Landing speed

[andrewc]

Why is landing speed defined as a factor of stall speed
(x1.3) rather than as a fixed number of knots above
your machines stall speed?

Thoughts...

-- Andrew

[Keygrip]

Is that "landing speed" or "approach speed"?

Was interested to hear Oxford are teaching Vat as 62kts for Pilot and aircraft, then adding 2½kts per passenger and 1 knot for every ten gallons of fuel on board at time of landing.....

so a/c, pilot, one pax and 30gall of fuel remaining would give Vat as 62+2½+3 = 68½ knots (Piper Arrow).

Never heard it worked out like that before.

[A and C]

The thing from Oxford is just to get the students to think like airline pilots.

The fact of the matter is that Vs is quoted in the flight manual at MLW so a Vat of 1.3 Vs is what should be flown to make the landing distance quoted in the flight manual.

It would be interesting to see how near the Oxford way of doing things is to the flight manual performance.

Comments from Oxford please !.

[Timothy]

1.3 times the stall speed is very much a rule of thumb to be used when you don't have the POH (perhaps because you are certifying the aircraft, or because you are flying it unexpectedly (eg pilot incapacitated)).

Under these circumstances you won't go far wrong with 1.3 x Vs, but if you do have the POH you should follow the guidance in that in preference.

Once you have flown the aircraft a few times you can then nail the approach speed for that particular example. If you are too fast you float, or even balloon, if you are too slow you thump rather than squeak.

There are times when you want to thump predictably (most obviously short fields, but also when there is a risk of aquaplaning...yes, it happens in light aircraft) so you reduce approach speed to that which you have pre-determined in your experiments.

There are other times when grace and artistry are important (on the pull, nervous passenger etc), and runway length isn't a factor, when you might want to go to the upper end of the speed range.

It is important also to think about wind speed, gustiness and cross-wind. You should add speed in strong, gusty winds. In a steady crosswind, if you use the kick-off-drift method, you should adjust down to be certain to touch down preditably, but if you are wise enough to use wing-down this is unnecessary.

Here endeth the diatribe.

W

[shortstripper]

I couldn't agree more with WCollins!

Gustiness, crosswind ect should all be taken into account. If you really wanted to be precise then the approach should be worked on a factor of wing attitude to relative airflow ect ect ... but hey! at the end of the day too much science can kill the beauty.

1.3X is close enough, perfection is not science, it's an art! every aircraft, pilot and situation is different. Obi Wan was right "feel the force" :ok:

[IO540]

Isn't it meant to be "1.3 times whatever the stall speed is in the current configuration"?

[Flyin'Dutch']

Yup you are right IO540.

And that answers AndrewC's question.

As WCollins said it is a rule of thumb only and the POH should be followed were available.

No problem coming in slower when skill/knowlegde and conditions allow.

FD

[Dude~]

The way I see it, everyone has missed the point so far.

The benefit of it being expressed as a factor (1.3) of the stall speed is so that approach speeds are proportional.

Take the PA28 for ex. Stall is say 49kts (from memory) and approach is circa 65kts. Thats a 16kt margin over stall.

A jet however that stalls at 110kts, should approach at 143kts (or 1.15 stall over threshold) giving a margin of 33kts. WOuld you feel safe approaching a jet 16kts above stall? I wouldn;t. Its all proprtional.

I realise I may not have expressed myself particularly well here and welcome all abuse as deemed necessary;)

[englishal]

Not forgeting of course, that an aircraft can stall in any attitude, any airspeed and any configuration, not just at the bottom of the ASI :D

[IO540]

an aircraft can stall in any attitude, any airspeed and any configuration

Only if the IAS is below the stall speed in that configuration :O

[Timothy]

IO540

Not strictly true, unfortunately. Stall is exclusively a function of angle of attack, and the angle of attack can cause a stall at above book speeds for your configuration for a number of reasons.

