.015 PSI, so you need really good resolution. But you also want to read 300 knots, which is 2.125 PSI, so you need both resolution AND dynamic range.

 

[dynonsupport]

In addition, locating the static ports on a forward mounted pitot tube keeps it out of disturbed air in nearly any possible mode of flight. And, they are easy to "tune" in that location.

So, come on Dynon, get with the program!

Indeed, if you ENGINEER it, as you're doing, you may well be able to find a location of an integrated pitot/static probe that works for a particular aircraft. We did start with the intent of having a pitot/aoa/static port a decade ago when the probe was under development. But, we found that the the probe-based static source was VERY VERY sensitive to minor changes in position, aircraft type, etc. Since REALLY understanding static-based position error is a pretty advanced topic, we decided to leave the proper static source location to to the builder (and by proxy the airframe/kit designer).