Menu Sign In Contact FAQ
Banner
Welcome to our forums

What landing distance

The formula is based on the principle of energy conservation. Kinetic energy (1/2*m*V^2) at touch-down is converted to potential energy (m*g*H) (climbing up a slope) and work done (F*L)by the brakes. If H is negative, the brakes have to do more work.

The formula doesn’t work well for trikes, because the brakes are on the back wheels – and therefore ineffective by design. Compare C150 or PA28 brakes with a car or motorcycle, and it’s clear that we’re not supposed to use them. On a heavy bushplane with double calipers and massive rotors, and we can stand the airplane on the front wheels, like a TT rider at the end of Sulby straight.

So I think for a trike we probably need to apply a factor representing the transfer of weight to the (unbraked) nosewheel under heavy braking. My gut feeling is that a factor of 2 or 3 would be about right – that would have your Warrior stopped in 200 – 300 m.

In answer to the question re. tarmac, there are surely figures for tyre friction somewhere. In a trike the above considerations apply more strongly. With conventional gear, factors like piloting skill, tyre compound and surface temperature may introduce wide variation, so if landing on a hard but possibly contaminated concrete surface like the old seaplane hangar floor slabs at Loch Doon, I estimate available µ of about 0.5.

Glenswinton, SW Scotland, United Kingdom

For more power-off elevator authority at low speed, we trim for level flight in the pattern with one stage of flap and leave it so. By attempting to trim out stick force on approach, we would reduce the effective area of the elevator by the area of the trim tab.

It’s true that holding off in ground effect can result in touching tail first, at lower airspeed than Vso. Useful though that may seem, I don’t think we give much away by applying brakes before touchdown, planting the wheels on at Vso, lifting the tail with the brakes against elevator.

Glenswinton, SW Scotland, United Kingdom

we trim for level flight in the pattern with one stage of flap and leave it so.

we would reduce the effective area of the elevator by the area of the trim tab.

The J3/Super Cub and C180/182 (through D)/185 have tailplane trim so not affected. The Mooney also enjoys tailplane trim.

Thread drift apology

Oxford (EGTK), United Kingdom

Airborne_Again wrote:

But actually I was considering the case of a fully stalled landning. In that case certainly the touchdown point will be extended

I’m not sure about that “certainly”. With lower weight you have lower energy (at the same speed) in the first place. So less energy to destroy before landing. Assuming that you can destroy energy at the same rate during landing, it is not obvious, that touchdown point will be later.

Let’s do a practical example from a POH of an arrow II I randomly found on the web (as my own plane has no weight vs. stall speed diagram in the POH).

At 2600lb stall speed is 63mph. at 1600lb it is 50mph. In SI-units that 28,16m/S @ 1180kg and 22,35m/s @ 726kg.
Therefore stall happens at 936 kJ remaining energy @2600lb and at 363 kJ @ 1600lb – sounds like a huge difference.

At (high weight) approach speed (1,4V_S of the heavier plane that is 36,6 m/s), the heavier plane has 1581kJ while the lighter plane has 973kJ.

Therefore the difference between approach and stall speed (assuming approach speed is the same) for the heavier plane is 645kJ while it is 610 kJ for the lighter one.

Conclusion: In this example of an Arrow II even if you fly approach speed for the heavier weight there is less energy to destroy before landing with the lighter plane. Therefore the lighter plane will actually touch down earlier in a full stall landing even though stall speed is substantially lower.

Last Edited by Malibuflyer at 02 Aug 16:51
Germany

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


EGTF, EGLK, United Kingdom

And here for a Mooney:

EGTF, EGLK, United Kingdom

Malibuflyer wrote:

Assuming that you can destroy energy at the same rate during landing

I think this may be quite a leap

More weight on the wheels should allow more friction and more braking force to be applied before the static friction is exceeded. Also any incline means more energy converted to gravitational potential.

Off_Field wrote:

More weight on the wheels should allow more friction and more braking force to be applied before the static friction is exceeded.

Sure – but that it only relevant for the part of the deceleration after touchdown. According to my back of envelope calculation above, the energy immediately after touchdown is 936kJ for the heavier plane vs. 363kJ for the lighter one. So after touchdown (the part of the landing we call “ground roll”) the heavier plane needs to destroy almost 3 time the energy of the lighter plane. It’s quite obvious, that it can’t make this up with more friction force.

wbardorf wrote:

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

That’s quite clear. quite clear. The original question was what happens if you give away the advantage of the lower approach speed of the lighter plane and use the same speed as for MTOW – would you still have a shorter landing distance?
Original argument of AA was: The ground roll is obviously shorter as you touch down at lower speed but the distance before touch down might be longer as stall happens later with the lower weight.

@Airborn:
An additional illustration for what is happening just came to my mind under the shower this morning: High performance gliders have water tanks to increase their weight for competitions as gliding performance gets actually better with higher weight. Therefore the gliding part to the touchdown point can actually be longer…

Last Edited by Malibuflyer at 03 Aug 05:45
Germany

Malibuflyer wrote:

An additional illustration for what is happening just came to my mind under the shower this morning: High performance gliders have water tanks to increase their weight for competitions as gliding performance gets actually better with higher weight. Therefore the gliding part to the touchdown point can actually be longer…

The glide ratio doesn’t change with increased mass, but Vbg does. You’re right that if you have a lower mass and fly at the Vbg for a higher mass, you will reach ground after a shorter distance compared to the higher mass case. But is this relevant to powered aircraft? For one thing speed is not constant but reducing into the flare, also when you make a normal (as opposed to deadstick) landing, you will use some power all the way to the flare.

ESKC (Uppsala/Sundbro), Sweden

It your passenger are okay you can just do a full dirty stall test before descent, and note your current Vso, you then have your 1.3Vso that are best for the conditions. it’s a current exercise in mountain where distances are keys.

LFMD, France
Sign in to add your message

Back to Top