True Stall Speeds

We got off the topic on "Un drooping droop tips" thread and got into stall speeds. Jerry Burr, Kase, and I did some stall tests and posted them. The test was to fly with the GPS north, south, west and east and average north / south and east / west to come up with a real stall speed.

Jerry in his modified J-3 averaged around 31 mph stall speed, Kase was around 43.5 mph and I was at 39.0 mph. It would be interesting to see what the rest of you come in at.

The test was run at 2000 rpm, no climb, full flaps. Two stalls in each direction. Just fly it down until it stalls and record the GPS ground speed, average them and report back the type of airplane, mods, etc that might have effected the outcome. Crash

Crash:

I gave it a go yesterday. 3,000 feet MSL, flights north, south, east and west at 2,000 RPM until stall from level, full flaps.

Average of the four: 37.5 MPH GPS.

I have a PA-18-150. Pretty stock except for VGs

Crash,

It seems to me, as an unlearned and unwashed one, aerodynamically speaking, that you'd want to run this test at idle thrust, to eliminate propeller differences. There is a substantial difference between the thrust delivered by an 82-41 prop and a 76-56 prop at 2000 rpm ( I think).

Not arguing here, simply asking the question. It seems that if you wanted to determine the stall speeds of various airplanes, idle thrust would remove more of the variables between designs and mods. Was there a reason that you chose 2000 rpm, other than that this is close to the rpm one might be holding near touchdown?

Also, other parameters would be helpful as well: where in the "stall" are you calling it stalled (like at the first buffet, when the nose comes through, etc), and, not to be insulting here, but is everyone using mph, vs knots. There are probably a few other parameters you might want to specify as well.

In any case, an interesting and worthwhile exercise.

Curious in Fairbanks,

MTV

Stewart,

That is in fact, part of my question. Also, the biggest variable is weight, as in all up weight, which I neglected to mention.

Note that Piper said the stall speed of the Cub, power off is 42 or 43 mph, depending on what source you look at and what year. That's at 1750 pounds all up weight. The Wipline 2000 pound conversion, on the other hand, specifies the stall speed at that weight as being 53 mph (or is it 52?).

Anyway, I do think this is a useful exercise, but I think you'd have to control the variables more, or at least specify parameters more for it to be even more useful, and fair.

Jerry's airplane, even though it's a different model (J-3) should have relatively the same stall speed as a PA-18, IF they were at the same weight and had similar mods, or had stock wings. Obviously, Jerry's airplane doesn't have stock wings, but my point is that the wing on a J-3 is, aerodynamically speaking, the same as the 18 (again, I believe).

If I'm reading the intent of this right, you're looking to evaluate the benefit of various mods???? If so, then you definitely need to specify weight.

MTV

Mike,
I deleted my post in order to rephrase it. Your response beat me.

For those that missed it, I implied that I could fly around all day at 2000 rpm in level flight and not stall. I assumed Crash to mean 2000' AGL, and reduce power to hold altitude until stall. My 12's not done so I can't report it, but I'm going to do it in my Cessna. I can't do it at 2000 rpm, though.

SB

When I did my test I wasn't at 2000 rpm. I just tried to fly as slow as I could and not loose or gain any altitude. Density altitude was around 7200'. Im assuming thats why my stall speeds were higher or else its my poor pilot technique. Stock wing with micro vgs.

Stall

Mike: I know there's a lot of variables as Jerry and I discussed on the other thread. I would say to fly light and do your best. I did mine at 2500' agl. Jerry mentioned that he tests at 2000 rpm, so that is where we agreed to run at. I flew it down until it would shake, and drop the nose. It's just a test for the fun of it. Get your Husky and join in. Take care! Crash

375handh said:

Crash:

I gave it a go yesterday. 3,000 feet MSL, flights north, south, east and west at 2,000 RPM until stall from level, full flaps.

Average of the four: 37.5 MPH GPS.

I have a PA-18-150. Pretty stock except for VGs

Click to expand...

Not bad, not bad at all. It would be interesting to have done this test before and after the VG's on the same day. If they make a 1 to 2 mph difference in your stall speed then they're well worth it. Crash

Stall

Hi Guys. Sorry for any mis-interpretation here. The goal is to fly as slow as possible at a constant altitude. For Crash and I it seems to be close to 2000 rpm. For you it may be different. Idle thrust is as far as I am concerned is something you do while getting in and out of the airplane. I (never) land in that mode. And I dare say anyone intending to land short or slow doesn't either. If you are flying as slow as you can and the VSI is centered, that is the speed we are looking for. Idle thrust in the air with a flat prop is like throwing the brakes on as compared to a cruise prop. If you don't believe it try a idle thrust descent and see who comes down the fastest. I talk in miles but can convert knots if that is what you prefer. The speed I would like to see is the slowest (flying) speed. If you are flying in ground effect and it stalls, you aren't flying. If it stalls in the air , you aren't flying. And the VSI isn't centered. Yes it's possible to fly around all day at 2000rpm. But not in slow flight. We're looking at the back side of the power curve not the front. If you would all prefer to use the stall break as the target speed that's great. It's easier to do, without the airplane bucking and bouncing. If any of you have the time, I would like to see a comparison of your altitude slowest speed and your ground effect slowest speed. As long as you test them the same way. Preferably level flight and just before the stall. And also what kind of wingtip you have. Crash really threw me a curve and would like to see more data to back it up. As far as weight is concerned just mention how heavy you were. Change in stall speed per weight is a easy formula. Sorry about addressing everybody in one shot here. Been gone. Jerry.

