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Daver

Well Known Member
My newly flying -9A flies great (past 5 hrs).

No issues really except for one:

The power off full flap IAS shows about 60 MPH!!!:eek:

Calibration runs at cruise speed using GPS indicate the ASI is about right on.

Could my ASI be accurate at cruise AND read this high at stall?

The plane weighs 1044 empty and the stall tests were done at about 1350 or so. I was expecting, I dunno, maybe 50 MPH or less.

The plane flies trim and is a real joy otherwise. Easiest thing to land.

I think I'll construct a simple manometer and see what's going on with the ASI on the low end of the scale.

My pitot system checks good. Maybe the static system?

I'm racking my brain trying to think of something I might have done to make it stall this fast. Could it be?

Any ideas or suggestions welcome.

Thanks!

Dave
-9A flying
 
Dave,

Guaranteed its not really stalling at 60 unless you did something REALLY unusual. I'm guessing its something in your pitotstatic system, or a miscalibration somewhere (are you electronic or steam?). Is your pitot tube built to standard? If it's too short (close to the wing), you may get interference from the bow pressure wave. I don't know if this is more of a problem at lower speeds vs. higher.

No other immediate ideas.

greg
 
MCA exercise - GPS

You could do a little work with your GPS, confirming a GS in MCA? Consider it part of your phase 1? I know stall is IAS and probably where the error is, but it will give you feel for it. Rick
 
still trying...

Its a simple steam gauge panel. The ASI came from Van's (it had all the right colored arcs).

I'll just keep trying to "smoke it out" until I fix it.

I just can't believe its really stalling at 60.

BTW, the CG is well within the middle of the range.

Thanks for the ideas!

Dave
 
Dave,

What pitot tube do you have?

Also, it is possible that the pitot tube may not be pointed into the relative wind when at a high AOA, giving you a bad IAS at the stall.
 
Your GPS will sort it out for you

Do 6 runs and record the numbers. Do a 360* 120* and 240* run at your desired altitude. Stall the plane and record the number at each heading. Do it all over again and record your numbers. Add them all up and divide by six and you will have your stall speed number.

Steve Barnes "The Builders Coach"
 
Anything peculiar about static port placement or design? Remember that the asi reads the pressure differential, so a low pressure at the static port will cause a high indicated airspeed.
 
Sounds too fast

What was you density altitude when you conducted the test?

I have a 21.5 ft wingspan RV-6A that is heavy and at ~1800 lbs. during the BFR at 4,000 ft last Saturday the CFI asked for a clean stall. 58 kts is where it occurred.

Bob Axsom
 
ASI

Hi

We found that high stall speeds on our 9 were actually the ASI reading 10mph too high across the whole flight envelope. After looking at the guage on the ground you could see the needle was displaced to the right so that it over reads 10mph. In fact its doing 10mph when its standing still its so fast :)

Ugh then you realise your 150 mph cruise is really 140......I will get round to re setting that needle some day but its only a number, so long as you know your aeroplane will be reading 60 at stall then keep the speed up on the approach ... no problem.


Well done with the 9
 
-9A IAS stall speed

My -9A stalls 60 IAS mph clean; 55 mph IAS 20 deg flaps; and 50 mph IAS full flaps.
 
armchair test pilot #274

I thought my -9a stall was not unusually fast, but seemed different each time...no doubt my technique mostly.
Anyone think that the elevator is stalling first, the nose drops, just like a 'real' stall? The main wing never reaches its critical AoA, or max lift, or whatever.
some simple runs of yarn and tape would tell the story here!
( something I need to do too!)

I've whacked the pitot tube with my head as well, so I've no doubt it is in a different postion than the last time I did stalls!.....a good reason to use a mast of some kind.
good luck!
 
ASI error

Well, I think I got to the bottom of my problem.

I constructed a manometer and checked IAS vs inches of water.

Sure enough, my Van's ASI reads about 10 MPH high. Check it at 40 kts - should have taken just over 1.04" of WC to show 40 kts but it only needed about 0.85 inches.

Didn't check the high range but I think I'll just get a new ASI.

The things you encounter in Phase I:rolleyes:

Dave
-9A
 
0.85" of water at 40 kt is about a 4 kt error. The error might be quite different at other speeds, as I found when I checked my analog ASI many years ago:


I rechecked the analog ASI error several years later and got very similar results.

My EFIS ASI was much more accurate, but even it seemed to have some error at higher speeds:
 
wrong table?

The table I was using for my manometer check shows 40 kts = 1.0469"

Is this wrong?

If its correct, 0.85" (for 40 kts) yields an error of closer to 20% or about 8 kts (about 10 mph).

At any rate, I think this error is my problem.

New ASI on the way!

Thanks all for your responses. This is way too much fun:D

Dave
-9A flying
 
Just as an FYI, my calibrated pitot static test box has a precision airspeed indicator as well, it's pretty easy to hook up and check the AS indicator for accuracy. In fact if somebody needs this done for a quicky check, I would be happy to do it no charge :)
 
Walt said:
Just as an FYI, my calibrated pitot static test box has a precision airspeed indicator as well, it's pretty easy to hook up and check the AS indicator for accuracy. In fact if somebody needs this done for a quicky check, I would be happy to do it no charge :)
Click to expand...
Walt, check your PM's.
 
IAS vs CAS vs TAS

I have a POH for a Diamond DA20. One of the tables is Indicated Airspeed vs Calibrated Airspeed. In a clean configuration, it reports the following:

KIAS 42 50 60 70 80 90 100 110 120 130 140 150 160
KCAS 52 58 66 75 83 92 101 110 120 129 138 147 156

IAS = CAS at normal cruise speeds (110-120 knots). At slower speeds IAS reads low and at higher speeds it reads high. As I undertand it, this is at least partially due to the angle at which the pitot tube points into the airstream. The airspeed system is calibrated so the IAS will be equal to CAS in the range that is desired.

I wonder if a similar situation happens with Van's airspeed system. Perhaps there is a range where IAS equals CAS. Outside that range there will be predictable error. If so, this may be what you are seeing.

Note that using GPS runs in different directions will give you something closer to true airspeed. Diamond's notion of Calibrated Airspeed is a bit different. It is "Indicated speed corrected for installation and instrument errors. CAS is equal to TAS at standard atmospheric conditions at 0 MSL".

In additon, True Airspeed is the "speed of the airplane relative to air. TAS is CAS corrected for altitude and temperature".
 
Daver said:
The table I was using for my manometer check shows 40 kts = 1.0469"

Is this wrong?

If its correct, 0.85" (for 40 kts) yields an error of closer to 20% or about 8 kts (about 10 mph).

At any rate, I think this error is my problem.

New ASI on the way!
Click to expand...
You are quite correct that 40 kt should be seen at 1.04" of water. My points were that your ASI only had about 4 ft of error, as .085" of water should give 36 kt. And, the error might be quite different at speeds of interest.
 
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