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Boost tabs - a boon for large-scale models

developed from information by

Roger Forgues

Roger Forgues' 35% Yak 54

One of many challenging aspects of building and flying really large models - say 25% or bigger - appears to be the question of precise, fast and responsive control of the control surfaces of the plane - ailerons, elevator and rudder. The aerodynamic forces brought to bear on large control surfaces are considerable, and such that multiple powerful, heavy-duty servos are often required for each control surface to be able to handle them. Such servos do not come cheap, either, with £50+ a time being fairly typical.

But it doesn't stop there! When multiple servos are ganged together to drive large control surfaces in this way, they need to be precisely matched, otherwise the servos will not move equally and will fight each other. To explain the problem, the following example was recently given in Model Airplane News. "Let's say you use two servos to drive a single aileron. Servo "A" rotates 55 degrees, and servo "B" rotates 60 degrees. When servo "B" tries to reach the end of its travel (60 degrees), servo "A" is already at the end of its travel (55 degrees). In essence, the servos are fighting each other, and that demands more energy from the battery".

To overcome this problem, some further control is required, and one such answer lies in JR's revolutionary MatchBox that allows you to easily make up to four servos move exactly the same amount, preventing them from working against each other. Intended for modelers of giant-scale craft and others who want to use multiple servos on a single channel, the JR MatchBox is an onboard electronic device that precisely matches servos used on a single channel - so that means a separate MatchBox at £46 each for each independently controlled control surface! For more on the JR MatchBox click this link. In some other solutions, two receivers have to be used to avoid running too much current through one.

Next comes the likelihood of the need for auxiliary battery packs, due to the huge demands being made on the batteries and the receiver - if the servos draw excessive current, the power to the receiver will drop off and cause it to work erratically. I recently read about a large-scale Ultimate that was using 15 servos - 14 of them digital - and this model was fitted with three separate batteries; one battery powers the servos, one is for the engine's ignition, and the third powers the receiver and the throttle servo. Now the costs are really mounting (and they don't necessarily stop there, either)!

But there is a very interesting alternative to this complicated and costly process which Roger Forgues has written to me about and which he has also been advocating on his thread on the Flying Giants website. Roger says that you don't need to install multiple servos on a control surface which would also bring the necessity to install MatchBoxes to calibrate the servos. You also do not need to install expensive servos that can bend steel! Batteries can be smaller capacity, no need for Lipos, so no need for regulators, and the list goes on. So, on a 40% aerobatic airplane as an example, Roger suggests you could end up saving close to $1000.00, you would have a lighter airframe, and have better control input.

And this is all achievable, says Roger, by the use of boost tabs.

Boost Tabs - what are they, then, and what do they do, and mostly, how can they be of help? Boost tabs have been around on full scale and model airplanes for quite some time, so it's not new, but judging from some of the responses on Roger's Flying Giants thread, there are a goodly number of large scale model builders who haven't heard of them. Basically, they are 'power assist' to your servos, similar to the power steering of your car.

A boost tab is a small auxiliary control surface mounted in the trailing edge of the main control surface to which they are offering assistance. They are actuated by a direct linkage which moves them in the opposite direction to their 'parent' control surface. Click the thumbnail image of Roger's large-scale (35%) Yak 54, left, and you can clearly see them on all three control surfaces. As Roger has done on his rudder, right, with some installations you may also install a tiny servo in the control surface to directly control the boost tab - click the pic for a close-up.

In an excellent article on the subject, Carl Risteen explains the action of the boost tab as follows.

"When an elevator moves to the "up" position, the boost tab moves "down" and tends to lift the trailing edge of the elevator - thus assisting the servo. The farther the elevator moves, the farther the boost tab moves in the opposite direction to counter the increasing aerodynamic force that opposes the elevator's motion... The result is a nearly linear assist (almost like hydraulic boost) that's far more effective than putting balancing area ahead of the hinge line, particularly at large deflections. Boost tabs can be retro-fitted without great difficulty to perk up sluggishly responding models. Thanks to boost tabs, one of my large models needed only one servo on the ailerons to get twice the roll rate that two servos had previously provided.

