IAS stall

[Jeff974]

Hi,

I say you my problem :

I have an ASI, an EFIS 100 and a GPS. During a flight, the speeds giving by these instruments are similar : 150 kph by ASI, 145 kph by Efis and 152 kph by GPS.

However, when I make some stalls, (the stall come from 65 kph by ASI, and it is the correct speed stall), the EFIS indicates 100 kph. I have a diifference of 35 kph, that I cannot agree.

Here is my installation :
the static line is common for ASI ans EFIS
the dynamic line is not common : each instrument has its dynamic line.

Do someone reach this problem ?

Thank's

 

[jaba-who]

First check the units the Dynon is showing.

What type of aircraft do you have. You say stall speed is 65 kilometres per hour - that is very slow. Do you mean 65 Knots (nautical miles per hour?)

Is the ASI showing kilometers per hour, Knots (nautical miles per hour) or miles per hour?

65 miles per hour is pretty close to 100 kilometers per hour.  Maybe it is showing the speed in Kilometres per hour.

 

[khorton]

Here is my installation :
the static line is common for ASI ans EFIS
the dynamic line is not common : each instrument has its dynamic line.

Tell us more about the dynamic (pitot) system.  Do you have two pitot tubes, with one hooked up to the ASI and one to the Dynon?  If so, are both pitot tubes both well clear of the prop wash?  Are both of them pointed at the same angle?  Are they both well away from the wing surface so they are out of the boundary layer (15 cm from the wing should be enough)?  Have you done a leak check on each dynamic system?

 

[Jeff974]

Hi,

thank you for answer. Here are some details :

What type of aircraft do you have : it is a Polaris FK14. It is an ULA which fly from 65 kph (full flaps) to 270 kph.
All the instruments are in kph. I have any doubt.

I have two pitot tubes, one hooked with the ASI and the other hooked with the EFIS. Each pitot tube has its proper tubing. The EFIS pitot has two tubes, one for dynamic port and one for AOA port.
Only the static port is common.

The ASI pitot is on the leadind edege, the EFIS pitot is back from the leading edge and below the wing surface at about 10 cm. I have no leaks.

I think that problem may be with the static port. Remember that the EFIS pitot has two probes : one for little angle of attack and one for big angle of attack. During normal flight (little AOA) the indicate speed is lower than ASI. During stall flight (big AOA) the indicate speed is upper than ASI.

If someboby already reach this...

Thank's

Jeff
Reunion Island