Determining Vspeeds

OkieAviator

OkieAviator

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OkieAviator
Today was a semi calm day and I decided to use that to get up and try to get my Vso. My plane is a new to me 1977 172N 180 (Penn Yan) conversion, with a Sportsman STOL kit and Flap Seals.

My plan was to get slightly above 4500 and slow flight a bit and do some power off stalls. Using WingX's new "BlackBox" feature along with watching my GTN 650 I tried to determine where Vso was.

So trying to get a direct crosswind and holding around 4500 my IAS was almost none exsistent.. maybe 35, not sure. Looking at blackbox deal I was a solid 46 GS before I completely pulled the power and let it stall out.

I guess my question is does it make sense to use the 2% increase of GS per 1000 ft to try to rough in Vso a bit? so at 4500 (3300 AGL), would this be about 43 Knot for Vso?

Any better way to do this?
 
I'm not sure what you are trying to accomplish. You get the stall speed slowing down progressively while raising the nose to maintain level flight. Once you hit the critical angle and the plane stalls, you have the stall speed. It is important that you are not climbing or descending in all this.

If the stall speed is low enough that the airspeed indicator is no longer reading, well that is a problem.

What are you going to do with the information? You can't change the limitations of the aircraft unless that is supported by the STCs for the STOL/flap kits installed.
 
You're doing it wrong silly :)

Slow to a stall as you normally would. Note the indicated airspeed where it stalls both with flaps up and with flaps 30. That's your stall IAS (duh).

Open the POH.
Convert that stall IAS to CAS
Multiply the result by 1.3
Convert the result from CAS back to IAS
That is you're new vRef for the given configurations and weight.
Actual speed should be vRef + half the gust factor

So lets say you stall at 44kts IAS. The POH coverts to say 52kts CAS. 52 * 1.3 = 67kts CAS. Now convert 67kts CAS back to IAS for a vRef speed of maybe 63kts (All made up numbers).

This is much more precise than the usual POH loose range of speeds. But remember, it is for the gross weight at the time. I suggest you do it while really light, really heavy, and typical. That will give you a good idea of how it will behave under the full range of conditions. It may only be a 3 knot difference in either direction, but someday those 3 knots might be critical.
 
Just trying to figure out at what speed the plane stalls. The airspeed indicator is showing well below 40. Which is why I looked at the GS and made the assumption I had no wind affecting it.(Probably bad assumption)

My end goal is to know if I should approach to land at 60, 55 or something lower...

Oh in regards to the POH, isn't that a bit worthless since the plane now has a 0-360-A1A in it?
 
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OkieAviator said:
Oh in regards to the POH, isn't that a bit worthless since the plane now has a 0-360-A1A in it?
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Only if you approach at high power.

At idle power, it's a weight.

You have an STC. If it makes a difference to the book stall speed limitation, you should have a POH mod to comply with the STC.

Having flown a couple of 172Ns with STOL kits on them, they don't make very much difference. They do look cool.

Do the 1.3 Vs0 (in CAS) as was mentioned earlier, but don't be too surprised if you come up with something pretty close to the 60 KIAS you already have for a short field approach.
 
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OkieAviator said:
Just trying to figure out at what speed the plane stalls. The airspeed indicator is showing well below 40. Which is why I looked at the GS and made the assumption I had no wind affecting it.(Probably bad assumption)

My end goal is to know if I should approach to land at 60, 55 or something lower...

Oh in regards to the POH, isn't that a bit worthless since the plane now has a 0-360-A1A in it?
Click to expand...

Do the test flight and calculations exactly as I described and all your questions will be answered.

The STOL Kit and 180hp engine do not effect your ability to read the airspeed indicator. And it doesn't change the IAS-CAS conversion table in the POH. See what number you get and do the process described above.

Remember IAS does not equal CAS does not equal GS. GS is meaningless for this. It is all about IAS to CAS conversions as described above.
 
MAKG1 said:
Only if you approach at high power.

At idle power, it's a weight.

You have an STC. If it makes a difference to the book stall speed limitation, you should have a POH mod to comply with the STC.

Having flown a couple of 172Ns with STOL kits on them, they don't make very much difference. They do look cool.

Do the 1.3 Vs0 (in CAS) as was mentioned earlier, but don't be too surprised if you come up with something pretty close to the 60 KIAS you already have for a short field approach.
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Okay, didn't know how much the STOL Kit really changed things. I'm going to take a CFII I know up with me who weights about 280. Maybe I can get it to stall at a higher speed and he can watch and record that. Then we'll do the CAS calculation and see where we're at. In the mean time I'll keep approaching at 60.
 
