Why 1.3 Vs for approach?
Joined: Sep 2002
Posts: 2,188
Likes: 6
From: La Belle Province
....Don't forget that most certification standards require a safe landing from Vref minus some thing - usually 5 knots. I have seen a few light aircraft which cannot comply (at 1.3vso minus x), so Vref ends up being more than 1.3Vso. There may well be transport aircraft with the same situation.
And ANY transport category aircraft which has speed adders for failure cases which do not directly impact lift generating capability are implicitly in the same category. (If I am to fly at Vref+X for a hyd system failure, that implies at least a chance that the a/c cannot easily flare with the presumably reduced elevator authority, or has lateral control issues, for example)
Thread Starter
Joined: Oct 2007
Posts: 107
Likes: 0
From: coventry
Is Vsr a mathematical modification of Vso data obtained by determining the stall str and level; or is some other flight test technique used to obtain the information; or (more likely) am I being totally naive and it is none of the above?
TIM
TIM
Joined: Oct 2004
Posts: 255
Likes: 15
From: australia
RansS9
When conducting stall testing for certification there are two very different series of tests:
1. Handling qualities stalls and,
2. Performance stalls.
The handling qualities stalls are a series which include, straight, turning, low deceleration rate, high deceleration rate, power on/off, sideslip (some standards), in all configurations - as required by the applicable certification standard. The stalling speed doesn't matter too much during these tests.
The performance stalls are done to determine the stalling speed for the configurations required for certification.
During the tests various paramenters are measured which will require correction, during the data reduction process, to a standard datum, because they change during the flight. Other items will require correction due to the measurement method. Examples of corrections made during data reduction would be; weight, CG, position error, mach number correction (to stall Cl), approach rate, normal acceleration etc. Also, as the standard stall deceleration rate is 1kt per second, all performance stalls are conducted in descending flight, so a correction is required here as well.
So, to gather data, for a 1g stall speed aircraft, it would not be necessary to test a fully developed stall such as for a Vs min stall. Provided an obvious "g break" is recorded (g reduction as Cl max is passed during deceleration) it may not be necessary to fly any slower.
Remember, that for transport aircraft the actual stall speed is not important, it just has to be established. However, in a number of standards there is a maximum stall speed requirement - can you imagine arguing compliance with the regulatory authority over 0.25 kts in stall speed
Hope this helps, if too detailed, sorry - its not meant to be an egg sucking lesson!
When conducting stall testing for certification there are two very different series of tests:
1. Handling qualities stalls and,
2. Performance stalls.
The handling qualities stalls are a series which include, straight, turning, low deceleration rate, high deceleration rate, power on/off, sideslip (some standards), in all configurations - as required by the applicable certification standard. The stalling speed doesn't matter too much during these tests.
The performance stalls are done to determine the stalling speed for the configurations required for certification.
During the tests various paramenters are measured which will require correction, during the data reduction process, to a standard datum, because they change during the flight. Other items will require correction due to the measurement method. Examples of corrections made during data reduction would be; weight, CG, position error, mach number correction (to stall Cl), approach rate, normal acceleration etc. Also, as the standard stall deceleration rate is 1kt per second, all performance stalls are conducted in descending flight, so a correction is required here as well.
So, to gather data, for a 1g stall speed aircraft, it would not be necessary to test a fully developed stall such as for a Vs min stall. Provided an obvious "g break" is recorded (g reduction as Cl max is passed during deceleration) it may not be necessary to fly any slower.
Remember, that for transport aircraft the actual stall speed is not important, it just has to be established. However, in a number of standards there is a maximum stall speed requirement - can you imagine arguing compliance with the regulatory authority over 0.25 kts in stall speed
Hope this helps, if too detailed, sorry - its not meant to be an egg sucking lesson!
Thread Starter
Joined: Oct 2007
Posts: 107
Likes: 0
From: coventry
Thanks for the information. Enough to know if I want more I'll need to start studying for that Degreee in Aeronautical Engineering!
Interesting approach to the stall 1Kt/sec in descending flight. If I remember correctly this was an exercise, recomended (unsurprisingly) by Mr. Farley in his book, to get those of us at the lighter end of aviation to know and love are wings.
I am not sure if my flying skills are upto deceleration rates of 1Kt/sec but it can't hurt to try.
TIM
Interesting approach to the stall 1Kt/sec in descending flight. If I remember correctly this was an exercise, recomended (unsurprisingly) by Mr. Farley in his book, to get those of us at the lighter end of aviation to know and love are wings.
I am not sure if my flying skills are upto deceleration rates of 1Kt/sec but it can't hurt to try.
TIM
Joined: Aug 2009
Posts: 3
Likes: 0
From: Uk
Not at all sure how all this works....
I remember we had to go down this line of digging at Boscombe for some project we were working on. You know how sometimes thinking too much takes you down some very unexpected roads. Luckly some body managed to pull out some old information from somewhere which showed that it was basically arrived at after a statistical analysis of a fairly widely scattered sample of data involving quite a wide range of aircraft types. Very interesting to read everyones thoughts, going to bed now as it is late.
Joined: Apr 2006
Aviation Qualifications: Military
Posts: 32
Likes: 0
From: Oxfordshire
Lots of good stuff here!
