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Overview of electric flight
Symposium by Keith Shaw
Public Newsletter Articles
How to rebuild an Astro motor
Weight reductions for electric conversions
A low cost thermal peak detection charger
Making Printed Circuit Boards
Aerobatics for Electric Airplanes
Nicad Care and Handling
The Shuttle ZXX
Getting the Most Out of Ferrite Motors
What Difference Does A Bit of Wire Make Anyway?
Using 1100AAU Cells for the Speed 400
"Squirrel" - Construction - Speed 400
A Weight Comparison of some Lightweight Coverings
Motor Comparison
Keith Shaw on Props
Building Foam Models
Electric Flight Box
Weight Reductions for Electric Conversions
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 engine kit.

Briefly, here is the design process for the aircraft:

Motor available:
- - 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 conversion:

Estimated weight of electric version: - - 8 Lbs (this assumes an airframe weight reduction of approximately 1 Lb is possible)

Calculated Wing Loading = weight ÷ area = 25 oz/sq ft. (A little higher than desired)

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 used.

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 combinations.

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:

CHANGE BEFORE AFTER SAVING
MAKE FUSE SIDES FROM BALSA 9.4 4.0 5.4
MAKE HULL BOTTOM FROM BALSA PLY 5.9 2.5 3.4
MAKE PYLON FROM 1/16 PLY WITH BALSA CORE 2.8 1.3 1.5
MAKE FUSE FORMERS FROM BALSA 2.1 0.8 1.3
HOLES IN WING RIBS 3.8 2.5 1.3
MAKE TIP FLOATS FROM BALSA 2.0 0.8 1.2
MAKE WING TRAILING EDGE FROM LIGHT BALSA 2.2 1.3 0.9
MAKE TOP REAR OF FUSE FROM BALSA 1.2 0.4 0.8
MAKE WING TIP PARTS FROM BALSA 1.2 0.5 0.7
MAKE TOP FRONT OF FUSE FROM BALSA 1.1 0.5 0.6
SHORTEN PLASTIC MOTOR POD BY 2" 3.8 3.2 0.6
MAKE MOTOR MOUNT WITH BALSA CONSTRUCTION 1.2 0.7 0.5
MAKE TIP FLOAT SKIN FROM 1/16 BALSA 0.8 0.4 0.4
USE BALSA LAMINATE MAIN WING SPARS 3.7 3.4 0.3
MAKE BUILT-UP FIN 0.9 0.6 0.3
MAKE BOW RETAINER FROM LIGHT BALSA 0.3 0.1 0.2
MAKE BUILT-UP ELEVATOR 1.1 0.9 0.2
MAKE BUILT-UP RUDDER 0.4 0.2 0.2
LIGHT PLY/BALSA LAMINATE NOSEGEAR BULKHEAD 0.3 0.2 0.1
TOTALS (oz) 44.2 24.3 19.9


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!