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Alan Simmons' Vickers Viscount

Article by Andrew Gibbs

Type 663 Vickers Viscount by Alan Simmons

I recently had the pleasure to meet up with Alan Simmons, who had with him a new model of an unusual prototype, a Vickers Viscount. As if that were not unusual enough, it’s a one-off experimental development version of the Viscount, the Type 663 Tay Viscount, which was powered by a pair of Rolls Royce Tay turbojet engines instead of the Viscount’s usual Rolls Royce Dart turboprops.

I was surprised to find out how old this design is; the turboprop Viscount first flew in July 1948, while the subject of Alan’s model first flew in 1950. This jet aircraft was the second prototype Viscount and was built as a test-bed. Sadly, the full size machine was withdrawn from use in 1958 and subsequently broken up. However, a number of Viscounts remain in museums and remarkably, three examples are still in service at the time of writing.

Prototype Type 663 Viscount

2 The prototype Tay Viscount was powered by a pair of Rolls-Royce Tay turbojet engines and first flew in RAF Markings as VX217 at Wisley on 15 March 1950.

Picture was sourced from Wikipedia:

Model Viscount

3 The Tay Viscount is a wonderfully refreshing shape. Alan’s done super job of preserving the aeroplane’s lines and its character, and the model has real presence.

The model is of all balsa construction. Alan chose to model this aircraft both because it appealed to him, and because he thought it would be a relatively straightforward project. However, he discovered that in aeromodelling, as in life, appearances can be deceptive! For example, during the design phase, Alan had to solve the problem of cooling the ESCs. Normally, these would be placed directly behind the fans, but for this twin motored, single battery model this posed a problem. It is unwise to lengthen the battery to ESC leads very much as this can lead to ESC problems, but in order to put the ESCs behind the fans this is exactly what he would have had to have done. The ideal location from the cable length standpoint was to position the ESCs in the wing itself, but this left the problem of cooling them.

EDF model Viscount

4 The model’s windows are moulded transparent plastic and incorporate some curvature. Each transparency is backed with black card, and this measure appears to give the windows depth.

EDF RC model Viscount

5 The model is finished in glass fibre, which adds a great deal of strength for a minimal weight penalty. The value of this was demonstrated during a take off accident – the model cartwheeled, yet remarkably sustained almost no damage.

ESC cooling plates

6 The flush mounted metal cooling plate used to cool the ESCs can be seen here, located under the leading edge of the wing. This system of cooloing works surprisingly well. The ESCs have no cooling air passing directly over them; instead they shed their heat to the metal plate with which they are in contact..

EDF model aircraft Nacelles

7 Each nacelle houses a WeMoTec minifan and Mega 16 series EDF motor. With only two winds, the motors have a high Kv of 3,350. Each motor draws about 35A on a three cell LiPo. The model’s intakes have been swelled a little from scale. This was necessary to make a viable model, and in my view, this deviation has not spoiled the model’s character at all.

Alan had the idea that perhaps the ESCs could be cooled sufficiently by having them within the wing, but in firm contact with a metal plate which was exposed to the airflow. The idea was that the ESC would shed its heat to the metal plate, and the plate to the passing airflow. Before finalising the design of the model, Alan carried out a test. He anticipated that the ESCs might well overheat, but was surprised to find that in practice they did not go above 33°C (91°F). Two additional factors make this result all the more remarkable; firstly this was on a static test, without the benefit of cooling air passing over the plates, and secondly the heat shrink covering was left on the ESC’s. Pleased with these results, this design detail was duly incorporated into the finished model, and no ESC overheating problems have been experienced. Alan’s next step will be to use an Eagle Tree data logger to record in-flight temperatures on the back face of the ESC’s, which he expects will be a bit higher.

The Tay Viscount’s nacelles looked particularly slim although this was something of an illusion created by their length – the centrifugal turbojets were fatter than the later axial designs, but Alan had to make the nacelles a bit fatter than scale to allow for the 72mm outside diameter minifans to be accommodated at the chosen scale. The intake and exhausts are enlarged by a smaller proportion, the result being that the ducting is divergent towards the front face of the fan, and convergent toward the tail pipe. This is common with EDF models, and has proved no problem in practice.

