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An aircraft is sometimes described as 80000 parts flying in close formation, and while most of these parts are fairly straightforward and found on most designs, the VC10 has some peculiarities that are not commonly found on an airliner. They deserve some attention. Periscopes Obviously when mentioning periscopes many people will think of submarines. They are however not the only vehicle using them. While it seems a bit strange the VC10 was designed to have a periscope included in its equipment. And not just one, but two! The reason behind this is fairly straightforward: as a pilot you sometimes want to see what's happening with your aircraft. And with that great big tail at the back, well, there's just no window through which you'll be able to get a look at what's happening up there. Normally this will not be a problem, but when you encounter icing conditions you might want to know how this is affecting your aircraft. The wings can be seen through the cabin windows, but not the tail and for this there is the periscope.
Normally the periscope will be stored in its box somewhere in the back of the aircraft. In A40-AB, the Sultan of Oman's aircraft, this box is kept in the aft galley. The periscope itself is about two and a half feet in length, and is a pretty sturdy piece of ironmongery. There are two mountings for the periscope in the roof of the fuselage, which are similar in design, both to be found just in front of the rearmost toilets in the aircraft. These can be revealed by undoing two quick-release catches on one of two small triangular panels in the roof. This provides the sight below.
The large hole on the right is the actual mounting into which the periscope is inserted. Once inserted partly, a catch is released and then the large lever can be pulled down which opens the small hatch and allows the periscope to be fully inserted, which has it protruding some three to five inches from the fuselage top. A catch keeps the periscope from falling down again, this catch is released with the small lever on the left, after which the hatch can be closed again. Preferably this is done before the periscope is completely removed from the mounting as it can get a bit drafty otherwise. In the photo above, the small notice reads: 'Periscope operation - The large handle operates the pressure flap - The small handle operates the catches'.
In its installed position the periscope can be rotated 360 degrees, and the control on the side tilts the mirror at the top about 60 degrees up and down. All together you can look all the way around the aircraft, except for straight up at the sky (but there shouldn't be much to look at up there anyway). By switching from one mounting to the other both sides of the fin can be viewed. From the lefthand mounting, looking to the right wingtip will have you looking at the base of the fin as the periscope sticks up right next to it.
This same periscope can also be mounted in two other locations. The first of these is in the cockpit roof. The mounting installed here was meant originally for use with a periscopic sextant, but will also take the periscope. During BOAC crew training flights where Dutch Roll characteristics were demonstrated trainees were sometimes given a chance to view the back end of the plane from here to see the effects. The second location is located in the bottom of the fuselage and was included to enable the crew to check the status of the landing gear if any uncertainty about its position was present amongst the crew. It consisted of an opening in the electronics bay access hatch located underneath the fuselage. Unlike the others there was no pressure shutter to cover this opening - it was just a plug which could be removed and replaced with the periscope - since it would generally only be used at lower altitudes where the pressure differential would be lower. Still it has been confirmed to me by an ex-BOAC First Officer that the plug could be removed when flying at 39000 feet, although this needed quite a pull and the result was quite noisy! And what about the second periscope then? To find this piece of equipment we will have to look around the back galley area. Somewhere in this area there will be a small piece of floor covering (about 6 by 6 inches) that can be lifted up. Underneath is then a small tube with a prism at the bottom that can be lowered into the aft cargo hold. The reasoning behind this small periscope is the fire-surpressing capability of the cargo hold. If a fire should exist in the cargo hold then the detection system will pick this up, but there are no extinguishing agents at hand to use. Basically the cargo hold is designed to be sealed so that the fire will die from oxygen starvation. As a fire warning can also be due to a faulty detection system the periscope is a simple device that can confirm or deny the existence of fire. Another reason for looking through it could be to confirm that a fire has actually died out, as a quick landing might be in order if it hasn't. Mind, I myself wouldn't want to be there on the floor watching a fire eating away at the aircraft I'm on. Thrust Augmenters These small pieces of equipment should be classified as belonging to the pressurization system of the VC10. A bit of background on this: the cabin of an aircraft is pressurized by pumping in compressed air at a constant rate and the cabin pressure is then regulated by varying the amount of air that leaves the cabin again through the outflow valves. On the VC10 the outflow valves are located at the left front of the fuselage next to the nosegear bay, and in the tail of the aircraft. The compressors on the engines provide the compressed air and this is constantly fed to the cabin via the airconditioning system which manages temperature and humidity.
From the above it is clear that there will be a constant flow of
air leaving the aircraft through the outflow valves. Someone in the design
office saw this as a waste of energy and designed the thrust augmenters to
recuperate some of this energy. The thrust augmenters are two small tubes
mounted at an angle in the fuselage side, one at the right front just behind the
nosegear, and another below the righthand side engines. These tubes are angled
towards the rear and through a switch mounted on the flight engineers panel
valves are opened in these tubes allowing cabin air to exit through them. The
tubes are about two inches in diameter and the amount of air passing through
them will result in the outflow valves closing fractionally to keep the cabin
pressure constant.
This same air flowing through the augmenters, being angled back will provide a small bit of forward thrust. I am guessing that the amount of thrust produced by this system cannot be measured in knots of speed gained, but still, over a longer flight it could well have saved a small amount of fuel with the added weight to the aircraft being fairly small. It does seem a bit strange to have this system installed on the right side only, but as the forward outflow valve is mounted on the left side, I'm guessing that the sideways force created by the angle in the thrust augmenters will oppose the same effect from the outflow valve. The next question will be why the outflow valves themselves are not angled like the thrust augmenters, making full use of the potential. The main reason for not doing this is probably because of the size of the outflow valve assembly. Also because of the range of pressure differentials that the system needs to care for the pipe from the outflow valve has a much larger diameter than the thrust augmenter and this will not provide the same amount of thrust as a smaller diameter pipe for the same pressure differential. The Engine/Airframe course notes for Ground Engineers have this to say about them: "The thrust augmenters are normally used above an aircraft altitude of 35,000 Ft to enable the exhaust airflow from the cabin to be usefully employed in giving a percentage of additional thrust to the aircraft”. In an article in Flight International from 10 May 1962 it is estimated that the thrust augmenters would generate a fuel saving of 50 Lbs/hr. A small addition to this subject from Martyn Taylor is the following thought: "It was always my understanding that the reason the thrust augmenters were fitted to the right-hand side of the fuselage was to offset the extra drag created by the fifth engine that could by carried under the starboard wing." Perhaps this was true, but I cannot confirm nor deny it, anybody else with more info? Oh, and more about the fifth engine pod later!
An interesting sidenote to this item is provided by the page reproduced here from the Concorde Component Location Handbook. In the chapter for Pressurization Control item 4 on this page seems a bit familiar: Thrust Recovery Nozzle. Mounted at a 45 degree angle (or thereabouts) behind the nosegear bay are two of these items and from the drawing it looks suspiciously like an improved version of the thrust augmenter described above. On the Concorde a louvred exhaust is included which is operated by a pressure switch. It is interesting to see that the ideas that were present when the VC10 was designed may have found its way into what is still regarded as the masterpiece of aeronautical engineering: Concorde. Another airplane which uses this idea is the McDonnell-Douglas DC-10 (built between 1970 and 1988), which is fitted with a Thrust Recovery Valve.
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