Isn't it cool how we learn new stuff (or stuff new to us) almost everyday? [sm=sunsmiley.gif]

Michael's explanation is probably correct because the Eagle Tree FDR manual talks about sealing the fuselage and that any openings can affect the static pressure sensor in the unit.

That implies that you really do need to do the silicone treatment to the wing saddle if you want consistent readings.

One of my concerns about the Eagle Tree unit was that it did not have a separate static port.

I wanted to clarify one thing on my previous explanation. Where I was referencing hole diameter and leaks was in the dynamic line itself. The hole size does not matter is all you are doing is putting pressure down a line to a sensor. It's a closed tube per say. Pressure is pressure. Blow into a 1" diameter or .1" diamter tube with a closed end. You'll get the same force. A static measurement must be free of pressure fluctuations in order to give accurate measurements. You'll notice in 99% of full-scale aircraft that static measurements are made with a pitot-static system that either has static ports perpendicular to the flow of airstream on a pitot probe or a set of static ports strategically placed on the side of a fuselage. About the only time they tell you to use static from inside the fuselage is if you need to break the altimiter instrument glass in an emergency, if the altimiter is stuck... Wonder if you can do that with the electronic EFIS's and still have altitude? lol

My opinion is that it is almost impossible to get good static measurements from inside a fuselage. I've got the experience and the data to prove it. You either have positive pressure or a vacuum and both can happen with an aircraft during flight. Especially if it is on a turbine aircraft.

Ohh, and you need to get a pitot probe far enough outside of the boundary layer (Several inches) in order to get clean dynamic air. It shouldn't flex or vibrate. Lastly, don't forget that humidity and air density also come into play! Enough variables? IAS is a good ballpark figure.

Michael

A lot of planes with EFIS also have an electric gyro artificial horizon and a barometric altimeter for backup.

When I was flying Cherokee 140's in training, I would not take off without working vacuum. I did not care that I was flying VFR and shouldn't need the attitude indicator.

I also learned a very important lesson about checking the pitot tube. I went flying in a Cherokee Six with some pilot friends. As soon as we took off, the pilot noticed that he didn't have altitude or airspeed. And this was a dusk to night VMC flight. When we got on the ground, they checked the pitot and it had a dirt wasp nest on it. If I hadn't been so happy to be on the ground, I probably would have been mad at the pilot or myself for not properly preflighting the plane.

That Cherokee Six was involved in a mid-air at 5500 MSL two weeks later with a Bonanza. I still get chills ... even though it was a different pilot at the controls.

Science Guy?

I think I read Popular Science too, once on a time. I’m still waiting on my flying car.

Michael is correct, you need a static port from the side of the fuselage just ahead of the tail. But I doubt that the ¾ oz package has a port for it. You could seal the box and just run another tube to inside of the box. They are probably using a Motorola differential pressure chip to measure static and dynamic pressure. I’m a bit surprised by the 9% error that they told you about, except they were talking about the error of the true airspeed. What is the error of the indicated airspeed? I’d have to know what sensors they are really using.

You can buy a E-6B flight computer from any FBO or Sporty’s that will allow you to determine the true airspeed from the indicated if you have the local conditions. Also available from that place about 5 miles away where the big aluminum sausages are filled with meat and luggage.

Jimmy Dean

Jimmy ,

If you have a Palm with Palm OS 3.0 or higher, or even a Pocket PC, you can download a number of free or shareware E6-B calculators from [link]http://www.palmflying.com/calce6b.html[/link].

[link=http://www.palmflying.com/copilot.html]CoPilot 4.8[/link] for Palm users gets high marks. I had an earlier version on my phone and it was pretty cool.

I'm betting most if not all of them will calculate TAS. I know that CoPilot will let you use knots or mph.

So it sounds like I should not use this data recorder to compare two different airplanes, unless I am sure they are exactly the same, it will still give me best set-up ie prop, engine, mixture setting and give me the fastest one, but when I start comparing two aircraft, the variables become to high... I am going to check this guy out and see if I could figure a way to run the static port to the outside of the fuse via tubing. That would take care of it. Another issue we had considered is the length of the small pitot tube, Duane's plane had it sticking straight out the leading edge about a 1/4"... My plane had the pitot tube coming out 1/2" back from the leading edge and just breaking the plane of the leading edge. Mullvera stated this could also effect readings???
I was told by the Eagletree tech the air at 5500' would have a lower reading of approx. 9% That is what I saw from sea level to Boulder
right at 9-10% slower... I wonder now is the plane really that much slower 20mph or is it mostly the density altitude reflecting the 20mph difference???

Any Takers???

