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High stall speed

Flying Canuck

Well Known Member
Patron
I did another stall test yesterday in my 9A. It broke at 53KIAS, same as my previous tests. The book stall speed at gross is 50 mph (43kt) or 44 mph (38kt) at solo weight. I would have been at about the halfway point between solo and gross. That means I'm stalling 12kt higher than the book suggests - that seems like a lot. Should I be concerned? Is there anything I can do?

My best approach speed and best glide speed look to be at about 70KIAS and both work well. I touch down at 65 KT without any floating. I don't lift off much below 60. These impact my landing and takeoff distances, but that's not a concern anywhere I'm going to fly.

Anyone else run into this with their -9A?
 
Airspeed readings can have errors due to the location of the static port. My RV-6 stalls at 38 KIAS. That is lower than what Van's Aircraft publishes. My error is attributed to my static port location.
 
It sounds like you have a pitot and/or static error.

My -9 stalls around 48 knots when heavy.

I don't recall where it stops flying when doing accelerated stalls.

I fly the entire pattern, with flaps down, at 55 knots when light an 60 knots when heavy and am way above stall at either speed.

How do your speeds compare to your GPS in no wind conditions?
 
If it were me, I'd look for instrument calibration error before I started thinking there was something wrong with the airframe.

I've seen a lot of certified airplanes where the indicated airspeed isn't very accurate when you get into the imminent stall neck of the woods.
 
I haven't analyzed my logs completely yet to check the pitot/static calibration, but I'll do that soon. On yesterday's stall, my calculated wind was nil (not sure at what point wind is displayed, but it was definitely under 5 kt). At point of stall the TAS was 59 and GS was 61. Not much difference there, nowhere near 12 kt and out in the other direction. On my 5 landings yesterday, the groundspeed - adjusted for calculated wind - matched my TAS exactly at or near touchdown every time.
 
Me too

Mine reads about the same as yours. Flaps up about 55KTS, flaps down about 52KTS. I just assume it?s an indication error. My IAS is about 5 KTS high at all speeds.

-Andy
 
+1

I added a little fence to my static ports to resolve an error (ASI was reading high) at cruise speed and lowering the indicated cruise raised my "indicated" stall speed by 8 MPH. It seems the errors get more pronounced at the lower end of the range, from my experience. The key is testing to determine "your" indicated stall speed.

Larry

If it were me, I'd look for instrument calibration error before I started thinking there was something wrong with the airframe.

I've seen a lot of certified airplanes where the indicated airspeed isn't very accurate when you get into the imminent stall neck of the woods.
 
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I just realized that I might be making an assumption that gives me a false impression that I don't have a calibration error. The Dynon Skyview display computes a wind vector and I didn't consider the fact that this is probably calculated by comparing the air data derived TAS, compass heading and GPS ground speed and track. Thinking about it now, I don't know how else they'd calculate it. This would mean that it's using the error to make the calculation and there is nothing in my raw logs that will tell me how the calibration is.

I guess I need to go up and do an old school dead reckoning calibration run at a few different indicated airspeeds to get a handle on the error.

What I'm a little worried about is that this means that my already low cruise speeds are not even as good as I thought.
 
It would be REALLY hard to build one of these that stalls 12 knots higher than the fleet. It?s probably an indication error at the low speed or a static leak.

If you don?t have th tools to check for a static leak, a simple flight check is easy. Take somone with you to do this test. Set you altimeter to ZERO when you are ready to depart the runway. Then come back and fly down the runway at 20?-50?, whatever you are comfortable with. Have the safety pilot read your altimeter. It should be close to your AGL altitude. If it reads higher than that, you have a leak.

Vic
 
...What I'm a little worried about is that this means that my already low cruise speeds are not even as good as I thought.

Not necessarily. It's likely that if you have an instrumentation error it's not linear. The high angle of attack near stall causes relative airflow to hit the pitot tube at a much different angle than in cruise. Likewise, you have spiral slipstream stuff going on and probably other factors as well.
 
