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2 pitot tubes?

P

prkaye

Well Known Member
Is it OK to have two seperate pitot tubes come together at a simple "y" junction, and feed the same instrument?

Here's why I'm wondering... Dynon still doesn't have a heated pitot/AOA probe (maybe never will). Their AOA probe is part of their pitot tube, it works on the difference between the pressure in the two holes. If the Dynon pitot freezes up, you not only lose airspeed indication, but attitude information as well, which could be really bad (this info from a Dynon employee who posted on a Dynon forum). So... could I just put a splitter off the pitot input to the Dynon, and have a second heated pitot feeding it as well?

My (rudimentary) understanding is that this should be OK, because the air pressure shouldn't depend on how many ports are there (nor on the diameter of the opening in the pitot tube)... but I'm wondering if there are other effects I'm not aware of?
 
I don't have personal experience on a aircraft installation, but for wind tunnel testing it is not kosher to have two pressure taps connected together. If there is a pressure differential, you will get flow through the tube that will affect the measurements. Having a valve to switch between the two is the way to do it.

That being said, perhaps the error in your case would be acceptably small. Hopefully someone else will chime in with personal experience.

Alex
 
prkaye said:
If the Dynon pitot freezes up, you not only lose airspeed indication, but attitude information as well, which could be really bad

QUOTE]


Attitude info.? I don't believe the Dynons work like that. If they do I wouldn't buy one. You would lose airspeed and the AOA indicator but the gyros (electronic or mechanical) would still work.
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Attitude info.? I don't believe the Dynons work like that
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Posting by a Dynon Employee at : dynon forum thread

"The attitude function of the Dynon EFIS relies on accurate airspeed data to perform correctly. If your pitot were to freeze over, the EFIS would no longer be a reliable source of attitude information. "
 
back to my original question, is Alex correct that in general it is bad practice to connect two different pitot (or static?) sources simultaneously to the same instrument ?
 
They all need some form of external speed

briand said:
Huh. I wonder if the same goes for the GRT? I guess I will ask and report back.
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Actually they all work that way. They need an external speed source. GPS, airspeed, fifth wheel, what ever. Without an external speed source they will slowly develop an offset. An IMU just keeps adding up data from the yaw and accel sensors to figure out where it is. The external speed source keeps it in check.

I use a $100k IMU (inertial measurement unit) for vehicle dynamics testing. It has a very accurate GPS with a base station and it still takes 10 minutes or so of driving around to get the IMU to settle down. It's just one of the problems with using inertial measurements to figure out where you are space.
 
I don't know about connecting them together, bit I've given serious thought to putting a separate pitot on the right wing hooked up to my second EFIS (GRT Sport) when I get it installed. That gives me redundancy for ice or bug ingestion in the pitot tube.

Sebastian Trost
RV-7A QB
Flying!!!!! 75 hours to date
 
prkaye said:
Is it OK to have two seperate pitot tubes come together at a simple "y" junction, and feed the same instrument?
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I've never seen this done before, but that doesn't mean it wouldn't work. Generally speaking, aircraft with two pitot tubes have them to provide redundant airspeed indication systems, and each pitot tube feeds a different ASI.

In principle, assuming both pitot tubes were properly located (i.e. not in the prop wash, not in the boundary layer, not in the wake of something else, and roughly aligned with the local flow) they should both be sensing the same pressure within a very, very small tolerance. If both tubes are seeing almost exactly the same pressure, then I believe that this would work. But, it would be wise to plan some sort of test to confirm proper operation, and to have a backup plan in case your testing shows that plumbing two pitot tubes together does not work acceptably well.

There are some unlikely events that could cause problems though. Imagine that one pitot tube gets hit with a bird, and now it is bent at a strange angle. Now your airspeed indication system is screwed up. If this type of event were to occur, it would be better to have a valve so you could select either pitot tube to feed the EFIS.

I recommend you consider having a valve to select between pitot tubes. Or just wait for Dynon to release their heated pitot tube and go with one tube.
 
I'm going to ask a dumb question here. Is there any reason why you can't take an aluminum static port (the nice flush mount ones you can get from SafeAir), mount it on the wing ala the AFS AOA system, and pump that to the Dynon? You'd probably mount the port on the bottom of the wing somewhere close the leading edge. Pressure's pressure, right? I'd be very surprised if you couldn't calibrate that and make it work. It'd at least be worth a call to Dynon. Being that they don't have a heated pitot, it'd probably be in their best interests to help folks that want to fly IFR and have AOA in their plane. It would certainly makes the Dynon unit a LOT more attractive, I think.

Anyhow, if that works you could have just one heated pitot and a much simpler system.
 
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You have to get the pressure sensor away from the airflow around the wing. The air is accelerated around the wing surface. This is how lift is formed. If you mounted it on the surface you would get the speed of the surface air not the speed of the aircraft.
 
jcoloccia said:
I'm going to ask a dumb question here. Is there any reason why you can't take an aluminum static port (the nice flush mount ones you can get from SafeAir), mount it on the wing ala the AFS AOA system, and pump that to the Dynon? You'd probably mount the port on the bottom of the wing somewhere close the leading edge. Pressure's pressure, right?
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No this won't work for us, unfortunately.

