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Stall speed

J

jbowerman

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I am flying the initial 40 hours off an RV-6 and find the clean stall speed to be around 65 mph. According to specs it should be between 49 and 55 mph. It could be that the ASI is not calibrated, but if there is something else I need to be considering I would appreciate your input.

Regards,
Jim Bowerman
N42HC
RV-6
 
jbowerman said:
I am flying the initial 40 hours off an RV-6 and find the clean stall speed to be around 65 mph. According to specs it should be between 49 and 55 mph. It could be that the ASI is not calibrated, but if there is something else I need to be considering I would appreciate your input.
Click to expand...

My CLEAN stall speed is around 59 mph IAS. Full flaps is around 52.
What kind of static ports do you have? The standard "rivet head" style seem to give the most accurate results.
 
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jbowerman said:
I am flying the initial 40 hours off an RV-6 and find the clean stall speed to be around 65 mph. According to specs it should be between 49 and 55 mph. It could be that the ASI is not calibrated, but if there is something else I need to be considering I would appreciate your input.
Click to expand...

My airplane is on the heavier side; but with landing and a constant speed prop, I figure that I need to be very close to the runway at 60 kias. Because from there, the airspeed will decrease extremely quick, and it will drop out from under me if I don't watch it. Considering your numbers are in mph --- it's close to reality.

L.Adamson --- RV6A
 
Step one - check for a static system leak. A leak in the cockpit could cause the ASI to over read.

Step two - confirm you have Van's recommended static port in the correct location.

Step three - make a water manometer and check the ASI instrument error. See an old Jim Weir column for info, and you might also want this spreadsheet that converts back and forth between inches of water and ASI readings.
 
jbowerman said:
I am flying the initial 40 hours off an RV-6 and find the clean stall speed to be around 65 mph. According to specs it should be between 49 and 55 mph. It could be that the ASI is not calibrated, but if there is something else I need to be considering I would appreciate your input.

Regards,
Jim Bowerman
N42HC
RV-6
Click to expand...

Jim, where is your CG? An aircraft with a forward CG will stall at a higher airspeed than an aircraft with an aft CG.

If that isn't it, you may have a problem in the pitot/static system.

FYI, my airplane stalls at 50 knots indicated clean and at gross. At light weight (1350 lbs) it stalls at 44 knots indicated with full flaps and minimal power. At light weight, full flaps, and full power, it stalls at 42 knots indicated.
 
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There are many kinds of ASI errors

My airspeed instrument is a GRT EFIS. According to the results of my IFR certification last week, the instrument is as accurate as can be measured (integer read-out vs. decimal actual) or less than 1/2 kt error.

In reality, my IAS readings are often as much as 4 kts wrong and always higher than reality. Static leaking is not a factor. Static port configuration and location, of course, could be.

Here is a way to check your readings in the real world after eliminating sources as described above by Kevin Horton and others.

First, ensure your OAT is accurate. Next, fly at a low but reasonable altitude. Hold that altitude as precisely as you can. Reduce OAT error by performing this test at slow airspeeds first. (see Paul Lipps article in June "Experimenter". Now do as he did and find the highest and lowest GPS speeds and average them. Even better is the NTPS 3-way test that Kevin Horton uses, but that requires a computer with a spreadsheet. Do this for a few speeds. For each speed, compute density altitude and use it to convert your TAS to CAS. Of course, you noted your IAS's while you were doing all this. Now you know the IAS vs. the CAS.

Although not necessarily true it seems to me that the error in the system will be somewhat linear and you can project the error at stall speed. The fly in this ointment is that very high angles of attack introduce a new source of error, but, heck, you can't solve all the problems at once. You can always test your skills by flying this test method at 2 mph above your stall speed. Oh, btw, be sure to record your aircraft's weight during the tests as the stall speed will - you know - change with weight.
 
My money is on Kevin Horton on this one. A open static port adds 10 or 11 miles an hour to your indicated on an RV. Ask me how I know. :confused:
 
Thanks to all of you for the valuable information. I was very concerned about what seemed to be high stall speeds, but it looks as though I am in the ball park, but need to review the static and pitot installations plus do some air work.

Other than the stall speeds the RV6 flies great. The only real problem is on these hot days in Houston it tends to float a lot and not want to slow down if the approach speed is around 80 mph. I need to get it slowed down over the fence.

Best regards,
Jim Bowerman
 
Welcome, Jim

Some of us have also reduced idle RPM to 650 or so, to reduce the float.

80 MPH is ok for two aboard but 75 works better over the fence solo with a FP prop.

Regards,
 
Notes from the Field

While sitting in the right seat, I recently let a highly experienced DAR with no prior RV time fly my -6A, the first and only time I have ever ceded the left seat to anyone. My -6A is fitted with Cleaveland static ports. I told him to expect a benign, straightforward stall at 60 MPH IAS clean and 55 MPH IAS flaps down. The predicted stall behavior and numbers I cited are exactly what he discovered.

qqxtp5.jpg
 
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Indicated airspeeds on the lower end are notoriously inaccurate for comparison purposes between planes. Especially with analog instruments. Differences in construction, static ports, tubing, etc.. can make quite a difference on the bottom (and top ends) of the speed indications. We've done a LOT of these RV instruments panels over the years (as well as a lot of Transponder/Pitot Static Checks) and I wouldn't be concerned with your numbers at all. I get to fly a lot of them, and regardless of the indicated stall, they all feel about the same relative to engine RPM and speed over the ground at touchdown. Everything is relative when speaking about indicated airspeeds. It's almost fruitless to compare 2 RV's without identical (and I mean absolutely identical) construction, static ports, pitot tubes, etc..

Anyway, in the end everyones RV's seem to stall at about the same "speed" regardless of what the individual airplanes indicators say. If you were up in the 70's or down in the 40's then I'd start to think you have some drastic error. At the point you are at, I wouldn't dink with it at all.

My 2 cents as usual.

Cheers,
Stein
 
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