Daily Archives: March 23, 2012

7 posts

The Prez’ Sez’ for March 2012

Please visit the Innov8tive Design website.


Electric Power System Presentation

Key Points and Rules of Thumb

1.    A power system consists of a Battery, Speed Controller, Motor and Propeller
2.    Each component should be matched together for a good power system.
3.    Li-Po Batteries have 3 ratings: Voltage (Cell count), Capacity and C-rating
4.    Li-Po Batteries are 4.2 volts per cell fully charged, 3.7 volts under load, and discharged at 3.0 volts per cell
5.    Li-Po batteries should be stored long-term at 3.7 to 3.8 volts per cell
6.    Battery capacity is measured in mah or milli-amp-hours.  1000mah = 1 AH
7.    Battery capacity is similar to fuels tank size in glow engines.
8.    Discharge rate = current ÷ battery capacity.  A 1C discharge will last for 1 Hour
9.    To calculate flight time use 60 minutes ÷ Discharge rate
10.    Speed Controllers convert DC energy into 3-phase AC to power brushless motors
11.    Speed Controllers have 2 ratings:  Voltage (Cell count) and Maximum Current
12.    You cannot harm a motor by using a larger speed controller
13.    Brushless Motors are 3-phase and bearings are the only wear item
14.    Brushless motors have 3 ratings:  Size, Power Rating (or Current Rating) and Kv
15.    Different manufacturers use different numbering schemes for their motors
16.    Electric Motors are basically the exact opposite of Glow Engines
17.    Electric Motors are constant RPM machines, Glow Engines are constant power machines
18.    Glow engines can be damaged by using too small a prop
19.    Electric motors can be damaged by using too large a prop
20.    Glow engines push power into the prop while the prop pulls power out of an electric motor
21.    Prop selection is critical for good performance in electric motors
22.    The power that an electric motor puts out is determined by the prop used
23.    When selecting batteries, try to keep the Current to voltage ratio between 2 and 5
24.    Always use the 80% rule when calculating power systems
25.    Get the proper tools for Electrics, it will save you time and money in the long run
26.    Learn how to solder!  It is the most important skill needed for electric power flying
27.    Never trust factory Pre-tinned leads, always re-tin them yourself prior to soldering
28.    Only use rosin core solder!  Never use acid core solder

1 cubic inch of 2-stroke Glow Engine is equal to 2000 watts of electrical power
Ex. A .60 Glow engine = 1200 watts and a .40 glow engine = 800 watts

1 cubic inch of 4-stroke Glow Engine is equal to 1500 watts of electrical power
Ex. A .40 glow 4-stroke = 600 watts and a .90 glow 4-stroke = 1350 watts

Power requirements for Electric:  Power Gliders need 50 watts per pound, trainers need 75 watts per pound, sport models need 100 watts per pound, pattern planes and warbirds need 150 watts per pound and 3D and competition Fun-Fly planes can use 200+ watts per pound

Selecting a Power System for a Model

A Simple 5-step Process

1.    Calculate the required Watts of power needed based on the weight and type of aircraft
2.    Select an appropriately sized battery to provide the desired flight time
3.    Choose the correct Motor based on the number of cells you are using and prop size
4.    Select a Speed Controller to match the Motor being used
5.    Select the right prop to get the desired performance of the model

Step by Step example from the presentation, a 6 pound, .45 size Ugly Stick power system

Determining Power Required:  A .45 glow engine = 900 watts ( 2000 x 0.45), alternately a 6-pound model with pattern performance needs 150 watts per pound x 6 pounds or 900 watts

Selecting the proper size battery:  From the chart below, 900 watts can be done with a 4 or 5-cell Li-Po battery.  Higher voltage equals higher efficiency so we will use a 5-cell pack (18.5 volts under load).  To get 900 watts from 18.5 volts requires 900 ÷ 18.5 or 48.6 amps.  Since 2/3 throttle is typically ½ max current, we will use 24 amps for calculating battery size. Battery capacity is determined by flight time at power load.  For 10 minutes we need a 6C discharge rate.  24 amps at 6C is a 4000mah battery pack.  For 80% rule we increase battery size to 5000mah.

2-cell Battery – 100 to 300 Watts            6-cell Battery – 1,000 to 2,500 Watts
3-cell Battery – 250 to 600 Watts            8-cell Battery – 1,800 to 4,000 Watts
4-cell Battery – 450 to 1,100 Watts        10-cell Battery – 2,800 to 6,500 Watts
5-cell Battery – 700 to 1,700 Watts        12-cell Battery – 4,000 to 10,000 Watts

Selecting the proper Motor:  Find a motor that will deliver 900 watts on a 5-cell pack at 48 amps.  Remember the 80% rule and get a motor rated for 1125 watts if you need 900 watts.  Do not forget to take prop size into consideration.  Prop charts make this easy if they are available.

Select a matching Speed Controller:  The speed controller should handle the full rated current of the motor, even if you are not pulling that much current.  When in doubt, always go up one size larger.

Selecting the correct prop:  In many cases several props will pull similar current, but give different results.  For example, a 10×10, 11×8 and 12×6 prop will all pull about the same current, but give dramatically different results.  The 10×10 prop will give the highest speed, but the least thrust and would be suitable for a pylon racer type plane.  The 12×6 prop with give a lot of thrust but greatly reduced top speed.  This prop would be good for a slow plane like a Piper Cub.  The 11×8 prop would provide a good compromise between speed and thrust, and would work best on a sport model such as an Ugly Stick.