Pitot errors

Like everyone else I doubt you have built an aeroplane that is not very close to Van?s data, this is probably a pitot error. I had a leak in my system which me@nt I was landing at much higher speeds than I thought.

4 ways you can check, do stalls in four different directions looking at you GPS readout.

have someone with a radar speed gun check your speed on approach.

Alter the angle of attack of your pitot see what effect that has.

Fly in formation with a buddy compare ASI readings.
 
Like everyone else I doubt you have built an aeroplane that is not very close to Van?s data, this is probably a pitot error. I had a leak in my system which me@nt I was landing at much higher speeds than I thought.
4 ways you can check, do stalls in four different directions looking at you GPS readout.
have someone with a radar speed gun check your speed on approach.
Alter the angle of attack of your pitot see what effect that has.
Fly in formation with a buddy compare ASI readings.

Actually the angle of the pitot has to be off a lot to alter airspeed reading.
 
Thanks for all of the input here. The link that DanH provided is awesome - I'll digest that and plan a test flight to gather the data. I've got data from previous flights that I can use in the spreadsheet, I did a few flights using my autopilot that should meet the required flight precision.

I really didn't think that I'd found a way to raise the actual stall speed without significant redesign. I did have my pitot/static system ground tested just ahead of my final inspection at the end of June - it was leak free. I don't believe there have been any changes to my indicated speeds thoughout my test phase, so a static leak may not be all that likely. We'll see what the data reveals. Then we'll see what can be done to fix the errors - if anything.

Thanks again.
 
Calibration error

To those that have calibration concerns, would you be willing to post the type and placement of your static port? Is it the standard Vans rivet or something else?
 
I initially had the Van's rivet based ports in the plans position - 2" below the longeron rivet line, 11/16" forward of the F-708 bulkhead rivet line. Before completing the fuselage, I moved to the SafeAir1 ports and located them an inch (I think) forward of the Van's ports which were left in place. I am, of course, plumbed to only the SafeAir1 ports.
 
Vans design.

I initially had the Van's rivet based ports in the plans position - 2" below the longeron rivet line, 11/16" forward of the F-708 bulkhead rivet line. Before completing the fuselage, I moved to the SafeAir1 ports and located them an inch (I think) forward of the Van's ports which were left in place. I am, of course, plumbed to only the SafeAir1 ports.

Would it be possible to plumb to the original spot and rivet? The reason I ask is location and type can make a difference. While I was building, I noticed a trend on VAF where those whom modified the original static design seemed to have an increase in calibration error. Your aircraft would provide some interesting data if you could use the original rivet. The data may not be perfect since you installed ports forward of the original rivet, but may still be interesting.

These links may help
http://www.vansairforce.com/community/showthread.php?t=153831&highlight=Static+calibration
http://www.vansairforce.com/community/showthread.php?t=145686&highlight=airspeed+error
 
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I might be able to relocate the new ports to the same location as the Vans ports, but that wouldn't be pretty or easy. Might be necessary though. The new ports are installed in a good sized hole that I'd have to patch.

I don't think re-plumbing to the vans ports would be easy either, the tubing is a different diameter so I'd have to come up with an airtight adapter.

In either case, it would require prying my oversized self back into the aft fuselage, something that I hoped to never do again. I guess we'll see what sort of errors I actually have.
 
Before you do anything else do a low and over the runway to check your altimeter as Vic suggested. A pitot static check can be perfect on the ground but errors often occur with the position, and or how far in or out the static port is from the fuselage skin. A litle “dam” before or after the port opening can change the results. Play with a few variations until the runway flyby test is close.
 
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GPS

I would check your IAS against GPS ground speed on a dead calm day.
I had an issue with a bad analog airspeed indicator years ago, and GPS speed helped me diagnose it.
 
Thanks for all of the input here. The link that DanH provided is awesome

Kevin is awesome.

I would check your IAS against GPS ground speed on a dead calm day.

