Where's the bast place for a static air port and other data collection instruments?

[Kenny Sharp]

I have an Eagle Tree Pro Recorder, and I was wondering where the best place for a static air port is.

I realize that the airspeed indicator relies on the preasure differential between the pitot tube which is pointed into the direction of travel, and the Static air port which is flush to the fuse, and is pointed perpendicular to the direction of travel.

I was thinking the static air port would be best if located at the center of preasure. Is this true? Does it matter?

Also, the GPS unit which measures 1 1/2 x 1/12 x 3/8, needs to find a happy home as well. Would the center of preasure be the best location for the GPS?

Any thoughts?

Thanks. :)

[vintage1]

With a leaky airframe, somewhere inside is probably as good as anywhere. Its only to pick up the ambient pressure after all. On full size with pressurized or at least sealed cabins its needful its outside the plane.

[Kenny Sharp]

With a leaky airframe, somewhere inside is probably as good as anywhere. Its only to pick up the ambient pressure after all. On full size with pressurized or at least sealed cabins its needful its outside the plane.

I am under the impression that airframes that have holes for cooling will cause the inside preasure to fluctuate. Much like the old VW Beatles did when you opened only one window just a crack.

Holes for motor, esc, and battery cooling, would constitute more than just a leaky airframe. Right?

[gouch]

I'm not familiar with the system, but would have figured the GPS would be used for speed measurement negating the need for pitot tube.......Ah...I guess you will only get ground speed with GPS, not airspeed.... Fair enough.

Can't help with the pitot problem, apart from sugesting a double pipe setup inner to forward, outer to the side. Would that not work? As vintage said, it's ambient that the "side looking" port wants to see

And a question back to you: Does it matter where the GPS unit goes?. It's only looking for sattellites isn't it? Apart from balancing issues, I wouldn't think it would matter....unless I am missing something?

Just as an aside: my brother in law built an altitude logger a few years ago using a pressure sensor and testing it in the car found that accelerating away from the lights (any acceleration actually) caused the figures to show he was going up or down!
T'was the pressure changing inside the vehicle due to the air mass shifting forward and aft. Confirmation of this can be seen with helium ballon hanging in the car, accelerate and the balloon moves forward, hit the brakes and it goes back.
You could win money with that experiment, as a lot of people just don't believe it till they see it.

Cheers
Paul

[Kenny Sharp]

[QUOTE=gouch Can't help with the pitot problem, apart from sugesting a double pipe setup inner to forward, outer to the side. Would that not work? As vintage said, it's ambient that the "side looking" port wants to see

And a question back to you: Does it matter where the GPS unit goes?. It's only looking for sattellites isn't it? Apart from balancing issues, I wouldn't think it would matter....unless I am missing something?
Cheers
Paul[/QUOTE]


I'm wondering if the ambient preasure reading would change depending on the pitot tube location. Or, where is the best location for accurate readings...if there is one.

As far as the GPS unit goes, I'm looking for a place that would disturb the flying charastics of the plane as little as possible. (If there is one?)

Thanks all. :)

[gouch]

Then the CofG is the best place for the GPS.

As far as the best place for the tube, where do the "big boys" put it? How about a tube sticking out the front of the leading edge away from any prop wash?

[Kenny Sharp]

Then the CofG is the best place for the GPS.

As far as the best place for the tube, where do the "big boys" put it? How about a tube sticking out the front of the leading edge away from any prop wash?

I'm planning on drilling a hole in the leading edge away from prop wash, and running the pitot tube inside the wing with the servo wires,and then, into the recorder.
I was more concerned with the static air port tube location.

Also, I'd rather not put the GPS unit inside the fuse, and I'd rather not have it on top of the wing.
I was thinking behind the wing saddle if this would be the area of least disturbance? I just don't know. :confused:

[Tom Harper]

kkens4,

I had problems with the static pick up inside of the fuselage. Perhaps I was getting ram air from some of the leaks. I mounted it flush with the side of the fuselage at the maximum width point. No special reason, just convenient.

I use a brass pitot tube for the velocity sensor. Run it through the leading edge and let it protrude a few inches.

Haven't had any problem with GPS reception inside the fuselage. I have one under the wing mount of an LT-40 and have not had problems yet. But, I think your precaution of putting it behind the wing saddle makes sense.

Tom

[Kenny Sharp]

Thanks Tom.

