Airspeed drastic change with decreased AOA

[THE MACHINIST 35]

second flight of my c model last night trying to figure out why my airspeed is within 10% during climb out at 30 degree angle or more (verified on gps by flying a box). As soon as I lower the nose it is off 20-30%.   Not sure if a static port is plugged or....................?

joey said he only has a pitot tube on his wing not a second tube which I am assuming is static  and his is very accurate 

 

thanks for any input

 

jamie

 

 

[marksires 484]

The second tube would not be static, pointing into the wind like that it would be pressurized just like the pitot.  The builder may have doubled it up for strength, or the second tube may be a vent for the fuel tank.   Either way it is odd, and the second tube in such close proximity to the pitot could be generating some unexpected pressure variances.

Mark

 

[1avidflyer 1983]

Is the second tube plugged off at the end with a small hole in the side of it?  If so, that is the static port.  Champs and Chiefs have a double line similar to what you have.  JImChuk

PS  You could unhook the static line on the back of your airspeed indicator, and see what the reading is then.  Another thought, maybe you have a bit of water in the pitot tube somewhere.  Unhook that from the airspeed indicator and blow it out.  A friend of mine had that happen in the fall, and shortly after take off, the airspeed quit.  Temps were right about freezing, and he ran into some cooler air and it froze up. 

Edited 21 Sep 2016 by 1avidflyer

[EDMO 1078]

Agree with what 1Avidflyer said above - But, as I see it in the photo, the tubes are in the wrong place - they should be located coming thru the rib at about 8" (not sure about that measurement - will have to check the manual) FROM THE LEADING EDGE, and yours looks like it may be by the rear spar instead.   EDMO

Edited 21 Sep 2016 by EDMO

[THE MACHINIST 35]

 

On my first crow hop the airspeed was inoperative so I took the hose off and blew it out from the inside out. (With gauge disconnected) didn't see anything bad come out but couldn't blow through it before with my mouth after using compressor on it then I could   

I didn't see a small hole in the second tube but I may have missed it.  I only see one line running from the wing into fuselage so it may be just for strength.   I posted a picture of the dash so you guys could see if you're running the same airspeed gauge.  I only saw one tube going into that as well so maybe the static port is just a hole in the back of the gauge.   I didn't think about it being too far rearward. That seems plausible as well. 

 

Thanks again for for all your thoughts 

 

Jamie

[EDMO 1078]

Jamie,  this double-tube setup is standard for combining the static tube with the pitot tube, but there should be a second line from the bottom tube to the ASI for the static.  There should be another threaded port in the ASI to put a fitting for the static line.   If it is not connected, then you just have a "stronger" pitot tube, but maybe in the wrong location.  Check the book.   EDMO

Edited 21 Sep 2016 by EDMO

[THE MACHINIST 35]

I saw a picture in the book but no spec 

I will have to look but I only saw one line going to ASI

 

 

[1avidflyer 1983]

I missed that the tube was so far back on the wing.  Main thing is that it is in undisturbed air.  That could very well be the problem.   Is the airspeed to low when you are flying level? JImChuk

Edited 21 Sep 2016 by 1avidflyer

[THE MACHINIST 35]

Airspeed is way too high when flying level

[akflyer 1427]

I would slip a tube over it and extend it out to in front of the wing just for a quick test.  I have my static in the cabin and it seems to work OK, but I trust my butt more on the lower range of the envelope than the ASI.

 

[THE MACHINIST 35]

That was my thought if no one else had another one to try

 

[lostman 198]

I agree that your issue is due to the fact that the pitot tube is off the rear spar. Most aircraft (I can't think of any that don't) have the tube in front of the forward spar. Some even have them in front of the entire wing (my experimental) so that it's not in disturbed air. 

Like Leni said I believe I would install another tube over that one and move it to another location to test the theory though. Personally I'd run it down on the strut since it appears close. That should at least let you know if it would fix the issue before moving anything permanently. 

Do you see two tubes going into the wing to even connect to the second one? Not entirely sure why they would have one other than what everyone else has already guessed.

[marksires 484]

Interestingly, the Piper Cherokee series all have the Pitot under the rear section of the wing, just about where this one is.  Undisturbed, pressurized air can be found in strange places on different airframes.

