Reproduced from the RCM&E, February 1979, with the kind permission of the Editor, Nexus Specialist Publications.
Redshift 60 tested
The first news of this innovative British 10cc R/C engine was revealed in the April 1977 Radio Motor Commentary when an illustrated description of the prototype was published. This was followed, last August, by a description of the excellently made production model. As the design and construction of the engine has, therefore, been adequately covered previously, our present remarks are primarily concerned with the Redshift's performance.
The Redshift is supplied without a silencer, but several current .60 size silencers will fit (or can be adapted to fit) and, for our tests, we used both the standard HP expansion chamber (as sold for use with the current Hirtenberger HP61 Silver Star and Gold Cup series engines) and the Webra 1100/ E-G6 silencer as intended for the Webra Speed 61 series. Of these two, the HP (at the expense of slightly reduced noise suppression) causes less power loss and was therefore used for most of our tests.
The manufacturer's recommendations in regard to fuel are a 75/25 mixture of methanol and castor oil for the first hour of running time, after which the oil content can be reduced to 20 per cent. Also recommended is the addition of 3-5 per cent nitromethane for 'improved throttling characteristics.' We used our standard R/C test mixture of 75/20/5 methanol/castor-oil/nitromethane. This added 200-300 r.p.m. with the engine propped for full-throttle speeds of around 12,000-13,000 r.p.m. static and markedly improved the Redshift's tolerance to mixture strength
variations, eliminating a tendency for the engine to abruptly cut out when the needle valve was leaned out too far.
Using the five per cent nitro fuel, HP silencer and a K&B bar-type long- reach plug, the following prop r.p.m. were recorded after running in: r.p.m.
16 x 4 Top Flite maple 8,000
14 x 6 Top Flite maple 8,900
14 x 4 Top Flite maple ,.10,500
12 x 6 Top Flite maple 11,400
12 x 73/4 Power Prop maple 11,600
11 x 7~ Zinger maple , 12,400
11 x 7~ McCann epoxy/glass fibre/carbon-fibre 12,900
11 x 6 Zinger maple ., 13,400
11 x 6 Top Flite maple 13,700
Even allowing for the moderate power absorption of the HP silencer, with its unrestricted 87sq.mm. outlet and 110cc volume, these are excellent prop r.p.m. figures and suggest that the Redshift is comparable with the most powerful imported engines currently available. In fact, subsequent open-exhaust dynamometer checks to establish its gross b.h.p. confirmed this to be so.
One thing that is essential, with the E.D. carburettor fitted to this engine, is a pressurised fuel supply because, despite Redshift designer Dick Fisher's efforts to maximise primary compression by an ingenious method of reducing effective crankcase volume, the carburettor will not pull fuel from the tank without assistance.
Of course, this is no problem, in the normal course of events, when one is using a silencer and can tap exhaust gas pressure to pressurise the fuel tank. Difficulties arise only when one removes the silencer in order to establish, for purely statistical reasons, the engine's gross output. For this, we resorted to a Robart, pump. (See Radio Motor Commentary, April1977 for details of the Robart). This, like the Perry Pump, is strictly a pump: it draws fuel from the tank and passes it on to the carburettor but, unlike the Perry, does not incorporate a regulator unit. It is very easy to
fit, since the only modification required to the engine is to drill and tap the
crankcase back plate to take a 1/16in. i.d. pressure nipple.
An alternative to the Robart, of course, would be to pressurise the fuelank from the crankcase, using the rotary-valve to provide only positive pulses, and to control the high pressure fuel flow so created, through a 'TK' Pressure System hook-up. TK systems, in fact, were favoured (in place of ordinary silencer pressure) by a number of Redshift users last season.
One or two changes have been made to the E..D. carburettor since our previous description. Instead of having a standard 13mm spigot diameter with an adaptor to fit the Redshift's 16mm intake boss, the latest engines have a new carb with a 16mm spigot. There is also an optional extra, in the shape of a modified needle-valve in which the spring ratchet is replaced by an O-ring and sleeve. Since, during the running-in of the engine, a month or two before testing, the original needle-valve had been found to leak, this latter modification was considered a worthwhile improvement and was therefore fitted to the engine.
Turning back to silencers for a moment: for the benefit of those readers who might have wondered what was inside the current type Webra silencer (earlier types were plain expansion chambers or vented front types), we are including a photo of the Webra 1100/E-G6 that was also tried on the Redshift. The two halves of this are 'glued' together, evidently with epoxy or polyester resin, which, in the presence of heat and vibration, may not be the best method of joining them. This was made abundantly clear when, with explosive suddenness, the Redshift shot up to 100 plus decibels as it spat the silencer's innards onto the workshop floor.
As can be seen from the photograph, the body of the silencer is divided from the tail section but the two parts are linked by a 25mm o.d. cylindrical baffle. Gases enter the cylinder through eight 4mm dia. holes (total area 100sq.mm.) and are then transferred through a similar series of holes into the tail cone of the silencer from which they escape via an 11mm i.d. (95sq.mm.) outlet nozzle. Incidentally, the two diametrically opposed holes in the rear part of the main body section and the corresponding screw holes in the tail section are non-standard. These were added later to enable the two halves to be joined more securely.
Below: the Redshift 60 with Robart fuel pump. This has a tubular diaphragm that is operated by positive and negative pulses tapped from the crankcase. A pumped or pressurised fuel supply is essential with the E.D. carburettor.
Right: the inside of the current Webra 1100/E-G6 silencer. See
text for details. (Redshift 60 tested).