Static pressure will be "by far" the most challenging measurement to get from any onboard data acquisition system without a professionally designed pitot-static probe. To derive air speed you need to measure "freestream" total and "freestream" staic pressure. Freestream referes to a non disturbed pressure(out of the influence of local accelerated flow due to things such as wings, fuselages, etc). Search Pitot-staic probe manufactures and you will find a variety of "off the shelf" probes wich will give you say 2-3% type air speed data. In general, a pitot probe will have both a total and static pressure taps. As Michael stated, the total(or ram) pressure is not real sensitive due to influences of the wings or fuselages, however the static pressure is. Most Pitot probes will have the static pressure orifices 6 to 10 probe diameters downstream of the total pressure port to reduce probe tip influences and also 8 to 10 probe diameters ahead of any mounting/attach structure. Trying to use a fuselage cavity pressure for a airspeed calculation is a crap shoot. Any static pressure measurement in the fuselage is not a "freestream" staic pressure but actually a measurement of the static pressure associated with a "local" fuselage velocity(higher velocity, lower static pressure). I would bet for 20 to 30 bucks you could pictup a 0.125 inch diameter pitot static probe that would get you 2-3% type data(staright and level flight only). Both total and static pressure measurements are Angle-of-attack sensitive. At 10ΒΊ AOA you start introducing another pressure measurement uncertainty that increases as AOA increases. To minimize AOA sensitivity you can purchase a shielded(more expensive) pitot probe which is less sensitive to AOA effects. Boring, I feel like I'm at work.

MaxQ

Tom Hegland
37c

Thanks for the information, it sounds like you are saying this is a system that is hard to get just right, yet we have tested the system againtst the radar gun and find it to be very close, within a mile an hour or two... I had trouble with one of my qm planes because I think I had to much air coming inside the fuse, but my quickee was dead on nuts right, it matched the radar gun exactly and so did my buddies qm without the front end holes, so they must be doing something right to get the recorder that close to the radar gun speeds, which have been with my Shotgun quickee about 155-162 mph on the course at the end of the straights and 170mph in a shallow dive... We radar gunned the qm's at 165mph coming into pylon #2 and the data recorder showed us about the same with readings of 163-168mph on the course and 178 in a dive... This was at 5500' in Boulder Co. We showed speeds of 175-185 at the nats and I know many people that have radar guned the planes and this is right in the range they should be going, so I think as long as I make sure I have no holes to let the air inside the fuse, my readings are close.... One thing I feel for sure is I can determine the best set-up, which is all I really want to accomplish, since whatever static pressure influence is created by flying the model, is the same on the same model, when I change props and one shows to be faster than the other prop, I am fairly certain this is accurate information on which prop is better, that is my main goal, which one works best... It seems to show slow down airspeed accurately too... We have checked the speeds in the turns around turn 2-3 and compared it to the radar gun readings and it matches the slow down speed as well, so all in all, it has worked fairly well in my opinion...
Thanks For the Information,
John W.

I'm just installing an Eagletree in a plane and am curious about how you anchored the magnets to the engine in order to monitor RPM. Did U use CA or epoxy? Any problem with out-of-balance? Any suggestions would be helpful.

Rich,

We haven't got that far yet. The initial plan was to drill a small hole in the spinner backplate and insert the magnet with a drop of epoxy, then cover the hole with tape until the epoxy cured to leave a smooth finish. That seems like it ought to work. But so far we've been focusing on airspeed only, haven't actually tried the in-air rpm.

Duane

Hi Folks,

Thanks for the feedback - we are listening and we appreciate it!

I just wrote a long reply regarding airspeed, hit POST, and then discovered somehow it didn't make it up. I'm going to lick my wounds and attempt to rewrite it now, then save it before I post. :-)

Airspeed is definitely a measurement that can be as simple or as complex is you want to make it. Variables such as pitot/static placement, temp, alt, speed, etc., can affect the readings. Depending on the degree of absolute accuracy vs repeatability, flying location, etc., these variables may or may not have much of an impact. With a static port especially, it's also necessary to factor in weight, installation difficulty, and additional cost, vs any gains.

Regarding static pressure, a fairly simple test to see if you would benefit from an external static port is to install our pitot tube as a static tube, in the same place you'd install it as a pitot tube. This is done by mounting the pitot tube at right angles with the direction of travel, out of turbulance. Then, the system will record any pressure differential between the pitot port (now a static port) and the actual static port inside the plane. Since this test could result in a negative pressure, and since the post-processed MPH # won't mean much with this test, we would need to look at the raw data here to see what effect this would have. Glad to do this for whoever wants to try this experiment.

We are also working with a couple of customers to see if there is benefit to a static port with their installations. In the coming weeks, depending on what we find with these customers and the above experiment, we will post instructions for adding a static port (will require soldering 4 connections), and/or offer an upgrade service for those who want it. Additionally, we'll definitely offer a version with both ports if this is benefical for a significant group of customers, meaning the accuracy gains are worth the additional cost/complexity for these customers.

Don't hesitate to email me at [email protected] if you want to try this experiment and/or have any other questions.

Regards,