I believe 1.3 was probably the average situation when a large number of aircraft recommended approach speeds was looked at. In practical terms, the manufacturer will recommend a speed for approach which gives satisfactory handling and takes into account the power available. In days gone by, view over the nose was also a factor. In the US Navy, the carrier approach speed also had to be able to cope with a gain of 50 feet by tweaking back on the pitch control and without adding power; in other words, the approach speed shouldn't be too far up the back side of the thrust required curve. With todays hi performance fighters, (eg F18) Vs is actually at a very high alpha, and tail strike would be significant at 1.3!
So with a conventional light aircraft, the manufacturer will probably start looking at the 1.4 area and reduce approach speed until he has a comfortable trade off of all the variables.
I believe 1.3 was probably the average situation when a large number of aircraft recommended approach speeds was looked at. In practical terms, the manufacturer will recommend a speed for approach which gives satisfactory handling and takes into account the power available. In days gone by, view over the nose was also a factor. In the US Navy, the carrier approach speed also had to be able to cope with a gain of 50 feet by tweaking back on the pitch control and without adding power; in other words, the approach speed shouldn't be too far up the back side of the thrust required curve. With todays hi performance fighters, (eg F18) Vs is actually at a very high alpha, and tail strike would be significant at 1.3!
So with a conventional light aircraft, the manufacturer will probably start looking at the 1.4 area and reduce approach speed until he has a comfortable trade off of all the variables.
Joined: Dec 2006
Posts: 44
Likes: 0
From: Australia
test pilots like to eat and be eaten by members of their preferred sex?
1.3 x 1.3 - 1.69
I am not sure if that will point you in the right direction
Silly/Stupid joke aside.
it is important to remember the V^2 component of the lift equation. and to think of other great points mentioned so far, such as roll out go around
1.1 x 1.1 = 1.21
giving only .21 of a g to arrest the flair and no margin for any other mishaps such as , Opps, there was a gust or sheer or we are landing short lets pull up to get inside the fence.
1.2 x 1.2x = 1.44
similar to 1.1 but not so dramatic, controles are still slugish?
1.4 x 1.4 = 1.96 now that's a great amount of lift and more than enough to arrest a decent. but may be the controls are a bit sensitive. and will we stay inside the fence at the deep end of the runway?
F=Ma
De accelerating an aircraft takes a lot of force/energy, and tires take a pounding when landing. break pads of current design get very hot.
I Like 1.3 x 1.3 = 1.69
This post best read while thinking of some thing else. (that way you may actually forget it)
1.3 x 1.3 - 1.69
I am not sure if that will point you in the right direction
Silly/Stupid joke aside.
it is important to remember the V^2 component of the lift equation. and to think of other great points mentioned so far, such as roll out go around
1.1 x 1.1 = 1.21
giving only .21 of a g to arrest the flair and no margin for any other mishaps such as , Opps, there was a gust or sheer or we are landing short lets pull up to get inside the fence.
1.2 x 1.2x = 1.44
similar to 1.1 but not so dramatic, controles are still slugish?
1.4 x 1.4 = 1.96 now that's a great amount of lift and more than enough to arrest a decent. but may be the controls are a bit sensitive. and will we stay inside the fence at the deep end of the runway?
F=Ma
De accelerating an aircraft takes a lot of force/energy, and tires take a pounding when landing. break pads of current design get very hot.
I Like 1.3 x 1.3 = 1.69
This post best read while thinking of some thing else. (that way you may actually forget it)
Joined: Nov 2007
Posts: 46
Likes: 0
From: Europe
POH Minumum Approach Speed less than 1,3Vs?
After an incident with a hard landing the following question arose:
The particular aircraft (light a/c normal category) has a published stall speed of 53 mph. The landing distance performance is based on approach speed of 60 mph according to the POH.
Consequently the Approach speed according to the POH is less than 1,3Vs.
Isn't 1,3Vs a design criteria for landing performance? Or is it likely that the definition "minimum steady flight speed" (as mentioned in post #5) is chosen as the reference speed on this small aircraft?
The particular aircraft (light a/c normal category) has a published stall speed of 53 mph. The landing distance performance is based on approach speed of 60 mph according to the POH.
Consequently the Approach speed according to the POH is less than 1,3Vs.
Isn't 1,3Vs a design criteria for landing performance? Or is it likely that the definition "minimum steady flight speed" (as mentioned in post #5) is chosen as the reference speed on this small aircraft?
Last edited by Crankshaft; 6th September 2010 at 23:10.
Joined: Jul 2010
Posts: 81
Likes: 0
From: Out there, somewhere
Mmmmm
I used to fly a light single, where the stall speeds were calculated without reference to weight, but the approach speed had suggested factors to reduce it at lighter weights.
It was possible to fly the approach less than 1.3vs and yet comply with the POH.
It could have looked tight (per your comment BOAC, which I understand), but the reality was that it was perfectly okay.
I don't have the POH to hand, so I can't honestly say if it was 1.2vs or 1.1vs, but you probably get my point.
I used to fly a light single, where the stall speeds were calculated without reference to weight, but the approach speed had suggested factors to reduce it at lighter weights.
It was possible to fly the approach less than 1.3vs and yet comply with the POH.
It could have looked tight (per your comment BOAC, which I understand), but the reality was that it was perfectly okay.
I don't have the POH to hand, so I can't honestly say if it was 1.2vs or 1.1vs, but you probably get my point.
