Monday, March 23, 2015

Building a Better Multirotor - Propeller Selection

Building something that flies? Start by selecting your props

Let me tell you why. On any aircraft without fixed wings, the propellers are what hold the vehicle in the air. If you wanted to build a plane instead, wouldn't you be picking an airframe that met your needs? High wing, low wing, low profile symmetrical wings, or thick flat bottomed wings? What kind of flying do you want to do? You pick the airplane with the wings to match. So, what kind of flying do you want to do with your multirotor? Let me explain the different wings, and why you'd pick them.

The Basics

To understand what props you want, you need to understand the basics of flight. How does any helicopter stay in the air? A helicopter works just like an airplane. It redirects air, creating a change in momentum of the air mass.

or Force = mass * acceleration. Notice that there are two variables here, mass and acceleration. Generally, your multirotor doesn't change mass while flying. So, in order to hover in the air we can easily calculate the force needed.

Force needed to fly = mass of multirotor * gravitational acceleration. Now we can look at the mass of air, and the acceleration needed to create to force that keeps the multirotor in the air. Guess what? We can control both of these variables. We can choose a large mass of air and accelerate it a little, or we can choose a small mass of air and accelerate it a large amount. Let me summarize this in terms of propellers.

A large propeller spinning slowly will create the same force as a small propeller spin very fast.

Getting a little more complicated

Now that you understand the basics, let us go to the next step, and understand some of the implications of having a large prop spinning slowly, or a small prop spinning fast.

Power Requirements

Let's pick a large propeller, and spin it real slow. How about this as an example?

Okay, now how about a small propeller spinning very fast.

Do you notice any difference in those aircraft? One is powered by a guy on a bike frame. The other has a monstrous jet engine. Both fly. In fact, once you boil down the math of flight, you can generate an equation that directly relates the effective size of the propellers (disc area) to the amount of power required to produce the force required to keep the aircraft in the air. Why is this? Doubling the acceleration of an object, keeping the mass equal, requires four times the energy. So, back to F=ma,

if we cut the mass in half and double the acceleration, the force stays the same. Even though we've kept the forces (F) equal, the second equation requires double the energy. Why? Look at the equation for kinetic energy.

Here we start with our initial kinetic energy E_k0 in terms of mass and velocity. Let's rewrite this in change of energy in terms of change of velocity.

For this example, let's double the velocity and compare the change in kinetic energy

Now substitute that into our previous equation and rearrange things...

Wow! A 2x change in velocity increases the energy by 4x!

Do you recognize what a change in velocity is? Change in velocity is acceleration. So, doubling our acceleration in F=ma means that we will require more energy to keep the same force. More energy in a given time means more power. More acceleration means more power to create the same amount of force. Now that we know this, we can relate our math to what this means in term of propellers. A larger propeller moves more air mass, requiring smaller acceleration, therefor requiring less power. Let me emphasize this.

Power required to fly is proportional to the size of prop

Now, let's flip that around a little. If we want to be able to carry a certain amount of weight, such as, say, a camera, you need less power if you simply use larger propellers. Here is an even larger realization.

Given the same battery capacity, larger propellers will yield longer flight times.

Yup. That simple. Do you want 1 hour flight times? Pick really large propellers and spin them incredibly slow. Want to move bricks from point A to B? Pick really large props and spin them slow.

Efficiency

You just learned that spinning a large prop slowly means longer flight times based solely on the difference in required energy. Guess what? A large propeller spinning slowly is also more efficient than a small prop spinning fast. Why? Have you heard of the sound barrier? As the tip of the propeller approaches the speed of sound, the resistance of air increases. The increase in resistance isn't just a straight line, either. Look at the equation for drag

Do you notice that v2 term? Have you seen that before? Drag force quadruples when you double the velocity. More drag, means more energy wasted, more energy not contributing to lift. Again, big prop spinning slowly will be more efficient than a small prop spinning fast creating the same amount of lift.