John’s DeHavilland Airco DH2


John & DH

In 2009 I built a 1/12 scale model (28 inch wingspan), using 6mm foam sheet for wings and 3mm for tail, carbon tube for the booms, foam block for the nacelle base and nose, and formed card for the nacelle cover. I put a scale pilot and Lewis gun in the cockpit. The 3S 1500 mAH battery extends under the pilot (where his legs would have been), the AXI 2208/34 motor driving an 8 x 4 prop is outside the rear of the nacelle, and between the pilot and the motor mount are 3 HS55 servos, both parts of a Spektrum 6 channel receiver with one antenna extending sideways, the other fore and aft, and a 10A speed controller.  It is a very tight fit; total flying weight 15.5 oz.

Inter-wing and inter-boom struts are balsa, landing gear is wire (and braced by cord from the front of the nacelle), and the wheels were originally sold as spares for a Neiuport. Those were light — too light, made of 0.016″ vacuum-formed plastic without a hub — and have been replaced by wheels built up out of the same foam used for wings and tail, with a plastic tube hub. The bracing wires are 0.020″ carbon fiber rod.

The aileron servo extends below the lower wing to drive push-pull rods going out to bell cranks near the aileron centers. The rudder and elevator servos each work pull-pull cords, each pair going out laterally as far as the vertical wing strut where the booms meet the wing and then through wire guide rings mounted on the boom struts to serve either rudder or elevator. The cords are set and tightened by tiny transparent sliders.

I first flew the DH2 in the fall of 2009, and felt that it was very difficult to fly, although its log book says that I had got as far as checking spin recovery as satisfactory. Then I put it in the hangar as being just too difficult. I looked at it every once in a while, promising myself to make another try, and each time delayed. Now, in January 2012, I took it out to the field. While waiting in the hangar that plane had matured into what history records as its later characteristics of being maneuverable and easy to fly. I flew circles and lazy eights and felt in complete control.

I had another perfect flight today, 8 minutes, doing some loops and spins as well. But the foam wheels that I built did not have a sufficiently strong connection to the plastic tube hub, which connection broke loose on landing, even though that was quite gentle. I have to make wheel hubs of wood, or similar, with the tube bearing inside, but large enough outside to have a large-area connection between the hub and the foam.

The Next Big Thing-Step 12

And you thought I was exaggerating.
Well, what can I say is, video testimonial can be a sobering thing. Since then I have broken many props and finally I broke my flight control board mounts so badly that I had to replace them. This time I said “screw it” and did not add the extra vibration isolation grommets, I screwed it on “hard mount” . Well, what do you know?  That extra layer of vibration isolation was what was causing my problems!?  Yes, the flight control board really does not care about vibration. The gyro board does, but it is already mounted on foam tape. As best as I can determine, the extra soft mounting caused an odd vibration input to the already-vibration-mounted gyro board and those gyros just got all confused and, well, tilted.
Wham! Snap roll, right into the ground!
I now have many flights with the hard mounted flight control board ( the big pink thing?) and everything is functioning correctly.
Who’d of thunk?

So now I am just practicing daily on my hovering and will soon transition to forward flight . Some observations are:

1: Cross winds cause translational lift and require the pilot to decrease throttle to maintain altitude.

2: Ground effect is more pronounced than with helicopters and occurs at a higher altitude than a helicopter and requires more throttle sooner.
( ground effect is opposite from fixed wing ! )

3. I really need a low battery alarm as the regular “pulsing-for-low-battery” feature will just cause a multicopter to ” CRASH! Right into the ground”.
( this just requires a couple of resistors and a buzzer as it is already printed on the circuit board. I just have to buy the pieces and do the soldering. )

4. I need to practice, practice, practice….

5. More practice

Here is a picture of the old problem grommets on the pink flight control board:

Tricopter Tail

Until the next time on: The Next Big Thing

Rocket Bob Kreutzer



Some fun videos

A swarm of nano quadrotors:



Airbus A400


My Debolt Champ Celebrates it’s 50th Birthday

The Bomb has had 3 different power systems; an Enya 19 unbaffled that I wore out, a muffler equiped Enya 19 that I wore out and now a Parkzone F4U outrunner power setup.

Many control systems have flown in the Bomb. First was a galloping ghost homebuilt radio with a Rand LR3, then a Digitrio, then a Man 234, then a Controllaire, a Citizenship, two Krafts, a JR, two Futabas and now a 2.4 Spektrum clone with Dx5.

The Bomb has seen many configuration adjustments.  The first was adding tricycle landing gear in 1966 when the first and only major rebuild of the nose occurred. Also the rudder was enlarged around this time to enable snap rolls. For a time I flew the Bomb with underwing flaps and also fired bottle rockets from the wing. The conversion to electric power happened in 2010 as did the addition of wing tip and tail lights for night flying. We also added a downward pointing light on the strut to help with landing flare; a poor mans radar altimeter.

The Bomb is a veteran of many competitions over the years. I first flew him in pattern when pattern classes were by airplane equipage.  I flew in the Rudder, Elevator and Motor class. I think that was when classes were numbered 1,2 and 3. I also flew the Bomb in Expert class in 1968 after crashing my competition plane, a Lanier Dart. It became the comic relief. The Bomb has won many fun flys including limbos, looping and spot landing events.

When the Bomb and I make visits to clubs that don’t know me (or the Bomb), it is fun convincing the safety inspectors that its safe to fly.  I actually failed once and pulled out a beautiful pattern plane.

Flying the Bomb is like wearing and old set of slippers, warm and familiar. No bad traits.





Here are two photos.  The first is the Champ in 1968 with my Lanier Pursuit that I flew to 3rd in Jr. at the 1969 Nationals. In the background is Ira Spear’s Sr. Falcon powered with a Merco 60 and Orbit radio. The second is the Champ today after a soccer field flying session.

Champ 1


Champ 2