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What landing distance

Seneca V

For a performance A type aircraft ASDA will be proportional to mass, because V1 cut speed will decrease with mass.

The take off safety speed of an MEP would need to be proportional to mass, but as it is derived from all up mass Vmc, this is not the case.

Oxford (EGTK), United Kingdom

I am guessing this chart, assumes you get airborne before you start decelerating. “Distance includes 3 second recognition time at abort speed” This of course is quite different to part 25, or a performance A calculation or the concept of V1. I have b58 and pa-44, I can’t find my PA-31 but they all show a shorter distance at lower weight, with identical abort speeds. These chart DO NOT introduce a recognition time, so you would likely still be on the ground, if you aborted at the notional abort speed.

Last Edited by Ted at 04 Aug 22:13
Ted
United Kingdom

wbardorf wrote:

The DA40 AFM shows lower approach speeds and lower total landing distances and ground roll distances with lower aircraft weight:

They also specify rather vastly different approach airspeeds depending on mass. The question was about what happens if one keeps the MLW approach speed at lower weight.

I actually don’t understand the approach speed variation with weight. I expected the square of the speed to be proportional to the weight (mass), but it is not. Not in the Diamond POH and not in the Mooney POH. Interestingly, the ratio increases with weight on the Diamond, but decreases on the Mooney.

ELLX

I actually don’t understand the approach speed variation with weight

Vref is a factor of the stall speed, usually 1.3 x Vs for the configuration. As the stall speed increases with weight therefore the Vref increases accordingly.

EGLL, EGLF, EGLK, United Kingdom

lionel wrote:

I expected the square of the speed to be proportional to the weight (mass), but it is not. Not in the Diamond POH and not in the Mooney POH. Interestingly, the ratio increases with weight on the Diamond, but decreases on the Mooney.

If linking speed to weight via lift at specific approach AoA, that should be the formula for Va(W) which is along Vref(W)= 1.3*VS(W) at max AoA, yes POH numbers for Mooney & Diamond do go out of fashion vs that exact formula but we are talking 3kts here

Maybe Diamonds can’t land slow at max MTOW (limited to 1210kg landing vs takeoff at 1280kg) due to weak gear design while on the Mooney can’t just land 5kts faster due to flat drag curve on higher speeds above 1.3*VS

Last Edited by Ibra at 18 Aug 12:03
Paris/Essex, France/UK, United Kingdom

Xlr8tr wrote:

Vref is a factor of the stall speed, usually 1.3 x Vs for the configuration. As the stall speed increases with weight therefore the Vref increases accordingly.

I was not speaking qualitatively (I understand that), but quantitatively.

  1. Straight-and-level stall (which is what defines Vs) happens when the critical AoA is reached.
  2. The stall speed is the speed at which, in straight and level flight, the wing is at critical AoA
  3. Since this is in straight and level flight, that is a situation where lift = weight
  4. Weight is proportional to mass
  5. So the stall speed is the speed at which, at the critical AoA, lift = weight; go any faster and either your AoA will be smaller or you will climb (or a bit of both).
  6. I was taught that, at fixed AoA and wing configuration, lift is proportional to the square of the calibrated airspeed (or maybe EAS if you get very precise, but that is beside the point here, the difference between EAS and CAS is negligible in the scenarios here discussed)
  7. Hence the square of stall speed “should” be proportional to the weight

But in these POHs, it is not.

ELLX

In the idealized case of a wing only airplane your model is close to reality. For real existing planes – especially small ones like ours – there are mainly two effects that make things more complex than your “square root law”:
“lift=weight” is only true if you take “lift” as equivalent of “total vertical component of aircraft lift”. The main difference of that from the lift which is generated by the wing is a) the pitch angle of the plane (as the wing lift is perpendicular to the body axis while the weight is perpendicular to the horizontal) and b) the (negative) lift of the stabilizer

Both are relevant at higher AoAs/lower speeds and b) is even depending on the CG.
Therefore The POH stall speed is depending on the CG diagram as the stall speed is always given at the allowed most forward CG (as the required negative lift of the stabilizer increases with forward CG).

I guess that is also the reason why some microlight planes that need to demonstrate low stall speeds in some regulations have a very narrow CG envelope and are very limited in terms of front CG at higher weights.

Last Edited by Malibuflyer at 18 Aug 16:18
Germany

Malibuflyer wrote:

there are mainly two effects that make things more complex than your “square root law”:

But why these don’t manifest on dynamic stalls in the turns which tend to accurately track those “square-root” law formulas?

My feeling they are mostly IAS, EAS, KCAS related aberrations and instrument errors near the stall than “deep physics issues” with the “square-root” model or zero-lift drag characteristic, for approach speed one will be far away for these and flying same AoA will give a square-root forumula between approach speed and weight

But there maybe more to it to deviate from an ideal square-root law, drag curve shape, forward visibility, approach stability in gusts vs weight, gear max/min touchdown speeds, changes of flaps config with weight

I flew a D31 turbulent, stall speed is 35kts but everybody decided the approach speed to be 60kts for any weight & conditions unless you want troubles

Last Edited by Ibra at 18 Aug 17:54
Paris/Essex, France/UK, United Kingdom

lionel wrote:

But in these POHs, it is not.

Perhaps there are other effects. Vref does not have to be 1.3 Vs, it just has to be above 1.3 in most cases. There might even be some aircraft where even this doesn’t apply. i.e. It just have to a safe speed. You have to check of actual certification rules applicable at the time the TC was issued.

Also some aircraft are not capable of full stall at forward CG at low weights power off. e.g. some model of the bonanza don’t have enough elevator authority. Some aircraft have some stability issues at very low speed this increases the minimum speed. i..e this becomes the limit used to calculate Vref not Vs.

Last Edited by Ted at 18 Aug 18:34
Ted
United Kingdom

Ibra wrote:

But why these don’t manifest on dynamic stalls in the turns which tend to accurately track those “square-root” law formulas?

You mean the effect that in a turn the stall speed increase (e.g. the factor you have to apply to the straight and level stall speed) is the square root of the load factor?

That already implies the answer to your question: The described aerodynamic effects are already reflected in the straight and level stall speed and therefore the increase is independent from them.

Germany
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