Speed

We've just learned that a big slow prop gives longer flight times and more lift. Why would we ever want a small prop spinning insanely fast? If you think of a spinning propeller travelling through the air, the tips would draw a screw pattern. Propellers have a radius, and a pitch. The pitch is usually given in distance the propeller would travel after a single rotation of the propeller. If you want to travel more distance, you either need more pitch or to spin the propeller more times in the same amount of time. There are limits to how steep we can make the pitch. The best compromise is to spin a small propeller faster. Simply put, if you want to fly fast, you need to spin your props fast. There is no way around it. You can't have your cake and eat it too. Real life always has trade offs.

Multirotors and helicopters have another problem. Forward flight. When you move your aircraft in a direction, a portion of the lift goes toward accelerating the aircraft in that direction. Say your prop pitch and prop RPM says that you push air at 50mph. You can't direct all that airflow into forward flight. Only a portion of it goes toward forward flight. Some of it has to be used for lift. Even if you can push air at 50mph, your aircraft will only go a portion of that speed.

One more thing... Multi-blade props

Why do multirotor aircraft often have propellers with 3 or more blades? We know that moving more air mass means more lifting power, right? Adding more blades to a propeller without changing the pitch or diameter means that you can move more air in the same amount of space. Now, there are diminishing returns. Adding 100 blades won't give you 100x more lift. However, a multiblade prop will act like a larger 2-blade prop in a smaller amount of space. Just to be clear, though, the equivalent 2-blade propeller will be more efficient.

Conclusions

  • Large Slow-fly propellers with low KV motors will give you long flight times and more carrying capacity
  • Small sport propellers with high KV motors will allow you to fly fast, but require more power and cut flight times
  • Multi-blade propellers can give you more lift in a smaller area without increasing power requirements geometrically

Ok, any person who flies planes is saying, "Duh. I already knew this." However, sometimes we think that helicopters work on the principles of magic and that the same rules don't apply. The same rules do apply. Hopefully next time you build a multirotor you will use this knowledge to build one that meets all your requirements.

Tuesday, October 2, 2012

The Fixed Pitch Tiltrotor - The Build and In Action

**Update** I will start my next build of a Mini-Tiltrotor. Check back for updates! The components list can be found on my board



A Little Background

Ever since I saw a video (at the bottom of this post) of an RC Tiltrotor on Youtube, I have wanted to build one myself. In addition, I have wanted to build my own autopilot. Why build my own autopilot when there are plenty of cheap ones out there these days? Well... Because it is a lot of fun. Anyway, I've had a video of my tiltrotor on Youtube at night. Since filming it, it burned out an ESC. I finally got around to fixing it, mostly so I could take it apart and scrap it to build a tricopter. Yes, I know, a tricopter is not nearly as cool as a tiltrotor, but it is a much better development platform for autopilots. Don't worry, my tiltrotor days are not over. Once I have a stable autopilot, I will build one that converts to full forward flight. I promise.

Advantages of a Tiltrotor

At the expense of a slightly more complicated autopilot and mechanical complexity, a tiltrotor has half the motors, ESCs, connectors, propellers, and power as a quadrotor. Essentially, a cheap tiltrotor can be significantly cheaper than a comparable quadrotor. And they are cooler. Yup. That's about it.

Flight Video

How It Works

Some of you might be wondering how this thing even flies. I plan on doing a multi part video on how things fly that includes something like this fixed pitch tiltrotor. For now, we will have to use a simplified diagram of the forces to understand how a fixed pitch tilt rotor can work.

Friday, January 27, 2012

RC Tilt Rotor In Action!

I was able to get my tilt rotor in action tonight. Hopefully tomorrow I will be able to get a video of it in a larger field in action. Here a quick video I had my 3yo take while I flew.

Monday, January 23, 2012

The RC Tiltrotor!

I finally got a prototype autopilot board up and running with a gyro and a small but powerful Arm Cortex-M3 microprocessor, so I decided to throw together a tiltrotor. I am still trying to figure out pitch, but there will be videos to follow.
The tilt mechanism is from cheap front wheel 4mm landing gear stuff. The motors are HXT750s from Hobbyking and I am using HK SS 25A ESCs. A video to follow! (Hopefully with some successful flights!)

