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Here's a puzzle. Looking at the PoH of my aircraft, it appears that the climb angle (distance to clear a 50 foot obstacle, starting from the end of the ground roll) is shallower at lower weight than at max gross.

That doesn't make sense to me - oughtn't the climb slope to be steeper at lower weight?

For example, the PoH claims that at sea level, 20 C and 1430 lb, the ground roll is 424 feet, and it needs 972 feet (total, including the ground roll) to clear a 50' obstacle.

At 1320 lb (also 20 C and sea level), it says ground roll is 210 feet and 782 feet to clear the 50 feet.

So the climb slope at 1430 lb is (972-424)/50, or 10.96:1.

And the climb slope at 1320 lb is (782-210)/50, or 11.44:1. Shallower, at lower weight.

I get similar results at all temperatures and pressure altitudes.

What's going on?

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    $\begingroup$ @RobertDiGiovanni PoH numbers are standardized; that's not how they work. The 972 feet to clear 50' is total, from being stopped. I'm pretty sure. $\endgroup$ Aug 10, 2023 at 21:51
  • $\begingroup$ @MichaelHall Have a look for yourself: superpetrelusa.com/wp-content/uploads/2021/08/… page 5-3 is for 1430 lb, page 5-4 is for 1320 lb. $\endgroup$ Aug 10, 2023 at 22:04

2 Answers 2

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There are three stages to a short field takeoff over an obstacle- the ground roll, where the airplane accelerates to the minimum airspeed necessary to leave the ground, a period of further acceleration to Vx, and then a climb at Vx. I believe your analysis is flawed because it ignores this acceleration period.

The ground roll is the distance from the beginning of the takeoff run to the moment the airspeed is sufficient to become unstuck. When the aircraft is lighter, the required airspeed to become unstuck is less. So the heavier aircraft will be going faster at the end of the ground roll than the lighter aircraft will be.

The climb angle of the lighter aircraft is certainly steeper once both have reached Vx. But the lighter aircraft is further from Vx at the moment the ground roll ends and spends a little more distance accelerating to reach the same airspeed.

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    $\begingroup$ So, briefly, (972-424)/50 is an oversimplification. "The" climb slope isn't constant from wheels up to obstacle cleared. $\endgroup$ Aug 10, 2023 at 22:04
  • $\begingroup$ @Chris That makes sense, but Vx is itself lower at lower weight (altho this is not shown in the PoH). But I suppose you have to be correct. $\endgroup$ Aug 10, 2023 at 22:08
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    $\begingroup$ @CamilleGoudeseune Right. Or to put it another way, lighter aircraft are better at climbing steeper but they're even more better at leaving the ground at low airspeeds. $\endgroup$
    – Chris
    Aug 10, 2023 at 22:08
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    $\begingroup$ @nerdfever.com True, but I'm not sure it's as big as a difference. More to the point, it's likely that the POH numbers are assuming that you are climbing at the published "Vx" not the true absolute best Vx corresponding to your actual weight. It would be rather a poor idea to publish required takeoff distances assuming you are traveling some airspeed they don't tell you how to calculate! $\endgroup$
    – Chris
    Aug 10, 2023 at 22:09
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    $\begingroup$ @nerdfever.com They may or may not have run some tests to figure out Vx at other weights, but unless they've published them they certainly can't publish performance figures using them, is my point. $\endgroup$
    – Chris
    Aug 10, 2023 at 22:13
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This is a very interesting question involving an amphibious aircraft, the Super Petrel LS.

Building on the answer by Chris, we see that take-off from water would involve "muddy/soft" field technique.

Indeed, the lighter plane would lift out of the water at a slower speed, therefore requiring more time to accelerate.

On dry land, it would be smarter to hold off on rotation until reaching Vx in any case, as rolling at low AoA is less draggy than flying. Only when field conditions create greater drag is it advantageous to haul it into the air at a lower airspeed.

This is why many planes take off "clean", no flaps, using a greater ground roll and climb out at Vy. Even at Vx with flaps, keeping drag as low as possible by delaying rotation can be the most efficient way to gain airspeed.

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  • $\begingroup$ Very helpful. Is this true in general, that the best way to clear an obstacle is to rotate only when reaching Vx? I've heard rotating earlier is recommended, followed by accelerating in ground effect until Vx. $\endgroup$ Aug 24, 2023 at 15:57

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