TSwezey said:
You have to get the pressure sensor away from the airflow around the wing. The air is accelerated around the wing surface. This is how lift is formed. If you mounted it on the surface you would get the speed of the surface air not the speed of the aircraft.
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The fact that the airspeed at the wing surface is different from the aircraft's speed isn't a problem. The airflow anywhere close to the aircraft is different from the aircraft's speed, but Bernoulli's Law tells us that as the air speeds up, its static pressure decreases. The pitot tube measures the total pressure, which is the sum of the dynamic pressure and the static pressure. The total pressure will be the same, even though the air is speeding up and slowing down. But, to make this work, we need to ensure the pitot tube is not in the boundary layer. This is the problem with John's idea - it puts the pitot tube at the wing's surface, deep in the boundary layer. Bernoulli's Law breaks down in the boundary layer, so the pressure measured would be too low, and the ASI would under read.

This idea could be made to work, but the relationship between pressure and speed would differ from that assumed by ASIs. The pressure would also vary with angle of attack, so we would need a very complicated calculation to find airspeed, given this pressure. The B-2 has flush pitot ports, but it has a very expensive air data computer to do the magic calculations.
 
Actually, I wasn't suggesting replacing the pitot with a port in the wing. I was thinking more along the lines of putting the AOA sensor in the wing. From what I can tell, the Dynon works by looking at the difference between the pitot and a second port mounted at an angle. Maybe I'm wrong here.

Anyhow, I thought that a static type port flush mounted under the wing and somewhat facing forward (under the leading edge, for example) would have more or less the same relationship between AOA and pressure as a port mounted at an angle directly under the pitot. That being the case it would just be a matter of calibrating the system (which I'm assuming the Dynon allows you to do).

Anyhow, I'll defer to Kevin since he's the man when it comes to this sort of stuff :)
 
I hate to hijack this again. Not really but... I received a response from GRT and this is what was said:


"Our AHRS functions with or without airspeed. If we have airspeed we use it to improve the solution. But if you did not hook it up or even make it wrong you would be hard pressed to see a difference.

The Dynon actually uses the airspeed for their solution, not just aiding like we do."



FWIW, I don't really plan to fly in ice but my main concern would be FOD in my tube.
 
jcoloccia said:
Actually, I wasn't suggesting replacing the pitot with a port in the wing. I was thinking more along the lines of putting the AOA sensor in the wing. From what I can tell, the Dynon works by looking at the difference between the pitot and a second port mounted at an angle. Maybe I'm wrong here.
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Ahh. I misunderstood what you were describing.

I doubt this would work very well with the Dynon, as their AOA calculation assumes the AOA pressure comes from a hole on an inclined face that is stuck out in the air stream a bit away from the wing. There should be a reasonably predictable relationship between AOA and pressures on the Dynon probe. If you used a hole on the bottom of the leading edge, the relationship between pressures and AOA would be much more complicated. I suspect this would cause the Dynon AOA to give strange results in some situations. It could probably work if the AOA system had been designed with this type of installation in mind.

But, these are experimental aircraft. There is no harm in trying out this idea, as long as proper testing is done before deciding to rely on it.
 
In an attitude reference system, the speed measurement is used to cancel the centripetal, and possibly the translational, acceleration terms from the accelerometer measurement, leaving you with just the earth gravity vector.

If you don't have the speed measurement, either from airspeed or from GPS, or some other source, and the gyros are low quality (i.e. high measurement bias) then a sustained acceleration (such as doing a high banked turn for a few minutes) could confuse the attitude estimate.
 
Heated Pitot

Phil, you may not have to wait long for a heated pitot tube from Dynon. I e-mailed them approximately one month ago referencing the same issue. Their response indicated that they have a new heated pitot/AOA tube in the works and should have it available this spring. Hope this helps.

Jerry C.
 
I hope so (that Dynon gets their unit out). But from reading on the forums, they've been promising it for a couple of years now, and keep delaying. I think I'm adopting a "I'll believe it when I see it" attitude.

Am I correct in assuming that I don't have to worry about installing pitot tubes and stuff before I rivet the bottom skins on my wings? Because of inspection requirements in Canada, it maybe quite a while before I put those bottom skins on... I'll likely just do the top skins and then move onto the fuselage. I'm assuming I can delay most of my pitot-tube, wiring and lighting decisions until I'm ready to rivet the bottom skins on?
 
prkaye said:
Because of inspection requirements in Canada, it maybe quite a while before I put those bottom skins on... I'll likely just do the top skins and then move onto the fuselage. I'm assuming I can delay most of my pitot-tube, wiring and lighting decisions until I'm ready to rivet the bottom skins on?
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Yeah, there is no rush to sort this out. It will be easy to fit a heated pitot later. You could even fit one after the bottom skin is riveted on, but it would be easier to drill the holes for the heater wire and pitot tube now.

And, don't take this personally, but most folks at this stage of the project aren't as close to having a completed aircraft as they think. So, you probably have several years for Dynon to release their heated pitot tube before it would delay your first flight.
 
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