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False Airspeed indication on 701

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lgold(at)quantum-associat
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Posted: Wed May 21, 2008 9:39 am Post subject: False Airspeed indication on 701

I need advice solving an airspeed  indicator problem.<?xml:namespace prefix = o ns =  "urn:schemas-microsoft-com:office:office" /> 

Ever since I compl<?xml:namespace  prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />eted my 701, (about 50 flight hours ago) I noticed a  sharp drop-off in airspeed as soon as I pulled back the stick. As I increased  the angle of attack, indicated airspeed would go from 80 to 40 MPH in just a few  seconds, and at times even drop  to zero, although I sensed no stall. At first I though this airspeed change  (other than the zero indication) was characteristic of the 701 since both a  steam gauge and a Dynon showed the same drop.  Both instruments have their own pitot  tubes, but are connected to a single static line that terminates in an open tube far outboard inside one wing.  Both instruments read OK when in cruse attitude  (80-85-mph). 

I just returned from a 1600-mile flight in my new 701, going  from the San Francisco area to Page Az. (It was a great confirmation of the  701’s flight capabilities, requiring flight over 10000’ for several hours,  crossing over mountaintops with turbulent 30-mph winds, going into Bryce &  the Grand Canyon, Monument Valley, etc.). I was flying next to a Challenger that could  not cruse faster than 75-MPH so I had to fly in a nose-high attitude. At this  speed/attitude, my buddy in the Challenger consistently showed 10-MPH higher  airspeed than I was showing, which  was confirmed by GPS.  

To try fixing the faulty airspeed  indication, during the return from  Page, I cut the static line under the instrument panel to check  whether possible pressurization in the wing at a high  angle of attack was the cause of the  problem; and did notice some improvement. I also bent  the steam gauge’s pitot tube down a little so that the wind would be  perpendicular with the airflow at a high angle of attack, and cut the bottom tip  of the tube on an angle, but this had not effect. 

As an aside, I know an angle-of-attack  /LRI indicator would help keep me out of trouble. (I get a little concerned with zero airspeed displayed on  takeoff). One reason I purchased the Dynon is that it does have an AOA  indicator. However I found it impossible to follow the four-stall  setup procedure required by Dynon to synchronize the AOA because the  Dynon never saw the expected sharp stall drop-off. 

What else can I do to fix my airspeed  indication problem? Suggestions would be  appreciated. 

Les

   [quote][b]

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psm(at)att.net
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Posted: Wed May 21, 2008 7:04 pm Post subject: False Airspeed indication on 701

Hi Les,



I think you are on the right track by cutting the static line and 

seeing some improvement or difference.



The big planes (C-182, etc.) have dual static ports that are mounted 

on each side of the fuselage where the airflow is straight across 

them.  That is, the fuselage side is lined up with the normal 

direction of flight.  I believe they are connected in the middle, but 

I am not sure about that.  They sample the outside air rather than 

any enclosed air such as your wing or cockpit air.



The static port for the XL in the kit I got is a variation on this 

theme.  It is a single tube sealed at the front end and with three 

little holes in the sides equally spaced around the tube.



I have heard of alternate static ports mounted inside the cockpit 

which can be of some use if your main static port is clogged or iced 

over, but these are not expected to give accurate readings for 

airspeed or altitude.  On the other hand this kind of degradation in 

these readings is better than no readings at all which is what you 

get with no static port at all.



I hope this helps.  I think you will do well to install a more normal 

static port system than the one you started with.



Good luck,



Paul

XL fuselage

At 10:34 AM 5/21/2008, you wrote:

 	  Quote: 	
		  Ever since I completed my 701, (about 50 flight hours ago) I noticed 

a sharp drop-off in airspeed as soon as I pulled back the stick. As 

I increased the angle of attack, indicated airspeed would go from 80 

to 40 MPH in just a few seconds, and

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klaus(at)utdallas.edu
Guest

Posted: Thu May 22, 2008 2:42 pm Post subject: False Airspeed indication on 701

Hi Les,



Paul has already addressed some important considerations.



Generally, the following holds. If the static line is disconnected in 

the cabine,

then the airspeed indicates higher, since cabin pressure is always less than

outside pressure. So there is no surprise that disconnecting the static

line in your cockpit boosted airspeed indication a bit.



It is easy to test integrity of the pitot line. Disconnect the line in the

cockpit. Put a balloon on the pitot end of the line. In the cockpit blow 

into

the tube, thus inflating the balloon. Seal the line in the cockpit. The 

balloon

should stay inflated for a considerable period of time, thus indicating

no major leak. If the balloon collapses quickly, then there is a major leak.



If I were to guess, I would say that your installation will pass the 

above test.

I suspect that the placement and length of the pitot tube is the culprit.

This is due to influence on airflow by the leading slats of the 701. To 

test for this,

simply extend the pitot tube so that the inlet is quite a bit forward of 

the leading

edge. Make sure that this extension cannot cause damage to the pitot tube

attachment in flight. Then fly and test if this solves the problem.



Happy flying,



Klaus



klaus(at)utdallas.edu

www.utdallas.edu/~klaus

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goodings(at)yorku.ca
Guest

Posted: Fri May 23, 2008 7:34 am Post subject: False airspeed indication on 701

Yesterday, Klaus Truemper wrote " . . . . since cabin pressure is always 

less than outside pressure."  In my 601HD, there is a 1/4" gap between the 

lower edge of the canopy and the upper longeron.  In flight, why is my 

shirt sleeve always sucked out through the gap (left or right side)?



John Goodings, C-FGPJ, CH601HD with R912S, Toronto/Ottawa/Carp.

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klaus(at)utdallas.edu
Guest

Posted: Fri May 23, 2008 10:32 am Post subject: False airspeed indication on 701

Hi John,



The pressure differential you mention at the canopy/longeron gap

is due to the fact that there is VERY low pressure above the

wing. So indeed, pressure over the wing is lower than

cockpit pressure. One time, that gap tried to "eat" my

sectional!



What I meant is atmospheric pressure,

which the static port(s) attempt to capture. That pressure

is higher than cockpit pressure.



Happy flying,



Klaus



-- 

Klaus Truemper

Professor Emeritus of Computer Science

University of Texas at Dallas

Erik Jonsson School of Engineering and

  Computer Science EC31

P.O. Box 830688

Richardson, TX 75083-0688

(972) 883-2712

klaus(at)utdallas.edu

www.utdallas.edu/~klaus

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