Comet 4c - Testing Update - Lift/Thrust/Drag

Comet 4C v0.94 was released to beta-testers today.

I am hugely grateful to the UK X-Plane Development Team for this testing. Now that initial bugs are out of the way, some point-to-point navigational exercises lead to several functional improvements. I’m also grateful to Mike Pyment, the owner of Comet 6407 (formerly G-APDF and XV-814) for allowing me access to his excellent cockpit. It’s all very well reading about these things in manuals, but with some gauges the penny doesn’t drop until you see them in action.

Otherwise this version is mostly about flight performance. A new version of the Rolls-Royce engine plug-in from David Plunkett triggered another wave of performance testing by me (countless take-offs and landings, climbs and descents, and hours on cruise). There is more work to do on fuel flow, but this is the current state of play:

Stall Speeds

Clean — 0.5% deviation from DH figures over 10 stalls;
Flaps 20º — 1.2% over 10 stalls;
Flaps 40º — 2.4% over 10 stalls;
Flaps 60º — 3.2% over 10 stalls.
Stalls became increasingly difficult to achieve because the drag from the flaps caused the air speed to drop so rapidly that stalls were harder to “catch”.

Rotate and Unstick Speeds

Rotate speeds were tested so that the minimum take-off safety speed can be achieved at a height of 35’ above the runway with one engine out of operation, as per specification. I found it extremely difficult to achieve consistent speeds manually, therefore, using the autopilot to maintain fixed pitch, I arranged a series of tests from 6º to 9º pitch at 0.5º increments, with data for landing gear loads, speed, altitude and distance travelled being logged in a text file every 1/10 sec. Currently, the Comet over-performs by 3% minimum take-off weight, and under-performs by 1% at maximum take off weight. In other words the X-Plane Comet needs to rotate at about 137.5 kts at MTOW instead of 136 kts to ensure it could “clear the hedge” if it took a bird in one engine.

High Speed Descent

The overall time from 40,000’ to 1,000’ for both the real and X-Plane Comets is 22 minutes. However, the X-Plane Comet has a slightly slower rate of descent at high altitude, and slightly higher rate of descent at low altitude.

High Speed Climb

The climb curve is a very close match with the real curve and the total times from 1,000’ to 37,000’ are within 2%.


For a range of speeds, altitudes and weights, cruise RPM is within 1.8% of the manual, with an average of 1%. Fuel consumption is 5-8% too low, and this will be attended to in the next beta.


The objective was to keep the engines at a fixed RPM from the landing checks (carried out at D10 and 150 Kts) down to the runway threshold. The real figure was 6,500 RPM ± 150 RPM (for minimum/maximum landing weights). X-Plane Comet v0.94 requires 6,250 RPM ± 150 RPM.


I spend a lot of time playing with all the permutations and combinations. It is usually possible to make one aspect of performance perfect, but at the expense of the others. For example, increasing low-altitude drag or reducing engine thrust would bring the approach RPM up to 6,500, but then the take off performance would be less accurate. This is the best compromise I have achieved so far and it feels pretty good.

Next steps: Adjust fuel flow, then get back on track with liveries and writing the user’s manual.

GMM-P (18/02/2012)

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