The three-leg groundspeed and track method is more accurate, and you don't have to wait for a calm day. Actually, it calculates the wind too. Just set altitude hold and heading mode, record track and GS after stabilizing on three different headings, then plug the recorded values into the spreadsheet after you land. You'll find links to several versions on Kevin's webpage.

Again:
https://kilohotel.com/rv8/index.php...ew=article&id=1100&catid=19&Itemid=138#method
 
Flying Canuck;13051 said:
...
I moved to the SafeAir1 ports and located them an inch (I think) forward of the Van's ports which were left in place...

I got lucky and surface mounted my SafeAir ports because of reports of bad readings when flush mounted. In 11 years I have not had any issues with this setup.


(Click to enlarge)
 
Kevin is awesome.



The three-leg groundspeed and track method is more accurate, and you don't have to wait for a calm day. Actually, it calculates the wind too. Just set altitude hold and heading mode, record track and GS after stabilizing on three different headings, then plug the recorded values into the spreadsheet after you land. You'll find links to several versions on Kevin's webpage.

Again:
https://kilohotel.com/rv8/index.php...ew=article&id=1100&catid=19&Itemid=138#method

I plugged in numbers from a flight a couple of weeks ago, the only problem with that flight is that my legs were only 45 degrees apart. The spreadsheet worked great and told me both altitude (55 feet) and airspeed (3.8 KT) error. Nice to have those numbers but I'll go and fly a few runs with 90 degrees between legs to get better numbers. It certainly looks to be a simple process and I can get all of the numbers after the flight from my data logs and concentrate on the flying.

I did notice from the logs of my last flight that my pressure altitude was dropping by about 15' while I was accelerating for takeoff, I saw from an old thread that this is not reliable because of ground effect. I'll throw in a couple of low and over runs to check my static system as well.
 
Keep in mind that a properly installed (but standard) pitot system will read lower than an ideal airspeed indicator at low speeds, due to the non-zero angle of attack on the pitot tube. Low speed comparisons should show IAS lower than GPS speeds, in no-wind conditions.
 
I am rather surprised by that.

Why?

When I bought those, others were having errors when mounting on the inside because they were flush with the skin. One solution was to glue a big rivet head with a hole in it over the static hole.

This method gets it up, out if the boundary layer, about the same height as the big pull rivet.

Note, the current production static ports seem to be longer than mine and should be mounted on the inside.
 

Because it is wrong to think that because a flush static port doesn't work, that any amount of protrusion will.

Air passing around an object can have varying pressure distribution around that object (think of a wing section). Just because an object doesn't have an aerodynamic shape, doesn't mean it doesn't have a variable pressure distribution with air flowing at high speed around it.

The hypothesis that flush ports don't work because the port surface is below the boundary layer is likely valid, but it can't be assumed that the flow disturbance produced by an irregularly shaped static port will have correct static pressure at the location of the opening. Based on my experience, my guess would be that in this case, it wouldn't.

Maybe you have done some detailed flight testing to prove they work. My primary concern (and reason for posting) is to get people to think twice regarding casual decisions with the static ports.
There is a lot more to it than TLAR...........
 
Because it is wrong to think that because a flush static port doesn't work, that any amount of protrusion will.

Air passing around an object can have varying pressure distribution around that object (think of a wing section). Just because an object doesn't have an aerodynamic shape, doesn't mean it doesn't have a variable pressure distribution with air flowing at high speed around it.

The hypothesis that flush ports don't work because the port surface is below the boundary layer is likely valid, but it can't be assumed that the flow disturbance produced by an irregularly shaped static port will have correct static pressure at the location of the opening. Based on my experience, my guess would be that in this case, it wouldn't.

Maybe you have done some detailed flight testing to prove they work. My primary concern (and reason for posting) is to get people to think twice regarding casual decisions with the static ports.
There is a lot more to it than TLAR...........
Thanks Scott. That is a well thought-out and reasonable response.

