by Rob Campbell
When embarking on a project to convert an available model design originally
intended for glow engine power, one initially makes an attempt to evaluate the
suitability of the kitted aircraft for electric conversion. The power system(s)
one has available, anticipated wing loading, and desired power per unit weight
are some of the primary considerations. Once a particular kit is selected, more
detailed work to accommodate the electric power system can begin. Most of the
modifications can be pencilled in right on the plans, for ready reference during
the construction phase. In almost all cases, some sort of weight reduction
exercise is in order to at least partially offset the added weight of the
electric power system - the motor battery in particular.
If you are past the kit trainer stage, but you are not yet a scratch builder
(ie. like me!), here is an example of what can be done with a standard glow
Briefly, here is the design process for the aircraft:
- - Astro 40G (geared)
Type of aircraft desired:
- - Seaplane
Readily available candidate:
- - Ace Seamaster 40
Published Statistics for Glow Engine version:
- - engine required 2-cycle .40-.45 or 4-cycle .60, weight 7 Lbs,
wing span 59.5", wing area 725sq in
Note that the weight (7 Lbs.) is rather high for a glow kit of this size. From
examination of the kit materials, the following was observed:
- - materials were very heavy - mostly ply construction - this and the large
fuselage hull will contribute greatly to the weight of the model - this also is
where the majority of the weight reduction potential lies
- - the wing leading edge is actually a heavy cardboard tube - although
replacing this would reduce the weight of the wing, it was decided this would be
more work than the builder wished to tackle!
Rough calculations were then made to determine the airworthiness of an electric
Estimated weight of electric version:
- - 8 Lbs (this assumes an airframe weight reduction of approximately 1 Lb
Calculated Wing Loading = weight ÷ area = 25 oz/sq ft. (A little higher than
Using "Cubic Wing Loading" better compensates for aircraft size:
- - Cubic Wing Loading = 11.3 oz/cu ft. (A little high) Between 9 and 10
oz/cu ft would be better.
(Note: Cubic Wing Loading is an empirical formula that takes into account the
affect of aircraft size on wing loading. As a general rule, larger aircraft can
handle higher wing loading than smaller aircraft. Cubic wing loading = weight
of aircraft in ounces divided by the wing area in square feet to the power of
3/2. [Scientific calculator required!])
Power to weight ratio for a standard Astro 40 power system in this aircraft =
425 - 450W/8Lbs = 53 - 56W/Lb. (Acceptable)
This model will fly but increased wing area and/or reduced weight will improve
its flight characteristics and make it easier to handle. Increasing the wing
area to 800sq in brings the Wing Loading down to 23 oz/sq ft and the Cubic Wing
Loading down to 9.8 oz/cu ft.
The actual measured wing area from the plans (not including wing tips) = chord X
span = 12" X 59.5" = 714 sq in. Note that this is a little less than the
published value of 725 sq in. Rearranging the equation above to solve for span:
Span = Area/Chord = 800/12 = 66.7"
Therefore, to achieve a wing area of about 800 sq in will require increasing the
span by 3 to 3 1/2" per side. This was done by simply increasing the rib
spacing. Since the wing is now larger and has more inertia in yaw, it wouldn't
hurt to increase the tail control surface areas to compensate. This can be done
using the highly scientific "looks about right" approach.
Since this airplane is on the large size for a 40, I also decided a little more
power, say 60 to 65W/Lb would be nice, so the cell count was increased to 20,
with an anticipated power in increased to 470 - 520W depending on the propeller
Even though this will be a fairly fast flying airplane, a gearbox is favoured
over direct drive since the cell count has been increased and good thrust at
take-off is desired. Even with good propeller selection, reduced propeller
efficiency can be one of the results of higher cell count/direct drive
A list was then created of weight reduction ideas, with items such as:
- - holes in wing ribs
- - replace main fuselage ply sheeting with balsa
- - replace ply bulkheads with balsa, etc.
Many of these ideas were tried. Another approach to lightening was to make
structural members from balsa sandwiched between thin Lite-ply. This was used
for the motor pylon and the servo rails.
The following data was collected during the build. You may find It useful as a
guide to the most worthwhile lightening opportunities for some kits.
Note that the large effort to remove material from the wing ribs yielded only a
small weight reduction. In contrast, it was probably no more effort to recut
the fuselage sides from medium balsa, yet this resulted in the single largest
weight reduction. Some other weight reductions were made on the fly, but the
builder lacked the discipline to record them all!
The table summarizes weight reductions achieved in ounces. The weight
reductions are sorted in descending order:
|MAKE FUSE SIDES FROM BALSA
|MAKE HULL BOTTOM FROM BALSA PLY
|MAKE PYLON FROM 1/16 PLY WITH BALSA CORE
|MAKE FUSE FORMERS FROM BALSA
|HOLES IN WING RIBS
|MAKE TIP FLOATS FROM BALSA
|MAKE WING TRAILING EDGE FROM LIGHT BALSA
|MAKE TOP REAR OF FUSE FROM BALSA
|MAKE WING TIP PARTS FROM BALSA
|MAKE TOP FRONT OF FUSE FROM BALSA
|SHORTEN PLASTIC MOTOR POD BY 2"
|MAKE MOTOR MOUNT WITH BALSA CONSTRUCTION
|MAKE TIP FLOAT SKIN FROM 1/16 BALSA
|USE BALSA LAMINATE MAIN WING SPARS
|MAKE BUILT-UP FIN
|MAKE BOW RETAINER FROM LIGHT BALSA
|MAKE BUILT-UP ELEVATOR
|MAKE BUILT-UP RUDDER
|LIGHT PLY/BALSA LAMINATE NOSEGEAR BULKHEAD
There are two ways of looking at this chart. One is to say that only a few
changes can contribute greatly to weight reduction. Another view, however, is
to say that a lot of small weight reductions can add up to a significant weight
reduction. I would summarize by saying that even the small weight reductions
are worthwhile if they are easy to do. Parts that are already small and light
can be left untouched.
In summary then, about 20 oz was trimmed from the airframe of the standard glow
kit. This undoubtedly improves the flight performance of the aircraft
considerably. Of course, some judgement is required to apply these changes
without weakening the airframe in an undesirable way.
Some of the weight reductions are small and, individually, may not seem to be
worth the extra effort. However, adding them together makes a very significant
contribution to the weight reduction effort. The actual final weight of this
modified kit is a little over 7 1/2 Lbs. If you have some other
weight-reduction ideas for those of us who like to convert glow kits to
electric, I would like to hear from you.
‘Till then, happy building!