Mark

 

[THE MACHINIST 35]

Joey sent me a picture of his pitot tube and it is in the same location.  According to him it is very accurate.  My only thought is I have no outside static source it's just take from the back of the ASI.   So I think I will see if my ASI can be set up with one and add an outside one 

 

thanks

 

jamie

[marksires 484]

Is your altimeter also just open to the underside of the panel?  How about the VSI?  If so, are they accurate?  If they are accurate, then the problem isn't the static source, it is either the pitot, or the gauge is just bad.  

Mark

 

[EDMO 1078]

Jamie,  These planes were designed with a cheap one-tube pitot, so there would only be one tube showing in the manual - Someone used a more expensive pitot/static tube combination, maybe because they already had one,  but evidently never connected the static line.  Open windows or doors in flight can also make a difference in what the ASI shows without an outside static vent.  Kitfox puts static vents on both sides back toward the fuselage tail, but one flyer on here has his static vents in the boot cowl and they work.   EDMO

Edited 22 Sep 2016 by EDMO

[THE MACHINIST 35]

Does my factory gauge have a port where I could add an outside static source to it?  I didn't see one but when I looked it was a cursory glance 

 

altimeter and VSI seem to work fine 

[1avidflyer 1983]

Your ASI must have a static port on it.  If the second tube was a static port and was hooked up, the instrument would compare the static port against the pitot tube, and should then be fairly accurate even if the pressure changes.    JImChuk

Edited 22 Sep 2016 by 1avidflyer

[EDMO 1078]

Your ASI must have a static port on it.  If the second tube was a static port and was hooked up, the instrument would compare the static port against the pitot tube, and should then be fairly accurate even if the pressure changes.    JImChuk

You may have to put a (1/8NPT?) threaded / push on fitting to the back of the ASI to hook up a tube.   EDMO

[1avidflyer 1983]

Right you are Ed.  It should have a hole/port that the fitting will screw into.   Some that I've seen had the ASI, VSI, and ALT static ports connected to one line that was open in back of the panel.  JImChuk

[THE MACHINIST 35]

If there is a port on the second piece of my pitot tube any good thoughts on how to get a tube out to it without slicing the wing up too much.  My thought is to use the ASI tube to pull it through but that will only work if it's not glued or zip tied too securely. 

[EDMO 1078]

If there is a port on the second piece of my pitot tube any good thoughts on how to get a tube out to it without slicing the wing up too much.  My thought is to use the ASI tube to pull it through but that will only work if it's not glued or zip tied too securely. 

If yours is like Kitfoxes, then the ASI / pitot tube goes from the aluminum tube in the wing across a rib, and into a hole in the rear spar and on into the root end of wing - that hole should be too small for two tubes to go thru it, and my book says to put a gob of silicone caulk in the hole.   I think slow, like some introverted Toolmaker/machinists, so I need some time to think about this - (It took the first Toolmaker a while to learn how to make sharp tools out of rocks!) 

One quick thought, is that you should have an inspection hole/ring close to the pitot, (even better if you have two), and you might be able to feed a rubber / plastic tube from the root end, thru the openings in the ribs, maybe inside of a small metal tube or taped to a wood stick, like quarter-round trim, and attach it to the static tube using the inspection hole, and untape it from the stick.   Maybe you can figure out something better?  

Another thought is to use an aluminum tube same size as pitot tube and stick it thru ribs from the root end and connect it to the static tube using a short rubber tube, and doing the same from the root end to the VSI static port.... EDMO

NOTE:  Some of these commercial pitot/static combo tubes have a flared or threaded end inside of the wing, and that really screws up trying to put a rubber tube on it - one way to solve that problem is to add a fitting or cut off the end and slip your rubber tube over what is left.  Ed

Edited 23 Sep 2016 by EDMO

[marksires 484]

If your Altimeter and VSI are working correctly, then the issue isn't the static source - even if they are all just open under the panel, they are all using the same static source.  Either your pitot tube is poorly placed, or the instrument itself is just bad.

Mark

[THE MACHINIST 35]

Anyone have a spare laying around I could test the theory with?

 

thanks

 

jamie

[akflyer 1427]

If no one else gets back to you, I can dig one up and shoot it to you that came off the amphib project I have so you can try it out but I wont be hone till after the 5th.