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Hangar 9 P51 D

Electric conversion project

Part 4 - Flight Performance

by Andrew Gibbs

Flight performance – first flight
Before flying the P51, I carefully checked the whole model over. Any areas of concern, however small, were rectified before setting off to the flying field.

At last an opportunity arrived when the combination coincided of suitable weather, a suitable venue and a cameraman (thank you George). The model was then range checked, and the batteries topped up while I 'got my eye in' on a well-known model.

The flying trials were carried out from a grass strip. The grass was not as short as the taxi trials venue, but was probably typical of a good average club field. The first attempt to take off proved that full up elevator on low rates did not provide sufficient elevator authority to prevent a nose-over. No damage was done, so a second attempt was made with more elevator throw. I gradually increased power, and as the model gained speed I progressively relaxed the elevator, allowing the tail to come up.

In spite of having more than 1,000 Watts on tap, the model showed little tendency to yaw during the take off which was a pleasant surprise. To prevent the possibility of a disastrous low-level stall on rotation, I made sure that only a hint of 'up' was present as the model broke ground.

The Mustang rose cleanly into the blue sky, climbing easily even though full throttle still had yet to be reached. The gear was retracted, and the model was found to require only a little trimming; just a few beeps of aileron and it maintained a wings level attitude. In pitch, I found the P51 needed a little down trim to maintain level flight at half throttle, which suggests that my initial assumption about the intended CG reference point was correct.

Half throttle was found to be a comfortable setting for cruising around. Being a reasonably large model, the P51 does not appear to be an exceptionally fast machine, but it was certainly no slouch, as it covered a fair bit of sky quite quickly. It looked very scale-like, which is surely much of the point of a model like this. I was very surprised at how soon I felt comfortable with this enjoyable model. I was quickly happy to make low-level passes and turns (on an otherwise unoccupied flying site) for the benefit of the camera.

After about 6 minutes the undercarriage was lowered, which as expected caused a slight nose-down change in trim. This was easily compensated for by adding some up trim. During the downwind leg, I added a little more up trim and reduced power so that the model was in trim at a lower flight speed, as appropriate for an approach to land.

Landing– first flight
The model was landed gently. As the speed decayed, up elevator was progresively applied, but this was insufficient to prevent a nose over.

Power system performance
I’d limited this first flight to 6 ½ minutes, as I felt this was probably close to the likely safe maximum duration with a 5,000 mAh pack. I was astounded to find that only 1,500 mAh was required to replenish the battery. This represents only 30% of the total battery energy, and indicates that the model was only consuming an average of approximately 14 Amps/300 Watts. This is a very surprisingly modest 34 Watts per pound of model weight, which seems to indicate that this is a particularly efficient combination of airframe and power system. Whatever the reason, clearly considerably longer flights were going to be easily possible. The battery was slightly warm to the touch after the flight, indicating that it was adequately (but not excessively) cooled and that it had plenty of performance in hand.

Second flight
During this flight I wanted to further investigate the model’s behaviour. The model was taken to altitude and deliberately stalled. I found it had to be slowed down to a remarkable extent before the stall occurred. On my example, the stall in a power off condition caused the left wing to drop. Being a relatively large model, a significant loss of height occurred after the manoeuvre. There was no mistaking the high angle of attack necessary for this model to stall in level flight. I was left feeling confident that this model would be quite difficult to stall inadvertently.

Next, a spin was tried; the model was slowed down as for the stall, and just before I judged it was going to stall, full rudder was applied. This caused the model to enter a fairly leisurely spin. Less than one further turn was required for recovery, which was accomplished simply by neutralising the controls.

Aileron control is very smooth and the model showed no sign of adverse yaw. Rolling manoeuvres look beautiful and the model is responsive, but not at all twitchy. There is plenty of power to help the model through large and impressive scale-like loops. Low, fast passes were simple, with the model feeling very stable and controllable.

After 11 ½ minutes the model was landed. Again the landing ended in a gentle nose over even though up elevator was progressively fed in as the model slowed. Fortunately no damage resulted. This nosing over tendency was partly due to the length of the grass, but I feel that a contributory factor is the proximity of the wheels to the model’s balance point. Overall though, I was thoroughly impressed with the Mustang’s handling and this factor is really the only thing I can fault about the model’s behaviour.

I was again impressed at how little energy this consumed, with the battery requiring only 2,700 mAh to recharge. This indicates that a 5,000 mAh battery has the potential to fly this model for a maximum of around 20 min. if required, although a shorter flight time would result if full throttle was used a lot.

Third flight
Prior to the third flight, 30 g of lead (about 1 ounce) was added to the tail of the model to move the CG rearwards a little. I hoped that this would improve the model's tendency to nose over on take off and landing. This worked, and the model was still stable in flight with no bad habits.

Performance Assessment
I would estimate that the performance is similar to a 0.60 i.c. engine on the installed electric power system. One of the beauties of electric power though is of course that the performance can be increased, within reason, simply be changing the prop for one which demands more current and hence power. I may well try a 16 x 10 and even a 16 x 12 in the future, although the model already flies very nicely.

Conclusions
This was a most enjoyable project, even if it did give me a little brain ache at times! A lot of work went into the project and it took far longer than I expected. In fact, the amount of work in planning and carrying out the conversion greatly exceeded the time required to assemble the parts of the model which did not need modifying. I estimate I could have assembled at least five of these models for i.c. power in the time it took me to build and convert this one!

However, the result of my efforts was worthwhile; I was rewarded with a beautiful, easy to fly and practical electric warbird (albeit in a civilian colour scheme in this instance) that flies very well.

The flight performance and handling of the model is excellent, and I have it on good authority that Hangar 9's other WW2 warbirds handle similarly well. The P51 is not the cheapest model of this size but in view of the quality of the model and its flight performance in particular, I feel its good value for money.

Hangar 9's evergreen Mustang has been around for a while, and I hope it will be around a lot longer so that many more modellers can enjoy its charms.

Lessons learned
Next time, I'd almost certainly choose 2 x 3S batteries instead of a single 6S example. This would slightly complicate charging, but would provide more flexibility in that either pack could do duty in a model requiring a 3S pack. Also, if in an accident the 6S pack was to become significantly damaged it would be of no further use, whereas with a pair of 3S packs one may survive.

A slightly smaller battery would still be perfectly suitable for this particular model, especially if continuous high speed was not required. This would result in significant cost and weight savings which are always welcome.

For the future
I plan on packing up the rear edge of the retract unit in order to get the leg to lean further forward when it’s extended. I may also make some new gear doors which fit flush with the lower surface of the wing, reducing drag.

Note: All the power system themes touched on here such as power loading, pitch speed and much more are explained clearly in the 3-part series on Electric Power Systems

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