The best known is being in a turn, which can dramatically increase the IAS at which a stall starts, but high speed stalls can also be acheived in any aerobatic manouevres which use inertia to move the aircraft in contradiction (as it were) to aerodynamics.

Also various wind conditions (notably microbursts) can cause the angle of attack to increase (because the airflow is coming from a different angle) despite the airspeed being kept constant.

It is for this reason that bigger/faster aircraft use AoA measurement rather than IAS to determine closeness to stall.

W

[Keygrip]

I think that what IO540 is getting at is the literal translation of an aircraft can stall in any attitude, any airspeed and any configuration
Let's take a sample aircraft, say a PA28 ('cause there's lots of them), trim it slightly nose down, final approach to a runway, nice shallow descent, say - 95 knots, clean or dirty (it matters not).

Now WITHOUT CHANGING THE ATTITUDE, SPEED OR CONFIGURATION - make that aircraft stall. How would you do that?

[Final 3 Greens]

Keygrip

Try flying through severe icing conditions ;)

[Dude~]

Eeeeyeees....

right.

I think what they are trying to say is that the stall speed increases with the root of the g you are pulling. Ie a 1 g stall in a PA28 is circa 50kts. Pull 4g and in increases by root 4. Inthis case 2. So stall speed in a PA28 under 4 g is 100kts.
At 9 g, it would stall at 150kts etc etc.

Of course an aeroplane could be pulling g in a vertical climb, or any other attitude which is what Collins is trying to say. (I think!)

[shortstripper]

Methinks we're all getting carried away again ... generally in normal conditions 1.3X indicated stall speed will suffice. However:-

Again, I agree with what WCollins says .... what he is saying is that AoA is the all important figure.

Airspeed (esp IAS) just gives an indication and should only be used as a guide. An aircraft will always stall at the same AoA despite airspeed. IAS is a rough way of measuring your AoA under normal flying conditions but that is all! Weight, g, thermal activity, gusts, ect ect will all effect the speed at which you stall ... but not the AoA.

Glider pilots learn to fly by attitude with the ASI blanked off ... still not infallable, but a darn sight better than just learning speeds!

IM

[Capt. Manuvar]

When learning to fly I was told to stick to a particular 1.3Vs, which worked well in a C152 with full tanks and two up.
But i discovered to my horror that using that same speed when i was solo and with say 1/2 tanks led to me floating like a dinghy :* .
Andrewc
The aim of a good landing is to touch down just above the stall (at least in the spamcans i fly)
If your stall speed at MLW is 50kts(1.3 VS=65kts) then you will have to bleed off 15kts in the flare
But if your stall speed at your reduced weight is 45 kts (1.3 Vs=58kts) and you approch at 65 kts the you'll have to bleed off 20kts (instead of 13kts) in the flare. And its the extra 7kts thatwill cause you float.
I'm suprised schools dont highlight this fact ( rather than telling people to stick to a mgic number) considering the number AAIB/NTSB reports i've read in my relatively short aviation 'career'.
But what do i know? i'm a college dropout.hope i answered your question though.

[Whipping Boy's SATCO]

Being really pedantic, should we not say that a given "aerofoil" will always stall at the same angle of attack?

For me, 1.3xVs (Vso) works.

[Final 3 Greens]

Capt M

It is rare to find an SEP POH that gives you the range of hard data to reduce the airspeed relative to the a/c weight, an honourable exception being the SA121 Bulldog.

Following that train of logic through, on what basis could a flight school provide this information to a student? Make it up using a rule of thumb?

You can just imagine what a lawyer would make of such advice where there any problems.

[Dude~]

The PA28R Arrow II I fly has an approach speed directly and linearly related to weight. It is between 80mph at 1800lbs and 90mph at 2600lbs (max weight)

Also flew a PA32 and the difference between max and min weight approach speed was almost 15 kts according to the POh.

Definitely worth taking in to account. Good airmanship to be aware of it too.