RC model main gear legs

8 The main gear legs of the left nacelle. These will both be made retractable, and will shorten as they do so. There’s not a lot of room to achieve all this, so it’s a good job Alan enjoys a challenge!

RC model nose gear leg

9 In time, the nose gear of the Viscount will be made retractable, in a forward direction. Compared to the main gear, it will be simple to make this leg retractable.

BAe146 gear leg

10 This is the left hand main gear leg on a BAe146 airliner. The leg retracts inwards, and shortens in length as it does so. This is achieved by the trailing link part hinging in an upwards direction.

RC model gear leg extended

11 The model Viscount gear leg will shorten in a different way. It is seen here in the extended position.

RC model gear leg partly retracted

12 The gear retracts in a forward direction, and is seen here part retracted.

Gear leg retracted

13 The Viscount main gear leg in its fully retracted position. Notice the way the leg has effectively been shortened.

The main gear is also squeezed into the nacelles. At present the undercarriage is fixed, although as can be seen above, an ongoing project is to make it retractable. Each nacelle houses two separate gear legs. Each leg has a single wheel, and will retract either side of the jetpipe. As per the prototype, the gear legs will be made to shorten as they retract. This particular detail means that the work required is of a complexity that renders the gear a separate project in its own right. This will be a design challenge, but I am sure Alan is well up to the task. Shortening gear legs are found on a number of aircraft including the BAe146 (see photos above).

For energy, the model carries a single 3-cell 4,800mAh LiPo battery, from which both Mega 16 series EDF motors are fed. At full power, the battery voltage falls to about 10V, with each motor drawing about 35 Amps. The total current draw is thus 70A, equating to a power consumption of around 700W. A current of 70A from the high performance 4.8Ah battery represents a discharge of under 15C, so the pack is not unduly stressed.

The model is still under development, and had only been flown once or twice when I saw it. However, Alan reports that two minutes of flight uses about 1,400mAh from the battery, so the expected practical maximum flight time is around 5 or 6 minutes.

The fan rpm is estimated like this; the motors have a kv of 3,350, so on 10V the no-load speed would be 33,500rpm. This can be multiplied by about 0.8 to estimate the actual rpm, giving an estimated figure of 26,000rpm. It would be interesting to compare this to a actual measured figure.


Vickers Viscount power system data
1 x 3S 4,500mAh LiPo
  RPM (approx) Voltage Current
Pitch speed
Full throttle 26,000 10 70 700 n/a
Cruising flight TBA TBA TBA TBA n/a


RC model receiver installation

14 The entire cockpit lifts off to provide access to the equipment bay. Alan has used the near 90 degree angle of the prototype’s aerials to good effect in mounting his 2.4GHz receiver aerials.

RC model rudder hatch

15 The dihedral tail is a distinctive feature of the Viscount. Alan added a small hatch at the base of the model's fin to allow access to its rudder linkage.


Alan's model incorporates a number of hatches. I was impressed at how simple and effective these were, adding very little weight to the airframe. The next few photos detail how these were made.

RC electric model belly hatch

16 The model incorporates several hatches. This belly access hatch allows Alan to connect the electrics after the batteries have been installed.

RC EDF model hatch latch

17 Each hatch has a simple and effective latching system. Here the latch is about to be pulled down into the open position, which will allow the hatch to be closed.

Hatch latch

18 The hatch itself is shown here in the open position, while the wire latch is in the closed position.

RC model inside construction of hatch latch

19 This is the back of the hatch, showing the simple wire latch. Ingenious!

Finally, here is a summary of the technical data for this fine model of the Type 663 Viscount:

Tay Viscount technical data
Span 1,571mm 62 inches
Length TBA TBA
Flying weight
(1 x 4,500mAh 3S LiPo)
2,500g 5lb 8oz
Wing Area 0.29 sq m 449 sq in
Wing Loading 00g/dm 28 oz/sq ft
Batteries 1 x 4,500mAh LiPo
Motors 2 x Mega ACn EDF; 2 wind, 6 pole
EDF unit 2 x WeMoTec Minifan
Max Power 700W
Power Loading (max power) 286W/kg 130W/lb
Power Loading (average power) TBA
Control functions Rudder, elevator, ailerons & throttle. Retracts are under development.

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