The approach speeds I used in my 150 that had a Horton STOL kit were substantially slower than published in the original POH, but then I always felt the POH speeds were too fast for the stock airplane...YMMV.

The problem you're running into with airspeed indication, though, is largely due to position errors with the pitot tube. Doing stalls at max weight will probably still result in "unreadable" airspeeds that can't be converted to CAS to compute a 1.3Vs approach speed. I just adjusted speeds until the airplane landed the way I wanted it to, and converted up to max gross weight.
 
OkieAviator said:
Okay, didn't know how much the STOL Kit really changed things. I'm going to take a CFII I know up with me who weights about 280. Maybe I can get it to stall at a higher speed and he can watch and record that. Then we'll do the CAS calculation and see where we're at. In the mean time I'll keep approaching at 60.
Click to expand...

Make sure you run a W&B with a 280lb CFI. I have not idea what you weigh or if you have rear ballast,mets so YMMV
 
OkieAviator said:
Today was a semi calm day and I decided to use that to get up and try to get my Vso. My plane is a new to me 1977 172N 180 (Penn Yan) conversion, with a Sportsman STOL kit and Flap Seals.

My plan was to get slightly above 4500 and slow flight a bit and do some power off stalls. Using WingX's new "BlackBox" feature along with watching my GTN 650 I tried to determine where Vso was.

So trying to get a direct crosswind and holding around 4500 my IAS was almost none exsistent.. maybe 35, not sure. Looking at blackbox deal I was a solid 46 GS before I completely pulled the power and let it stall out.

I guess my question is does it make sense to use the 2% increase of GS per 1000 ft to try to rough in Vso a bit? so at 4500 (3300 AGL), would this be about 43 Knot for Vso?

Any better way to do this?
Click to expand...
GS is not only irrelevant but a mistake to use. Use the book numbers, that's why they're published in the POH.
 
murphey said:
Use the book numbers, that's why they're published in the POH.
Click to expand...

Trouble with the book numbers is they're not exactly accurate for the modded airplane, and the mods basically say "your performance will be the same or better than The stock airplane."

While it doesn't deal with approach speed directly, there is a method to determine the effect of the mods on other performance parameters...
http://www.avweb.com/news/airman/182410-1.html?redirected=1
 
All I care about is a consistent IAS reading so I can fly slow approaches with a suitable margin above stall. The actual value is unimportant.
 
Pedals2Paddles said:
Do the test flight and calculations exactly as I described and all your questions will be answered.
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I think you left out one step. Although it's hopefully implicit, if he's looking for stall speed on approach to landing, he should (changing your post)

Slow to a stall as you normally would in an approach to landing descent. Note the indicated airspeed where it stalls both with flaps up and with flaps 30. That's your stall IAS (duh).
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Level stall speed will be different than descending stall speed due to higher AoA.

And of course, the number he gets is only valid for the weight at the time of the test.
 
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Level stall speed is different than descending stall speed? The only way your stall speed should vary is with differential power. At stall you're behind the power curve. Pitch controls speed, thrust controls altitude.
 
What you really are looking for is an Angle Of Attack instrument. It shows where you are in relation to the stall at any weight, density altitude, speed.
 
Stewartb said:
Level stall speed is different than descending stall speed? The only way your stall speed should vary is with differential power..
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The only way your stall speed should vary is with angle of attack (the religious pitch vs power debate is irrelevant).
 
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Whatever you say. Since I live and operate in a predominantly STOL environment dominated by big-tire Cubs and Skywagons who routinely land at high AOA with power on? I'll respectfully disagree. Carry on.
 
Stewartb said:
Whatever you say. Since I live and operate in a predominantly STOL environment dominated by big-tire Cubs and Skywagons who routinely land at high AOA with power on? I'll respectfully disagree. Carry on.
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Are you sure they are at high AOA or high pitch angle? The two are not synonymous.
 
Stewartb said:
Level stall speed is different than descending stall speed? The only way your stall speed should vary is with differential power. At stall you're behind the power curve. Pitch controls speed, thrust controls altitude.
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Removing instrumentation error, yes the speeds are different. In a descending stall the AOA is higher due to relative flow of the wind impacting the surface of the wing.

Stick and Rudder argues the concept of impact force of the wind on the wing contributing the lift, which differs from what we learned in groundshool about Bernoulli's principle. IIRC it's in that section where descending stall and AOA is discussed.
 
Jaybird180 said:
Removing instrumentation error, yes the speeds are different. In a descending stall the AOA is higher due to relative flow of the wind impacting the surface of the wing.

Stick and Rudder argues the concept of impact force of the wind on the wing contributing the lift, which differs from what we learned in groundshool about Bernoulli's principle. IIRC it's in that section where descending stall and AOA is discussed.
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That's otherwise a really good book, but if it's arguing "impact force," it's wrong. Impact force models don't stall.

