VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics


Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 06-17-2018, 01:32 PM
macrafic's Avatar
macrafic macrafic is offline
 
Join Date: May 2006
Location: Rochester, MN
Posts: 394
Default At What Point is Stall Speed Captured?

I have had this question for as long as I have been flying (1969) but have never pursued the answer because I have only ever flown certified aircraft with a manufacturer POH. However, I am flying our own-built RV-7A and the question is now front and center.

When determining the stall speeds of a homebuilt aircraft, when is the actual speed captured and what speed is used?

Is the speed captured at the pending stall (buffeting, etc.) or is it captured at the moment the wing falls? Is IAS or TAS generally used?

During testing, I captured the IAS at the moment the wing fell but, since I am calibrating an AFS AOA, which is dependent on the identified stall speeds, I want to make sure I am as accurate as possible.

Appreciate input!
__________________
Rich and Cindy Macrafic
Rochester, MN
Flying
Reply With Quote
  #2  
Old 06-17-2018, 01:42 PM
Ironflight's Avatar
Ironflight Ironflight is offline
VAF Moderator / Line Boy
 
Join Date: Jan 2005
Location: Dayton, NV
Posts: 11,701
Default

Ahhhh...stall speed. While we are taught that it is simple, it really isn’t, and every airplane type has different characteristics.

Part of the problem is that the entire wing doesn’t always stall at the same time - or the same way twice. Plus - if you look at a plot of AoA vs. Lift Coefficient for an airfoil, the peak is a curve, not an abrupt angle. So you are still generating a little lift at some point after Max Cl (until you’re not, which is just a little later....).

Then of course, you have airframes that simply don’t have a stall break - they just sit there nose high and start to sink. Short wing RV’s generally don’t do that, so if we’re talking RV’s, take the stall when the one nose drops at the break. And, of course, that is IAS becasue that is traditionally what a pilot is going to see.

Paul
__________________
Paul F. Dye
Editor in Chief - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com
Reply With Quote
  #3  
Old 06-17-2018, 01:52 PM
RV3bpilot's Avatar
RV3bpilot RV3bpilot is offline
 
Join Date: Oct 2012
Location: New Ulm, Minnesota
Posts: 268
Default

I agree with Paul, when one or both of wings quit giving lift and the nose drops that is the stall speed to write down.
__________________
Robin Mckee
New Ulm, MN 56073
RV3b N219BB
318 hours and counting
Reply With Quote
  #4  
Old 06-17-2018, 02:41 PM
macrafic's Avatar
macrafic macrafic is offline
 
Join Date: May 2006
Location: Rochester, MN
Posts: 394
Default

Terrific! Thanks all!
__________________
Rich and Cindy Macrafic
Rochester, MN
Flying
Reply With Quote
  #5  
Old 06-17-2018, 03:17 PM
Saville's Avatar
Saville Saville is offline
 
Join Date: May 2014
Location: KBVY Massachusetts
Posts: 827
Default

Quote:
Originally Posted by macrafic View Post
I have had this question for as long as I have been flying (1969) but have never pursued the answer because I have only ever flown certified aircraft with a manufacturer POH. However, I am flying our own-built RV-7A and the question is now front and center.

When determining the stall speeds of a homebuilt aircraft, when is the actual speed captured and what speed is used?

Is the speed captured at the pending stall (buffeting, etc.) or is it captured at the moment the wing falls? Is IAS or TAS generally used?

During testing, I captured the IAS at the moment the wing fell but, since I am calibrating an AFS AOA, which is dependent on the identified stall speeds, I want to make sure I am as accurate as possible.

Appreciate input!

Just this week I went up to establish the power off stalls - flaps up and flaps down - as a prelude to working on short field landings.

I took buffet onset as the stall speed.
__________________
Flying RV-8 N880BC
Double the 2017 Dues - happily paid.
Reply With Quote
  #6  
Old 06-17-2018, 03:31 PM
control control is offline
 
Join Date: Feb 2006
Location: Stockholm, Sweden
Posts: 554
Default

Not for practical use but I think I remember that for an AC, the definition of stall is when an increase in AOA produces less lift... that is probably a knot or two faster than when the nose actually drops.
__________________
RV-14 kit #140138
I have entered the 90% done, 90% left to do zone
Http://magnusrv.blogspot.se
Reply With Quote
  #7  
Old 06-17-2018, 04:35 PM
penguin penguin is offline
 
Join Date: Jan 2005
Location: England
Posts: 1,051
Default

When measuring your stall speed it is important to maintain 1g flight, that means slowing down slowly. All the books quote 1kt per second, which is much more slowly than a typical 'CFI stall'. But there are 2 aspects, first ensure the airplane does nothing too sporting when it stops flying, and then figure out what the stall speed is. Sounds like you are already passed the first part :-).

Start at about 75kt and trim, ensure you have plenty of height. Close the throttle and let the aircraft slow down, don't try to maintain altitude and expect the aircraft to descend. Concentrate on slowing down gradually, assuming you will stall at 55kt indicated, it will take 20 seconds to reach the stall. The stall is the minimum speed reached before the nose pitches down, a wing drops or you cannot raise the nose any further. Once either of those happens recover and try again, but from 10kt above the stall to save time. Ensure you are not continually losing height. It will take a few attempts to be able to note the minimum speed, and to get repeatable results over 2 or 3 tries. Try again with full flap.

Will be necessary to climb after a few attempts, warms the engine also. If the stall is approached too quickly the wing will be supporting more than 1g and the stall speed will be higher than the minimum attainable. try to fly in reasonably smooth air.

Pete
Reply With Quote
  #8  
Old 06-17-2018, 04:36 PM
rocketman1988 rocketman1988 is offline
 
Join Date: Jun 2010
Location: Sunman, IN
Posts: 1,145
Default Stall speed

Iím not sure that using the buffet as the stall speed is a good approach. If you trying to be accurate, that buffet speed will typically be a bit faster than the actual stall. If you are using the stall speed to determine other speeds, like approach speed, the error will compound and you will always end up fast. Not especially good in a short field scenario. Each additional knot will require additional runway, and it adds up fast...
__________________
Bob
Aerospace Engineer '88

RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
Doors - Done
On Gear
290 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...

Dues Paid 2018,...Thanks DR+
Reply With Quote
  #9  
Old 06-17-2018, 04:42 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 5,355
Default

Definitely IAS, since it ( stall speed in IAS) is independent of density altitude.
Reply With Quote
  #10  
Old 06-17-2018, 05:07 PM
Saville's Avatar
Saville Saville is offline
 
Join Date: May 2014
Location: KBVY Massachusetts
Posts: 827
Default

Quote:
Originally Posted by rocketman1988 View Post
Iím not sure that using the buffet as the stall speed is a good approach. If you trying to be accurate, that buffet speed will typically be a bit faster than the actual stall. If you are using the stall speed to determine other speeds, like approach speed, the error will compound and you will always end up fast. Not especially good in a short field scenario. Each additional knot will require additional runway, and it adds up fast...
I noted buffet speed and then the speed at which the stall broke.

I wanted to know both. But to be conservative I will think buffet speed.

From Vans "Stall Testing":

"Allow the speed to bleed off until you feel a slight buffet. Note the airspeed and recover......

...the buffet which does occur does so within just an mph or two of the fully developed stall. "

He does not say note the airspeed at which the stall fully develops.

If 1 knot is going to make the difference between landing and not landing I won't be trying to land there unless I have to.
__________________
Flying RV-8 N880BC
Double the 2017 Dues - happily paid.
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 01:43 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.