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Stall Speeds Low??/

Y

yankee-flyer

Well Known Member
N143WM made her first flight today. Needs a big wedge on the left side of the rudder but the big surprise was the indicated stall speeds. Gross weight at the time I started stall tests was about 960 pounds-- OWE 732, pilot 160, Fuel 11 gallons. Indicated speeds at 4000 feet were in the low 30's at buffet and high 20's at break. Is there a problem with my Dynon? It's noy holding the altitude correction well (ALTADJ). I'd appreciated low weight stall indications from others who've flown. Thanks for ALL the help, guys!

Wayne 120241/143WM
 
I'm not a 12 pilot but sounds like you might have a static port problem. I would check plumbing and the actual port outlet. Have you compared your IAS for accuracy, either by GPS and 3 leg course or with a chase plane?
 
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A pitot system leak will make all of your indicated airspeeds low.

Check (or recheck) your pitot system per the PAP.
 
Your first flight...

Wayne, Nice going and glad to hear you have your first flight completed. Thats a major milestone.

Tony
 
A static (on the ground) static check does nothing to guarantee accuracy of your ASI other than verify your instruments accuracy and check for leaks.

The other major component of this is in flight...how good is your static source? Static position error can cause you to be way off on your IAS. I went thru this with my new airplane and was really shocked that mine was as far off as it was....

There are dozens of threads on the subject here in the archives and Kevin Horton seems to be the resident expert on the subject.

One warning...if it does turn out to be static position error causing your pain, please realize that your altimeter is also going to indicate incorrectly as well and as a result so will your encoder telling ATC what altitude you are at.
 
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Brantel said:
A static (on the ground) static check does nothing to guarantee accuracy of your ASI other than verify your instruments accuracy and check for leaks.

The other major component of this is in flight...how good is your static source? Static position error can cause you to be way off on your IAS. I went thru this with my new airplane and was really shocked that mine was as far off as it was....

There are dozens of threads on the subject here in the archives and Kevin Horton seems to be the resident expert on the subject.

One warning...if it does turn out to be static position error causing your pain, please realize that your altimeter is also going to indicate incorrectly as well and as a result so will your encoder telling ATC what altitude you are at.
Click to expand...

Brian,
The RV-12 has standardized systems.
Assuming the builder has assembled the airplane per the plans he has a system that has bee proven to be accurate CAS vs IAS.
If it has been built per plans, the error is likely caused by either a leak or a problem with the D-180.
 
Anyone have the stall sppeds from flight card #1

and the weight when they were doing stall tests (About 30 poounds less that TOGW)??

Thanks

Wayne
 
He states that the system is verified leak free and that a static check has verified the accuracy of the Dynon so that leaves position error as one of the only remaining variables....

rvbuilder2002 said:
Brian,
The RV-12 has standardized systems.
Assuming the builder has assembled the airplane per the plans he has a system that has bee proven to be accurate CAS vs IAS.
If it has been built per plans, the error is likely caused by either a leak or a problem with the D-180.
Click to expand...
 
yankee-flyer said:
and the weight when they were doing stall tests (About 30 poounds less that TOGW)??

Thanks

Wayne
Click to expand...

Test Card 1 -- pilot plus 0 ballast, 11 gallons fuel per PAP (987 pounds)
No flaps - 38 knots indicated
1/2 flaps -38 knots indicated
full flaps - 37 knots indicated

(POH - No flaps - 41 @ 1050, 45 @ gross wt.)
(POH - Full flaps - xx @ 1050, 41 @ gross wt.)

Tony
 
Brantel said:
He states that the system is verified leak free and that a static check has verified the accuracy of the Dynon so that leaves position error as one of the only remaining variables....
Click to expand...
He has confirmed no static system leak, but he hasn't yet said he did a pitot system leak check. A pitot system leak would fit the symptoms, so it is definitely worth checking.
 
Does altimeter agree? Pitot incidence?

Does the chase plane confirm your altimeter?

I'd be inclined to look at the pitot system. The pitot has more pressure differential across the inside to the outside of the tube, so any leak is going to leak more.

Of course if that were the case, as you speed up, more p.d. and you'd think even more error, so....maybe not.

The pitot face? Is the angle of the face per the plans? I don't have a plan set, but I'm guessing it is cut at an angle to maintain cross sectional area at high angle of attack.
 
Sorry, I misread his post claiming no leaks...he did not confirm pitot. My bad!
Kevin Horton said:
He has confirmed no static system leak, but he hasn't yet said he did a pitot system leak check. A pitot system leak would fit the symptoms, so it is definitely worth checking.
Click to expand...
 
rvbuilder2002 said:
A pitot system leak will make all of your indicated airspeeds low.

Check (or recheck) your pitot system per the PAP.
Click to expand...

yankee-flyer said:
No leaks at all..
Wayne120241
Click to expand...

Kevin Horton said:
He has confirmed no static system leak, but he hasn't yet said he did a pitot system leak check. A pitot system leak would fit the symptoms, so it is definitely worth checking.
Click to expand...

As I posted previously, and Kevin pointed out again, it is likely a pitot line leak causing your problem.

You posted that you rechecked the static system and it has no leaks....have you tested the pitot system?
They are two totally different tests.
 
Pito tube opening taper orientation

just a thought. Is the taper on the pito tube oriented so that the top of the tube is forward of the bottom edge of the tube? One of the early articles on the RV12 mentioned that the tube was cut at an angle to improve low speed accuracy.

Also, [by my calculation {ratio of weights proportional to ratio of squared airspeeds}, which of course could be wrong] an airplane that stalls at 45 KIAS when it is 1320 lbs, should stall at 38.4KIAS when at 960 lbs. [40 @1320. Would correspond to 34.1 @ 960]

-Dave
 
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No leak-down

in either the pitot or static system. Pitot oriented as per plans-- high side up. Hope to run more tests tomorow but I'm suspecting the Dynon.

Thanks everyone.



Wayne
 
Homemade manometer

Brantel said:
It is very easy to verify the accuracy of th Dynon... Do a manometer test.
Click to expand...
Kevin Horton made a spreadsheet that converts airspeed to pressure.
http://www.tcwtech.com/airspeed%20conversion.xls
You can make a manometer out of 3/8" plastic tube bent into the shape of a "U". Be careful not to get water into the pitot line.
This article describes using GPS to accurately establish True Airspeed.
http://www.cockpitgps.com/other_articles/GPS_TAS/TAS_FNL4.pdf
Here is an article by Jim Weir describing calibrating the ASI.
http://www.google.com/url?sa=t&source=web&cd=2&ved=0CBgQFjAB&url=http%3A%2F%2Fwww.rst-engr.com%2Frst%2Farticles%2FKP89JUL.pdf&rct=j&q=%22jim%20weir%22%20manometer&ei=002kTKSDH4SAnwfS_YCRAQ&usg=AFQjCNEfa9rFMDCZDrttWekltkNh8SajAQ&cad=rja
Joe
 
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Pitot leak

I recently finished an Aventura HP and couldn't get more tha 39KIAS. I found a leaking pitot line fitting. You can check the system with a 3' piece of clear plastic tubing filled with about 10" of water to form a loop seal. Slip the end over the pitot tube and raise the free end to get a few inches difference in level of the loop seal. Clamp in place and watch your airspeed indicator. It should stabilize and hold for at least 5 minutes. If it begins dropping you have a leak.
 
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