Early on, I did a lot of speed and RoC verification test flights (11 years ago) and found it to be spot on. When I replaced the original O-290D2 (great engine for the -9, BTW), I again did a bunch of test flights, more than I probably needed to and it was accurate

(Addition: I mounted these in the recommended position.)

Just dumb luck, I guess.

I am curious to know if others have done the same thing.
 
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Static port location

Methods for accurately determining airspeed error are well established and particularly easy now with GPS, and in addition altitude error can be determined with such methods as those described in Kevin Horton's excellent work, already referenced in this thread. Pitot errors are rare with established tubes such as those from Garmin or Dynon. But the determination of where to locate static ports and the understanding of the effects of mechanical aspects - shape and installation method - are poorly understood and subject of conjecture. In the certified world, the determination of static port location is typically a matter of substantial trial and error.

Faced with a static port location dilemma on my plane and not wanting to drill numerous experimental holes, I came up with a new method of finding the best port location, along with another method of determining altitude and airspeed error. It's in a Kitplanes article, October 2017, link below:

http://www.kitplanes.com/issues/34_...ocation-and-Altitude-Calibration_21924-1.html

If you are interested and can't get it from the link, let me know and I will send you a .pdf of it.

Reinhard Metz
[email protected]
 
...
Faced with a static port location dilemma on my plane and not wanting to drill numerous experimental holes, I came up with a new method of finding the best port location, along with another method of determining altitude and airspeed error. It's in a Kitplanes article, October 2017, link below:

http://www.kitplanes.com/issues/34_...ocation-and-Altitude-Calibration_21924-1.html

If you are interested and can't get it from the link, let me know and I will send you a .pdf of it.

Reinhard Metz
[email protected]
You need to log in to KitPlanes to open the link.
 
Methods for accurately determining airspeed error are well established and particularly easy now with GPS, and in addition altitude error can be determined with such methods as those described in Kevin Horton's excellent work, already referenced in this thread. Pitot errors are rare with established tubes such as those from Garmin or Dynon. But the determination of where to locate static ports and the understanding of the effects of mechanical aspects - shape and installation method - are poorly understood and subject of conjecture. In the certified world, the determination of static port location is typically a matter of substantial trial and error.

Faced with a static port location dilemma on my plane and not wanting to drill numerous experimental holes, I came up with a new method of finding the best port location, along with another method of determining altitude and airspeed error. It's in a Kitplanes article, October 2017, link below:

http://www.kitplanes.com/issues/34_...ocation-and-Altitude-Calibration_21924-1.html

If you are interested and can't get it from the link, let me know and I will send you a .pdf of it.

Reinhard Metz
[email protected]

I'm a subscriber and am logged into the magazine's website but for some reason it won't let me view the article despite having activated web access, etc. I would like to have a copy of the article, if you don't mind. I'm sure I read it at some point but I look forward to being reminded!

Lars Pedersen
[email protected]
 
Static port article

Sent via email. Anyone else not able to get it on the Kitplanes portal, let me know.

Reinhard Metz
 
I am rather surprised by that.

Perhaps not if one does not look for accuracy. My old static port was flush surface and I thought it was all good till I decided to check the accuracy of my airspeed. I end up pulling the flush head port and replacing it with similar to VANs port which proved the most accurate one.
 
Static port placement test

The very first thing that has to be verified is the static port design or placement before anything else can be verified...
All the flying different headings, calculating speeds, and stall speed indications etc is for naught until the static system is determined to be correct...
The ONLY possible way to do that is the way Vic Syracuse suggested on his 11-26-18 post. Until that is done, nothing else means anything,
including knowing that you are at the altitude that your altimeter and encoder, are telling you and the controllers.
I have built several RV's and have found that Van's static port (a rivet) and it's placement is usually very close....
 
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The very first thing that has to be verified is the static port design or placement before anything else can be verified...
All the flying different headings, calculating speeds, and stall speed indications etc is for naught until the static system is determined to be correct...