The degree of assist is adjusted by varying the relative travel of the boost tab to that of the control surface. This can be accomplished by experimenting with various linkage holes in the control horns..."

This picture is of an aileron boost tab installed in one side of the elevator assembly of the empennage of Roger's Yak 54. You can see that as the servo mounted in the fixed main surface at A moves the principle control surface B in one direction, the boost tab C will be moved in the opposite direction by virtue of the control rod fitted from the main surface to the control horn mounted on the tab (click the picture for an enlarged view). This is a purely mechanical linkage operated automatically by the movement of the main control surface.

Concluding his article, which you can read in full here, Carl Risteen says that for those who may have been struggling with overloaded servos on a giant-scale model, slaved boost tabs (as described above) are well worth a try. They will reduce the load on the servos by about 85%. Direct, servo-operated boost tabs, athough they're a bit more complicated, open up new vistas of really gargantuan, high-power models with tiny servos providing all the control response the pilot could desire. Roger Forgues recommends them as a cost-effective answer from the start.

Below, you can see stages in the boost tab construction of one side of the wing of Roger's Yak.

Roger Forgues of Forgues Design develops, manufactures and markets specialist products for large radio control model airplanes - visit his website at


C. Rupert Moore, A.R.C.A.

A man to be remembered

Likely to be remembered mostly by free flight rubber power devotees of the older generation within the model flying fraternity, and more widely as the artist who created many of the covers appearing on Aeromodeller magazine from the mid 1940s, C. Rupert Moore was a truly innovative and inventive model maker, as well as a creative artist.

Not having that particular model flying background and having only come into model flying in my retirement years, I had not heard of Rupert Moore until he was mentioned by Robin Tuff in respect of Joe Scordato's search for a rubber-driven twin-engine drive system, Mr Moore having actually patented such a system in 1938. Patented under the wordy description "Means for transmitting power from a central power unit to one or more airscrews mounted externally of the unit housing" it was known as the Moore Drive, depicted in this wing cutaway illustration, below, by Rupert Moore himself which appeared in an article on the system in Aeromodeller February 1943. (Later - certainly by the mid-fifties - the magazine changed its name to Aero Modeller and is now produced again as a supplement to Aviation Modeller International.)

Click the image for a slightly enlarged view

There is further information on the drive itself on this month's talking point, but I was keen to find out more about the man himself. My own initial attempts to find information on Rupert Moore were not too productive, but Colin Stevens kindly went surfing on my behalf and his results, together with links from Robin Tuff and my own bit of surfing, provided me with enough to piece together a short, albeit somewhat 'gappy' biography of this interesting man. The various sources of information used here are acknowledged at the foot of this article.

C. Rupert Moore was born in 1904, at the very outset of controlled powered flight, within twelve months of the Wright brothers having achieved their first historic flight. Pioneer aviator Claude Grahame-White was touring around the country showing everyone his wonderful flying machine, landing on common land and thrilling the crowds. It is recorded that these heady days of pioneering aviation were to leave their mark on a small Rupert who would pick up the pieces of fabric from these aeroplanes and take them home for his wall, observing the colours and recording them in his many notebooks and apparently producing wonderful sketches and evocative scenes.

Suitably impressed he would try and emulate the flight of these machines by producing A-Frame pushers. Made by bracing together two bamboo rods with a wing made from oiled silk, small propellers were twisted from tin with a bearing made from a Colmans mustard tin - the surprise was they flew fairly well despite their heavy construction.

Rupert Moore's artistic abilities led him to attend art school and his collection of artifacts grew. He was good friends with someone who flew on test flights with the MAP (Ministry of Aircraft Production) and would frequently be entertained as a De Havilland Mosquito beat up his house! The same person would provide yet more fabric and colour samples to add to his growing collection, the bits usually coming from crashed aeroplanes but gladly snapped up. MAP was not founded until 1940, by which time Rupert Moore would have been 36 years of age but I have been unable to determine what Moore was doing at this time.