Jerry,

Thanks for the clarification. I understand now what you're up to. If the smoke ever clears, I'll give it a shot.

Dang smoke.

Did I mention the smoke?

MTV

3500 FT
30 GAL GAS
2000 RPM
STOCK WINGS W/VG'S

54.4 E
25.1 W
35.0 S
44.5 N

AVG 39.75

Slowest speed I saw on my GPS just above stall. maintaining level flight
ironically if I just avg N/S or E/W they come out identical.

S2D

S2D: Thanks for joining in with your numbers. Crash

mvivion said:

Jerry,

Thanks for the clarification. I understand now what you're up to. If the smoke ever clears, I'll give it a shot.

Dang smoke.

Did I mention the smoke?

MTV

Click to expand...

We got your smoke down in Anchorage and the Valley for a couple of days last week. Anchorage-ites were complaining it made their eyes all watery and their throat scratchy although it was only a blue haze. Did you feel sorry for us??? Crash

Crash,

If I could see you, I'd feel for you.

Better today, 4 miles vis. Still 1/16 mile in Central. If we start feeling sorry for ourselves, all we gots to do is look at Central weather. They've been all the way up to 1/8 mile at least once in the last three weeks or so.

I'd be on a respirator, or more likely, in Montana, if I were a Central type.

MTV

180 Hp, 8 ft flaps, power off, alt.2000ft,around 1750 lbs (+/- 30lbs) gross, temperature ?? but it was summertime , stall comes with 41.5 mls, with former 5 ft flaps same set up 43 mls. It does not sound much but the landingrun was always ever around 30 feet shorter.

Did the stall test today

Model PA 12 Stock with wheel paints,no flaps
Weight 1,580 lbs
one pilot
full fuel
Temperature 76 F at 2000 agl
RPM for steady stall and 0 VSI 1700 give or take
Avereage just before stall speed 40.25 kts gps.

Test was performed as specified, I was flying the buffet, and though it maybe the slowest just before stall speed.I wouldn't want to take my mind of what I was doing. Like a sneeze stall spin. But that's not the nature of the almighty 12.

J-5

Hi
I tried this 2 times and will do it once more.I am trying to get a baseline on speeds before trying VGs.
I am nose heavy with my 5 and i think this is hurting my speeds,I am having a hard time getting a break,she just starts to mush.
+6C @5000 ,1400lbs i came up with a average of 46 m.p.h.
This seems fast to me and i will try it again loaded the same as i was and then try it again with some weight in the back.
also my gps seems to have a bit of a lag to it and i was having fun trying to fly as slow as i could without it stalling while waiting for the GPS to stabilize on one speed.
What do you guys think,Does my speed sound to fast or about right?
Thanks,Aurele

Thanks for joining in. You're at 5,000 feet altitude and that will make a difference. Your numbers are most likely right in those conditions and not that bad. Take care. Crash

Two thoughts:

1) To allow direct comparison, these results really need to specify density altitude.

2) In any non-zero wind from any direction, arithmetic averaging of 4-way directional gps speeds always underestimates the actual true airspeed. The underestimation can be substantial depending upon the windspeed.

JimC

JimC said:

Two thoughts:

1) To allow direct comparison, these results really need to specify density altitude.

2) In any non-zero wind from any direction, arithmetic averaging of 4-way directional gps speeds always underestimates the actual true airspeed. The underestimation can be substantial depending upon the windspeed.

JimC

Click to expand...

I understand #1, but you'll have to explain #2 a little more for me Jim.

SB

Fly 360 record airspeed.
Fly 090 Record airspeed.
Fly 180 Record airspeed.
Fly 270 Record Airspeed.

Add all airspeeds up and then divide by four. This will average out your tailwind, crosswind and headwinds into a TAS.

Tim

Well, there is a standard procedure for computing true airspeed from 4-way gps readings, which involves vector artithmetic on all 4 legs. However, for a quick visualization of the problem, lets simplify the situation by saying that the wind is out of due north and that the 4 legs are due east-west and due north-south.

First discussing the east west legs, the direct north cross wind will cause the ground speed to be less than the airspeed when traveling east or west.

Second, discussing the north south legs, lets take an extreme example in order to accentuate the problem and assume that the plane is cruising at 100 knots and that the wind is doing 105 knots. Ground speed on the southbound leg is 205 knots, while ground speed on the north leg is -5 knots. You never complete that portion of the pattern -- completion time becomes infinite (if you don't intend to back up around the world .