The ONLY possible way to do that is the way Vic Syracuse suggested on his 11-26-18 post. Until that is done, nothing else means anything,
including knowing that you are at the altitude that your altimeter and encoder, are telling you and the controllers.
I have built several RV's and have found that Van's static port (a rivet) and it's placement is usually very close....

This is not true. In fact the way it is done commercially at some cost is to drag a probe that is known to deliver exactly the static pressure at the altitude flown. That is then compared to the pressure delivered by a static port, and ports get tried in multiple locations until one matches the probe.

Vic?s approach is good to determine if your altitude indication is correct, as long as you are satisfied with the amount of error included in your estimate of height over the runway v.s. the actual height. Further, while it may include a leak as a possible cause of error, that is not the only possibility. The port may inherently be in the wrong place and picking up a higher or lower pressure than actual static pressure at that altitude, due to the air flow over the fuselage.

It is however true that static port and associated altitude indication right is imperative to start with, as airspeed then uses both static and ram pressure to determine airspeed, so if altitude is wrong, then speed is pretty much guaranteed to be wrong.

Also, if you fly a triangulated gps course and calculate airspeed from that, that WILL be correct, and if it does not match true airspeed shown, then the static port is most likely incorrect, and thus also the altitude. (Since the pitot ram is seldom off)

Finally, if the Van?s ports indicate the right airspeed, then altitude is likely correct,and it?s because the port location and any aberrations due to rivet shape just happen to deliver a correct representation of the actual static pressure at that altitude.

Reinhard Metz
 
Test results

Finally managed to find a good day to do some test runs.

I did the static leak test that Vic described - couldn't set my altimeter down to 0 (the field is just under 3000'), so I set it to 1000' and took it around for a low pass. Altimeter read 1080 for the whole pass, that seemed high to me as I didn't think I was 80' AGL. Looked at the GPS altitudes for the same point, on the ground and on the low pass. That showed 114' difference, I really don't think I was that high. Obviously I'm not a great judge of height, I thought I was about 50'. The GPS altitude on the ground matches my published field altitude exactly, so I suppose I should trust it to be close. This suggests (to me) that I don't have a static leak since my altimeter was reading a little low, and not high.

I then proceeded to do 3 good runs for the calibration spreadsheet from Kevin's site. I did runs with an IAS of 126 and 116 and then a low power 2/3 flaps run at 71 KTS using 90 degrees between legs and my autopilot to maintain altitude. The results showed CAS errors of 4.5, 2.7 and 1.4 KTS (from high IAS to low). Altitude error decreased with IAS as well, 61, 33 and 10 feet.

Those numbers don't seem all that bad to me, and the run closest to stall speed only showed 1.4 KTS, that doesn't account for the 10 KT that I'm stalling above the book numbers.

Anyone able to clarify this for me?
 
Claude,

For the coarse check (high speed, low altitude pass), set the altimeter to read known field elevation (threshold elevation off an approach plate or taxi diagram works). This is the QNH setting, not QFE (altimeter reads zero) . The pass should be flown at WOT and Vmax (as fast as the airplane will go, not to exceed published limits) at the lowest altitude you’re comfortable. Altimeter reading should be very close to what you read prior to takeoff during the pass if your system is tight.

If you email your (Kevin Horton) static source pressure error spreadsheet, I can take a look and may be able to offer some suggestions. During our AOA flight test work, we’ve found the SkyView pressure sensors to be tight (i.e., almost no instrument error). Your raw data (csv or excel format) for the test runs will also help. IAS error increases as stall is approached (coefficient of pressure dirived AOA accuracy actually increases in the same regime) for a typical installation.

Static port configuration and or leak will effect CAS error. Every airplane is different. Also, any speed analysis depends on an accurate gross weight. It’s possible to bias flow around the static port with washers, fences etc. to adjust IAS.

PM or drop an email to vacf15 at yahoo if interested. Happy to share lessons learned :)

Cheers,

Vac
 
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