Lessons from the Electra
(Article submitted by Peter Fundarek
& Walter Gray)
The art of building and flying model aircraft consists of a considerable amount of
trial and error as various options are pursued to determine which ones will provide the
best flight characteristics. Since the main aim of a club is to share information, I
would like to present some of the information that I have gained from building and flying
the Goldberg Electra.
The Electra comes complete with an "05" ferrite motor and a prop which is
meant to be installed as a direct drive system. This is a mistake. The Electra
is a slow flying glider which is designed to operate in a speed range of about 15-30 mph.
A simple equation for calculating the approximate theoretical flying speed is pitch
x (RPM/1000). For a 7x4 prop spinning at 13,500 RPM, this translates to about 54
mph. This means that at best, the prop is cavitating and you are chewing up valuable
battery capacity just trying to get it to move. I first tried the Electra with a
direct drive prop and often I could simply not get any altitude at all. A better way
is to use a gear drive to reduce the speed of the prop thereby allowing a larger prop to
be used. The gear drive allows the motor to operate at high speeds which is more
efficient while allowing the power to be applied to moving the larger prop. This
past summer I experimented with a 2.5:1 gear drive running a Graupner 11x7.5 prop at 5400
RPM. This translates to a flying speed of roughly 40 mph. I have decided to
slow the plane down even further by installing a 3.5:1 gear drive. This should give
me about 3800 RPM and I will select a prop with a lower pitch of about 6.5 or so.
This should reduce the current drain on the batteries (longer flight time) while still
maintaining the speed in the useful range for the glider (about 25 mph). When you
use a gear drive, be sure to lubricate it well (I use a mixture of Molyslip 'E' and white
lithium grease) and use a speed controller. The simple on-off switch supplied with
the Electra will easily chew up the gears when starting it due to the inertia from the
large prop - you need the soft start supplied by a speed controller.
Replace as much of the wiring as possible with Astro 13 gauge wire or equivalent and
remove any unnecessary switches or other connections. I mount the fuse holder on the
outside of the plane and use it as the arming switch. The only connectors that I
have in the circuit are for plugging in the battery. And speaking of connectors, use
Sermos connectors instead of the Tamiya connectors that are supplied. Sermos
connectors have much lower resistance and are easier to use. Don't eliminate the
fuse! It is vital to have it there and best installed on the negative wire
immediately after the battery connector.
The motor could be replaced with an 05 or 035 cobalt motor but for this plane, I do not
believe that the extra expense is warranted. The motor is not on long enough for it
to make a difference. Save the cobalt motor for a plane with more performance
requirements. The motor supplied with the Electra is sufficient but be sure to break
it in properly and lubricate the end bearings at least once per season.
Whatever the prop that you select, spend the extra amount and get one that folds.
Graupner are good but expensive. Master Airscrew put more of a current drain on the
batteries and are less efficient. Not only will the folding prop prevent damage to
the motor on landing, during the flight when the motor is stopped it will fold back and
minimize the drag associated with a wind-milling prop. This will translate to
increased flight times and more flying satisfaction.
Build light. This is not a gas powered plane and not subject to the kinds of
vibrations that you get with such an engine. Therefore, use less glue and
reinforcement as you want to minimize the amount of excess baggage that the motor has to
haul around.
I have made some modifications to the structure based on my experience and some less than
perfect landings. I found that the front bulkhead ('former A' on the plans) tends to
get beaten up quite badly. I have replaced it with a copy made from plain printed
circuit board and this has proven to be quite good. It is almost as light as lite
ply but is stronger and more resilient due to the lack of grain. In a hard landing,
this former will just pop off the front of the plane, minimizing the damage to the rest of
the structure. Since it is stronger, more air holes can be provided to maximize the
air flow over the speed controller and batteries.
Another problem with repeated hard landings is that the cowling seems to really take a
beating. I have minimized damage to this unit by holding the unit on with either
tape or hatch latches similar to that used on the battery compartment. In a hard
landing, this allows the cowl to slip off and therefore minimizes the damage.
Don't install the optional landing gear as it only adds weight and does not improve
landing performance. A perfectly satisfactory landing can be made without the
landing gear, just remember to pull up slightly just before landing.
When holding the wing on, use at least 10 elastics as the minimum of 6 that is recommended
is not enough. The elastics will still allow the wing to suffer minimal damage in
the event of a hard landing.
To balance the aircraft for the stated centre of gravity, do not add any extra weight to
the plane to make it balance. Instead, move the motor battery forward or backward as
necessary to achieve balance. I installed a strip of lite ply across the fuselage in
front of the servo actuators to protect them from the motor battery. Installing the
servos so that the actuators are as far toward the tail as possible provides a bit more
room to manoeuvre the motor battery.
I also locate my radio battery in front of the receiver, as opposed to the arrangement
shown in the plans. This prevents the batteries from crashing into the receiver on a
hard landing. To minimize weight even further, it is possible to replace the
standard battery pack with one that has a smaller capacity (for example, 250 mAh).
The servos, if installed properly, only require 250 mA to operate and most of the time
they are not operating so this battery pack will provide at least one hour of flying.
Check your receiver pack before each flight and you will be fine. I do not
advocate using the battery eliminator circuit from some of the speed controllers as they
can be unreliable and I have heard of too many crashes associated with this feature.
I have found that 1/4" foam rubber is sufficient to wrap the batteries and
receiver as there is little if any vibration from the motor to be concerned about.
The canopy typically gets beaten up a bit by regular use and replacements can be
expensive. A cheap alternative is to buy a sheet of clear polythene and cut out your
own canopy. Take what remains of your current one and press it flat on the new
sheet. Trace the outline onto the new sheet and cut it out. To achieve the
proper curve, simply hold it over the outer container from a 2 oz size bottle of Zap (or
equivalent) and heat it with the air gun used for shrinking the covering. The
plastic will drape around the container and give you a reasonably good canopy. I use
a bit of electrical tape to hold the canopy in place during a flight.
Make sure that the battery compartment hatch is always in good condition and that it
closes securely. The last thing you want is for the motor battery to drop out of the
plane during a flight. If the hatch gets damaged or broken, replace it rather than
repairing it. A new one can be fashioned from lite ply quite quickly and easily.
Remember to cut the ventilation holes though.
I hope that you find this information useful. Making the changes as noted above has
significantly improved the performance of my model and I hope that it will help you to
enjoy this plane and the hobby.
(Article submitted by Peter Fundarek
& Walter Gray)
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