In actuality, you always have a more general situation where the wind isn't aligned with any of your legs, so that you have a crosswind on all 4 legs so you have to do the vector artihmetic on all 4 legs and you find that groundspeed is less than airspeed on all 4 legs. When the wind is relatively calm it doesn't make much difference. But when the wind is strong, there is a substantial difference between arithmetically averaged 4-way ground speed and airspeed. Again, there is a standard procedure for doing the vector arithmetic required by 4-way runs, and even it is simplified a bit in that it usually assumes the wind velocity remains constant during the entire run.

All the best,
Jim

JimC
Only problem I have with your argument is if you do an actual into the wind/ with the wind computation. they should be accurate.
Lets take your example:
100 Kt aircraft in a 105 Kt wind.
Your readings on gps should be 205 Kt one way and -5K the other way.
205 + (-5) = 200/2 =100 K exactly what the aircrafts airspeed is.
The fact that we are never gonna get there has nothing to do with this computation.
I do understand how a crosswind computatin will skew the results.
Brian

Brian, that was my sense of humor showing through. For actuallity, read my last paragraph:

"In actuality, you always have a more general situation where the wind isn't aligned with any of your legs, so that you have a crosswind on all 4 legs so you have to do the vector artihmetic on all 4 legs and you find that groundspeed is less than airspeed on all 4 legs. When the wind is relatively calm it doesn't make much difference. But when the wind is strong, there is a substantial difference between arithmetically averaged 4-way ground speed and airspeed".

Let us take an example that is still somewhat simplified so that we only have to do the vector arithmetic on 2 of the 4 legs, where the wind is from due north at 30 knots, our airspeed is known to be 100 kts, and our 4 gps tracks are east-west and north south, beginning with west.

We have a wind vector toward 180 at 30 kts and our airspeed is 100 knots,

so our aircraft alignment on the first leg toward the west is 270 + asin(30/100) = 287 degrees 27 minutes 27 seconds, and our ground track is 270 degrees at a gps ground speed of (100^2-30^2)^0.5 = 95.3939 kts.

On our second leg toward the south we have a tailwind and our aircraft alignment is 180 at a gps groundspeed of 100 +30 = 130 kts at 180 degrees.

On our third leg toward the east, our aircraft alignment is 90 + asin(30/100) = 107 degrees 27 minutes 27 seconds, and our ground track is 90 degrees at a gps ground speed of (100^2-30^2)^0.5 = 95.3939 knots.

on our fourth leg toward the north, we have a headwind and our aircraft alignment is 0 at a gps groundspeed of 100 - 30 = 70 kts at 0 degrees.

So, if we arithmetically average gps speeds for all 4 legs, we get

(95.3939 + 130 + 95.3939 +70)/4 = 390.7878/4 = 97.697 kts
BUT, we already know that we were doing 100 kts on all 4 legs, so for this example the 4 way arithmetic average underestimated our airspeed by 2.3031 kts.

We will get analogous reduced estimates of airspeed for any wind direction and speed, and any choice of 4 way ground tracks.
All the best,
Jim

Crap, I can't ever get my typing fingers to do right.

On the third leg, when I wrote, "On our third leg toward the east, our aircraft alignment is 90 + asin(30/100) = 107 degrees 27 minutes 27 seconds, and our ground track is 90 degrees at a gps ground speed of (100^2-30^2)^0.5 = 95.3939 knots", it should have read "On our third leg toward the east, our aircraft alignment is 90 - asin(30/100) = 72 degrees 32 minutes 33 seconds, and our ground track is 90 degrees at a gps ground speed of (100^2-30^2)^0.5 = 95.3939 knots"

Should have been a minus sign after that 90 instead of a plus sign. The gps groundspeed on that leg, all the other numbers on all legs, and the final averagegps groundspeed and discrepency remain unchanged.

Oops and all that,
Jim

JimC: Jerry and I were not proposing that this test be done in strong wind or adverse conditions. Just pick a nice, fairly calm day and run four opposite directions for a distance and slow fly it to stall a couple of times in each direction. I average my east and west speeds and my south and north speeds then averaged the two. At first I divided by four and it came out a little different but not by much. Take care.. Crash

P.S. If you want to calculate that east-west gps groundspeed using a different technique, it is also equal to 100*cos(asin(30/100)) = 100*cos(17 degrees 27 minutes 27 seconds) = 100*0.953939 = 95.3939 kts, the same answer as before.
Jim

Crash said:

Thanks for joining in. You're at 5,000 feet altitude and that will make a difference. Your numbers are most likely right in those conditions and not that bad. Take care. Crash

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i thought altitude didnt effect IAS stall speed? Is that incorrect?

if it is correct wouldnt you adjust your approach speeds based on field elevation?

Crash’s test used GPS ground speeds, not IAS.

stewartb said:

Crash’s test used GPS ground speeds, not IAS.

Click to expand...

Exactly.