Comet 4c - Take Off Flight Path

Net Take Off Flight Path 600 px (© Guy Montagu-Pollock 2016)
Fig.1: Net Take-Off Flight Path (click to enlarge).

Stall tests helped me tune the aerofoils and flaps for lift. Take-Off tests are another confirmation that the lift is good, and because the de Havilland manuals specify speeds, distances and gradients for each stage of the take-off path, they enable me to check and adjust flap drag, landing gear drag and wheel rolling resistance to hit the marks as accurately as possible.

Obviously, this requires flying exactly to the procedure in the manual. With reference to Fig.1, this is described below:

1Start of run. Flaps 20. Full power on all engines. Release brakes.
2V1  :  Decision speed, up to which the aircraft could still be stopped safely on the runway.
Quoted performance and tests include one engine failure at V1
3VR  :  Rotate to lift off at the scheduled unstick speed (usually 7-10 kt. less than VMU).
4VMU  :  Minimum unstick speed (lift-off). Distance to lift off is the "Take-Off Run".
5Landing gear raised immediately after lift-off.
6V2  :  Free-air safety speed (flaps 20). The aircraft must attain or exceed this speed and clear any obstacle at the end of the runway by 35'.
7Climb at full thrust on the remaining three engines at the free-air safety speed (flaps 20) to 1500 feet (AGL), then level out and let the aircraft accelerate to V3.
8V3  :  Free-air safety speed (flaps retracted). Raise flaps.

My own consistency with a yoke and pedals is not good enough for this, so I added a take-off test to my burgeoning flight-test plug-in. The plug-in used the built-in X-Plane autopilot, changing the autopilot settings, throttles, flaps and landing gear at the appropriate points. At the rotate speed, the autopilot was set to fixed pitch at a pre-determined angle. The tests went through angles from 6-12 degrees at one-degree intervals.

The autopilot had some limitations. When the aircraft is very light, inertia is less, so the rate of change of pitch is higher. On the other hand, the airflow over the control surfaces is so much slower that the controls are less effective. Flying manually, I found big, bold movements on the controls were necessary at low speeds, backing off as soon as the nose began to lift. At higher speeds, the nose would lift with much smaller inputs, but the inputs needed to be sustained or increased gently and gradually. The autopilot could not make such a distinction. At very low weights, the angle of attack at the scheduled unstick speed was never high enough to produce enough lift, so the aircraft failed to leave the ground. The only way to compensate was to rotate earlier than the manual suggested, to achieve a pitch of about 6 degrees at VMU, increasing to about 10 degrees at V2. The opposite was true at very high weights, where I found it necessary to delay rotation.

The table below shows the optimum results. During tests, I tried a number of variations, and it was interesting that the factory recommendations really did produce the best results. For example, raising the nose earlier, or more steeply, resulted in an earlier lift-off, and more clearance "over the hedge". However, the enormous amount of drag produced by increasing the angle-of-attack was far, far greater than the rolling resistance of the wheels, so V2 was not achieved. Keeping the nose down longer increased all the speeds, but also the take off run and take off distance. The figures quoted in the manual are an ideal compromise between airspeed and gradient.

The tests were carried out at sea level, and ISO (or ICAN) temperatures (i.e.: 15ºC at sea level), and no wind.

Take-Off Weight (lb.)100,000120,000140,000160,000
Optimum Pitch Set (deg.)101098
V1 (kt.)8192103110
Optimum VR (kt.)94 (VMU–15)109 (VMU–10)122 (VMU–7)130 (VMU–5)
Lift-Off / Unstick (VMU)
— Target speed (kt.)109119129135
— Actual speed (kt.)108.9118.3128.4134.5
— Pitch @ take-off (deg.)
Take-Off Run
— Target distance (ft.)1800260037005100
— Actual distance (ft.)1788259336144489
V2 @ 35' AGL
— Target speed (kt.)114125135145
— Actual speed (kt.)118.0124.7134.5143.8
— Pitch @ V2 (deg.)10.310.29.28.0
Take-Off Distance
— Target distance (ft.)2600350048006500
— Actual distance (ft.)2565347247806406
Distance to 1500 ft. AGL
— Target distance (ft.)9000125001800025500
— Actual distance (ft.)8884120551783723666
1500 ft. @ V3
— Target distance (ft.)10000140001900027000
— Actual distance (ft.)8884123641920025618

Fig.2: Table of take-off performance, with engine failure at V1 (tests made with XP 10.45).

Next step: climb and descent tests.


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