C. Rupert Moore's original "Castor", incorporating the Moore Drive and immaculately restored and displayed here by Mike Beach. Photograph by Ken Croft.

Certainly, by the mid-1930s Rupert Moore was well established in the hobby of model airoplane design, building and flying. In 1936 he set out to make twin engine rubber powered model airplanes and by 1938 he had invented the Moore Drive - and made it work. He reports that in 1938 he flew a scale "Castor" in the Frog Cup Elimination and averaged a remarkable 54 seconds. Moore also designed and developed the Moore Diaphragm - a device to prevent rubber bunching causing CoG change induced trim shift in his big, rubber-powered scale models like his 44" Tiger Moth and others.

Another of Rupert Moore's innovative designs was a Hawker Typhoon which had an ingenious retractable undercarriage which was raised when the rubber was fully wound and lowered as the power was expended. Moore's original model pictured above left, is another one that has been restored by Mike Beach. This mechanism was also incorporated into other model designs.

By the mid-1940s, Rupert Moore was working for the Aeromodeller magazine who had offices at The Model Aerodrome, Eaton Bray (see also 'A day at Eaton Bray', club scene, August 2007), where his job was to provide the all-essential artwork for the covers of the magazine, a job he did to much acclaim right through until the late sixties. His cover artwork had a remarkable quality, accurately recording the design and colours of his subjects and at the same time portraying them in a dramatic and evocative style.

Remembering those covers with some affection, regular modelflight reader and contributor Colin Stevens writes, "What I do particularly recall all those years ago was the eager anticipation of the next issue of Aeromodeller, to see what Moore's picture was going to represent, and then finding enough in his illustration to gaze at it for many minutes.He not only depicted an aircraft accurately, but he could also conjure-up an atmosphere, and therein lay the charm." The June 1948 issue cover (right) was notable in that it depicted not the usual artistic impression of a full size aviation scene, but was used as publicity for Jetex's new micro rocket motors when it depicted Rupert Moore's impression of a twin Jetex powered Gloster Meteor model in steep ascent.

Two examples of C. Rupert Moore's Aeromodeller covers

C. Rupert Moore's artwork for the Aeromodeller covers was really only the tip of the iceberg in terms of his aviation art legacy, though. During the same period that he was producing the Aeromodeller covers, Moore also provided the illustrations for a number of notable aviation publications. The first edition of his own book Aircraft Paintings was published by Harborough Press around 1945 and it contained 16 large prints with descriptive notes and included a 12-page biography of the man. He also produced the artwork for the dust jackets and frontispieces of Owen G. Thetford's seven-volume Aircraft of the Fighting Powers as well as the cover paintings and coloured plates of Thetford's Camouflage '14 - '18 Aircraft and Camouflage 1939 - '42 Aircraft. It appears, too, that there were some paintings and prints, one of which is stated to be held in the Royal Aeronautical Society Library's art collection and which was also produced as a fund-raising jig-saw. Still with an aeronautical interest, Rupert Moore was responsible for some black and white illustrations in Bird Flight for Bird Lovers by Jack Parham - published in 1944 this title set out to compare bird flight to aeroplane flight.

Opposite the entrance doors of St. Colomba's Church in Grey Lynn, Auckland, New Zealand, and dedicated to Mrs Henrietta Woodhall in 1969 and her husband John Woodhall in 1972, who served in World War Two, are two stained-glass windows. The left-hand light depicts the Mother of Jesus and the infant Christ and the right hand depicts Christ's baptism. On the right wall of the nave there is a further stained glass window which was dedicated in September 1975 to the memory of John Woodhall; the left-hand light depicts Jesus crucified whilst the right-hand one portrays Christ resurrected. Unfortunately, the folk at St. Colomba's could tell me little more about the origins of these windows and, in particular, their artist. His name? C. Rupert Moore.

C. Rupert Moore died in 1982 at the age of 78, relatively unknown outside of model aviation circles, perhaps, but in the comparatively small world of our hobby, a special man indeed.

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