Wednesday, December 7, 2011

New Crazy Creation! The Dual Motor, Differential Steering RC Airboat Utilizing the Turnigy 9X User Mixes

Finally got around to throwing a video together of my latest creation, the dual motor, differential steering RC airboat!

The Video

Explanation of the Turnigy 9X Mixes to Accomplish Differential Steering

I had to use 3 mixes in order to accomplish the differential steering. First, I had to pick a channel for the other ESC. I used channel 7 and made sure none of the switches controlled channel 7.

User Programmed Mixes

  1. Throttle (Channel 3) -> Channel 7 (Aux)
  2. Rudder (Channel 4) -> Throttle (Channel 3)
  3. Rudder (Channel 4) -> Channel 7 (Aux)

Quick Video of the Mixes



***Warning!***


Using differential steering can catch you off guard! Once you set the mixes, if you bump the rudder, one of the motors will turn on! Try not to lose a finger. Also, as the battery drains, you will lose the ability to steer. Don't get caught downriver without a paddle!

Wednesday, November 16, 2011

Complete Guide - Getting into RC Airboats for a complete Newbie

Every time I take 'Das Boot' out and put it in the water, I get people wondering how to get started in the exciting (and relatively cheap) way to get into the world of RC boats. This is a complete guide to acquiring all the components needed to build a cheap airboat.

The Parts of a Cheap Airboat

Electronics

The first step to building a cheap airboat is picking a cheap motor. Once you have selected the motor, the rest of the boat will fall into place. This may seem like a long list, but most of the small parts come in multi-packs. Once you have a supply of the small parts, all you need are a few main parts and you can build numerous boats.

Motor

The motor and prop set up sets the size for the entire boat. To keep the boat small and manageable you want to select a fast spinning motor with a small prop. This generally also helps the boat be fast. A good start is the AX 2306N 2000kv brushless Micro Motor from HobbyKing. HobbyKing rates the motor up to 9A, but it handle more with ease. You always want extra motors. Grab a couple of these.


ESC

Next, lets just pick the cheapest ESC that we can find. the Hobbyking SS Series 25-30A ESC is a good choice if it is in stock. Otherwise, the Hobbyking SS Series 15-18A ESC will work. As with the motors, the more ESCs the merrier. These tend to get damaged by water, so grab at least 2 ESCs

Propeller

Monday, October 17, 2011

The PinPiYak - Home Depot pink foam flying pizza -yak

The Picture

I had a bunch of these hexTronik DT750 Brushless Outrunners sitting around for use in a multi-rotor project I have yet to make a lot of headway on. I decided I wanted to make something that took advantage of the thrust of the motor, had 3D capabilities, and was dirt simple to make.

After searching through RCGroups, I found the PiYak. A circle with elevons and a rudder. Perfect! I grabbed my sheet of pink foam, and threw something together using the proportions given on the rcgroups thread.

The Parts

  1. hexTronik DT750 Brushless Outrunner 750kv
  2. TGY Slow Fly Prop 12*3.8SF w/ shaft adapters
  3. ZIPPY Flightmax 2200mAh 3S1P 20C
  4. HobbyKing 939MG Metal Gear Servo 2.5kg/ 12.5g/ 0.14sec
  5. FlyZone 25amp ESC
  6. Turnigy 9X 2.4GHz 8Ch Receiver (V2)
  7. Hex locknuts M4 10pc

The Results



The plane has plenty of power. I tried to balance the TGY SF 12x3.8 prop, but the thing is grossly out of balance. The motor doesn't seem to care, however, and the plane shoots vertical with ease. The 3S 2200mAh battery lasts too long considering that the recommended battery for the motor is a 1300mAh. However, the larger battery keeps the CG in the right ball park.

I can't wait to put these motors in some multi-rotor configuration!