Max Holste 152 (oz16175)

About this Plan

Max Holste 152. Radio control scale model, for electric power with GWS geared motor.

Quote: "Full-size plan feature. Max Hoist 152, by John Ralph.

Half a century on from George Woolls' original Max Holste MH-152 (oz1837) rubber-powered model from the pages of April 1954 Aeromodeller magazine, John Ralph's 50% enlargment makes an excellent basis for lightweight and even Indoor RC scale, and could be enlarged even further for bigger electric motors or IC power. Anyone game for an even larger version?

My objective with this project was threefold, firstly to set the boundaries of model specification, which result from exploiting recent advances in battery technology. Secondly, to identify off-the-shelf lightweight RC systems, that can use the new batteries and finally, to present a converted free flight scale model from the past which is typical of the many elegant designs still available in the Aeromodeller archives.

Batteries: Nicads OUT, Li-ion IN! For decades, we have relied on the ubiquitus rechargeable Nicad and more recently NiMh cells for our radios and electric flight. For a while, in many cases, we will continue to do so, but there are same alternative 'New Boys On The Block'.

A couple of years ago 'Tadiran' (LiMgO2) cells started to be offered by some specialist suppliers and in the last year or so Li- ion and Li-Poly types have appeared. I will not take much space discussing Tadirans since they are being displaced by the Lithium Ion types. Suffice to say that the unit cell voltage of the Tadirans is about 3.2V and the size available is/was AA, with a capacity of 750mAh. This means an energy density some five times that of Nicads.

Li-ion cells have a similar energy gain over Nicads, but have an even higher voltage of 4.1V. The most useful sizes available are 550 and 700 mAh, The former, which I have been using, weigh a mere 30g for a two-cell airborne pack.

I've been having fun for a year or so with these cells in a simple test model. It is 30 in span, weighs 5 oz all-up, with three channel radio and can fly for about 30 minutes under the power of its small electric motor. A similar weight and voltage NiMh pack would only manage about six minutes! Put another way, five times as many flights from a single charge.

The downside: The first clue is the size of my test model. This is because the new cells are limited to about 2 Amps current draw and thus small electric motors (by R/C standards) are required.

Secondly, charging has to be done slowly - at least 5hrs from the specified minimum voltage. The latter is about 6V for a two-cell pack. The cells will be damaged if they are discharged much below this level, but R/C speed controllers provide the necessary voltage cut-off insurance for this.

Thirdly, the charging needs to be at constant current, again to a specific voltage of 8.4V for two cells. A special charger is thus needed. On balance? If you are a free flight scale enthusiast, or just like the traditional approach to model aircraft of a moderate size, mini R/C is made for you. The restrictions on the new cells simply mean that the suitable models are more likely to appeal than some of the monsters around these days. Further, these small, lightly loaded RIC models are easy to fly. So, provided calmish conditions are chosen for early trials, the 'Black Art' of R/C can soon be learnt (even for "Old Dogs'!).

Welcome aboard: I am hoping that the pictures of my Max Holste will have recruited some converts to mini R/C and for those, as well as others perhaps flying ARTF plastic wonders, a few notes of guidance won't come amiss.

The new Max Holste 152: This is 50% enlargement of Max Holste MH-152 (oz1837) plan that appeared in April 1954 Aeromodeller, and is your full size free plan with this issue of FSM. The general construction of the original has been retained but a few mods were necessary.

1. Wood sizes in most cases have increased with the scale-up, but not all. The main fuselage sides are still made from 1/16 balsa and the top and bottom sheeting fore-and-aft of the cabin are also maintained at 1/32. For the tailplane, a built-up structure as shown replaces the plain sheet of the original, but the fins are still cut from 1/16 sheet.

2. Control surfaces Double rudders are used, hinged with silk strips. These are operated by pull-threads attached to a bellcrank in the rear of the fuselage The said bellcrank is attached, via a closed loop, to a servo mounted under the floor of the cabin. The rudders are spring loaded inwards with light shirring elastic, thus achieving two-way operation. This simple arrangement is light and allows the empennage to be unhooked and removed from the fuselage, as was the case on the original. The elevator is operated by a thin push rod (I used 1mm carbon) by its servo, mounted in the centre of the fuselage alongside the rudder one. Control movement of the elevator was about +-30 degrees, with about half this for the rudders.

3. An additional former was needed on which to mount the electric motor (F1A). This was made from two cross-grain laminations of 1/16 balsa with a further piece of 1/64 ply on the front, just a bit larger than the motor mounting lugs.

4. Wing Mounting George Woolls' simple method for the original needed some revision for the larger R/C version. I have had one very bad and very public experience of a modified free flight model wing mounting failure. It was around 1964 I think when my Aeronca Sedan 'clapped its wings' after a sticking reed servo (What's that Dad?) caused a near vertical dive, followed by full up coming in as I fought to regain control. The crowd loved, it but I was very embarrassed! Anyway back to the present! I have still made the wings removable and flexible fore and aft to absorb landing knocks, but now hooks in the roots take retaining bands which pass across the inside of the cabin. This pulls the wings onto the cabin where tubes set in the latter mate with short pins set in the root ribs. The all-important struts fix the dihedral and take the flight loads. They are safely retained by wire springs at either end.

5. Undercarriage. I wanted a model that could be packed into a small box for trips up country, so I made the undercarriage removable. It's quite an easy modification - just fix a pair of tubes to F3 where the 'V' is. The torsion length is halved with bent up inside ends plugging into the tubes..."

Max Holst 152 from Flying Scale Models, March 2004.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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