Perhaps I worry too much about the fore/aft location of the static air port. :)

Just to clarify, I plan to have the brass pitot tube extend 1 in.-1 1/2 in. past the leading edge, approx. 1 ft. out from the spinner. I'll be using a 16 x16 prop. I would think that that is enough both ways.

[Tom Harper]

Wow - that's a real prop!

A foot out on the wing sounds good. I just measured my LT-40. The tube is 9" out on the wing and protrudes 2" ahead of the LE. I'm using a 10X4 prop.

Your set up sounds fine.

When I get the results of a flight I prefer to use Excel instead of EagleTree's software. Excel will load the files if you select 'delimited by spaces'.

Tom

[Kenny Sharp]

Yeah!, And for F5B ships, it's considered moderate. :eek: For me though, it will do. :D

That said, I'll try it in a Filip hotliner first, then the Surprise 14.
I may have to re-think running the pitot tube alongside the servo wires. Drilling even a small hole in the main spar has be cringing just thinking about it.

Thanks for posting your measurements. They are very helpfull.
Thanks for the software tip as well. :)

When I finish my install, I'll post some pics, OK?

[jkettu]

In full-size sailplanes a popular location for static ports is about halfway along the tail boom. Four holes spaced equally around the boom, all connected to a single point (to reduce sideslip/AoA error) and from there a single line to the instrument panel. Probably an overkill in your case.

[Brandano]

I'd actually use 2 tubes sticking out of the LE. One with the port on the tip, your dynamic pressure port. The other with the port('s?) on the side, your static port. Probably the pressure on this will also vary with attitude, speed etc like on any other position, but at least most of the "incognita" from the placement on the fuselage can be zeroed out and the same setup will behave the same way on several planes. Using 4 or more holes should have them balance each other out as the AOA or slip angle varies too, and fitting a sponge in front of the sensor might dampen out a few spikes as well

[Tom Harper]

Brandano,

Thanks, sounds like a good approach.

[pd1]

Powered full sized planes mostly have one of three locations for the static port.

Location one, mostly Cessnas, left side of fuselage ahead of the wing and just below the windshield, ahead Of the door.

Location two, Beechcraft and twin Cessnas, left side of fuselage behind the wing, half way between the top and bottom of the fuselage.
Usually there is a duplicate on the right side of the fuselage if flight into icing is a possibility.

Location three, almost all Pipers, on the rear of the pitot tube.
The pitot tube on pipers is a cast aluminum part that has the air inlet about 3 inches below the wing.

I flew an experimental racer once that we had pitot tube issues.
The pitot tube was made to extend from the leading edge of the wing, as you described.
We had to extend the tube to over a foot long before we could get accurate readings.

A by product was it created a wing dropping tendency when the plane stalled.
It was easily corrected with a stll strip on the oppisite wing in the mirror location.

If you want some pictures of pitot tubes and or static ports send me a PM.
I might even have a pitot tube or static port laying around you could have.

Paul

[Kenny Sharp]

Thanks Paul.
That would be great if you could please post them here. :)

[pd1]

Pictures are of static ports from King Air, Becchcraft Skipper and Beechcraft Sierra. All static ports for these planes are located on rear of fuselage.
Lake LA 4 static vent on rear of pitot tube.
The duplicate ports, on other side of fuselage are to compensate for slipping/skidding.

[Kenny Sharp]

Thanks for the pictures. It seems the location of the pitot tube is critical as well.
It makes me wonder if instead of in front of a model wing, perhaps low and below the wing is better.
With a top mounted wing, such as a hotliner/F5b, the fuse is sure to protect a dangling pitot tube somewhat.
Anyway, I've got lots to think about here.

[biber]

From what I know, the static port shouldn't be near the wing surface for obvious reasons.
A prandtl tube is very accurate, if manufactured carefully.
The dynamic pressure port isn't too sensitive to varying angles of attack anyway,
but the static port is, so try to keep the tube aligned with the airflow within about 5° or so.
With the probe's nose rounded hemispherical, a good place for the static port holes
is about 4 - 5 times of the probe's diameter back of the nose.

biber

[Kenny Sharp]

Here's what I came up with to test the altitude and airspeed.
Start small and work your way up, I say.
Also, the graphed results of 3 flights, including 7 climbs.
I feel satisfied with the data results.

[Kenny Sharp]

Here are another couple of graphs. This time with lift.