The part you and Mark are missing is that the lift is pointed some 4 deg further forward by the different pitch angle, usually with thrust reduced to compensate. Denker's book has a nice discussion of force balancing in other than straight and level configuations.
 
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Someone should make a pitot tube that rotates into the correct alignment during a change in AoA, then you don't run into CAS issues.
 
EdFred said:
Someone should make a pitot tube that rotates into the correct alignment during a change in AoA, then you don't run into CAS issues.
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Why? It's far simpler to install a AoA indicator.
 
midlifeflyer said:
The only way your stall speed should vary is with angle of attack (the religious pitch vs power debate is irrelevant).
Click to expand...

Stalls occur at the same AoA regardless, but stall speed does vary with power due to a number of factors. Plane stalls at a slower speed with power on
 
EdFred said:
Someone should make a pitot tube that rotates into the correct alignment during a change in AoA, then you don't run into CAS issues.
Click to expand...

The pitot tube on my Maule was mounted under the rear half of the wing...amazing how little position error there was with that.
 
True airspeed is GPS speed in a no wind situation. So you can calibrate your airspeed indicator if you want to.

There is a way to do it with wind also, but not while landing, unless the wind is directly down the runway. While flying, turn until you see your slowest GPS speed. Then fly for one minute in that direction. Note your GPS speed. Turn 180 degrees and fly in that direction. Note your GPS speed. Now average the two speeds. That is your TAS. Afterall, TAS is just GPS speed +- the wind.
 
dell30rb said:
Because if the pitot remains aligned with the AOA, you wouldn't see errors in the IAS during slow flight
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Okay, asking why was not the point and I should not have asked, instead stated "what's the point" and {here's what's easier/ better}
 
Jaybird180 said:
For the purpose you propose, I'm stating AoA works better than a movable pitot and is far simpler to engineer and maintain.
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Really? The purpose for which I propose is to have an accurate airspeed indication. How does an AoA indicator give me an accurate airspeed?

Enlighten me.
 
EdFred said:
AoA indicates airspeed? Who knew.
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The AoA indicator can help adjust the stall speed for actual conditions. The stall AoA is the same regardless of power, weight, or load factor. So, if you're in a very light Cherokee 6, for instance, you can approach at the slower speed with no extra math and conversions to and from CAS.

It more directly answers the critical question, "how close am I to stall right now?" with much more finesse than a stall warning.

They are also rather simple bits of equipment. Some of them just have two pitot-like openings and no moving parts.
 
MAKG1 said:
The AoA indicator can help adjust the stall speed for actual conditions. The stall AoA is the same regardless of power, weight, or load factor. So, if you're in a very light Cherokee 6, for instance, you can approach at the slower speed with no extra math and conversions to and from CAS.

It more directly answers the critical question, "how close am I to stall right now?" with much more finesse than a stall warning.

They are also rather simple bits of equipment. Some of them just have two pitot-like openings and no moving parts.
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That's not what I want. I want a pitot tube that provides no need for CAS/IAS calculations. I don't give a crap how close I am to stalling or the CAoA.

"Can I get a 2H pencil?"

"Why do you want a 2H pencil, you can use a pen that blah blah blah."

"Because I want a ****ing 2H pencil."
 
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EdFred said:
That's not what I want. I want a pitot tube that provides no need for CAS/IAS calculations. I don't give a crap how close I am to stalling or the CAoA.

"Can I get a 2H pencil?"

"Why do you want a 2H pencil, you can use a pen that blah blah blah."

"Because I want a ****ing 2H pencil."
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Well, adding extra moving parts reduces reliability. It's another adjustment to be made (and perhaps screwed up) at every annual. It's not at all clear what you should use to align it with the wind. Perhaps a trailing vane? That would make it an AoA indicator as well. Plumbing the pitot tube through a universal joint without affecting its mechanical properties would be a challenge.

That ASI isn't needed for much more than distance from critical AoA. If you want to know how long it will take to get somewhere, that's ground speed and it can be more readily gotten from a GPS or DME.

But if it's really what you want, design and test it. Maybe you'll find a market.
 
EdFred said:
Really? The purpose for which I propose is to have an accurate airspeed indication. How does an AoA indicator give me an accurate airspeed?

Enlighten me.
Click to expand...

I recall you being firmly entrenched in the camp of 'we don't need no steenkin' Airspeed Indicator to fly the playne':lol:
 
Jaybird180 said:
I recall you being firmly entrenched in the camp of 'we don't need no steenkin' Airspeed Indicator to fly the playne':lol:
Click to expand...

I'm pretty sure I was on the other side of the argument.
 
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