Project “Dropship glider” – introduction

When I saw this video from rctestflight I knew I will build something like that for myself. A voila, few months later it is done. Here is Dropship Glider.

Dropship glider - depron FPV delta glider

It is 29cm long with 20cm wingspan. Weights 97g AUW and has 21g/dm^2 wing loading. So, in theory, should glide. Somehow… If I got center of gravity right. And did not made ailerons too big. Or…

Dropship glider - depron FPV delta glider

Delta 6mm Depron “wing” is attached to 6mm carbon fiber rod and has some quite big dihedral: 15 degrees.

Dropship glider - depron FPV delta glider

The biggest problem was radio link and mixer for ailerons. I could not use my Taranis: I need that for a carrier and only radio control link I had was EM-16 with PPM output only and no way to setup any kind of mixer. The radio just has no “features” like that…

So, took one Arduino Pro Mini and wrote short program that acts as PPM decoder and mixer for ailerons.

Dropship glider - depron FPV delta glider Arduino

  • power is supplied by 1S LiPo taken from my Tiny Whoop
  • FPV AIO Eachine TX02 also taken from my Tine Whoop
  • 5V is supplied from cheap, regulated, step-up converter
  • 3rd servo is to release tether

First flight, or rather drop, tomorrow. There will be a video from the event of course…

Marabou Stork decommissioned

Depron airplanes are easy to build, but also do not last long. Few weeks ago I decommissioned by Marabou Stork. It did not crashed, it was flying almost fine, but it had at least a few problems:

  1. Motor was overheating. Just like that
  2. Wing that I originally designed for something weighting around 450g, now had to work with twice that much weight. Take offs were, at least, problematic
  3. It was big and slow and was not giving enough fun

Marabou Stork Depron FPV airplane

So, to paraphrase Monty Python:

This is a late marabou stork

It’s not a complete end of this design. Wing is still in pretty good condition and I have a plan to reuse it in experimental twin-motor design. But that later this year… In a mean time, last video footage.

Pitot tube is coming to INAV

One of the things that INAV was missing, was a decent support for Pitot tubes, or more generally speaking, airspeed sensors. Autonomous flight, or landing, without knowledge about airspeed can easily lead to a stall. Stall can lead to crash. A crash leads to rebuild. Rebuild of big airplane is a nightmare. Although, for some time now INAV was able to use digital PX4 Airspeed Sensors (I2C, based on MS4525), but they are quite expensive and airspeed was only reported in blackbox logs. Not very useful, right?

Now, this is changing. Next release of INAV (1.8 probably) will bring at least support for much cheaper, analog, APM Airspeed Sensor based on MPXV7002 chip. Although some simple additional electronics (2 resistors to be precise) will be required, but this pitot tube should be available for all flight controllers with free ADC input (Current or RSSI). Fancy ADC remapping will allow to use any ADC without built in dividers (Vbat has dividers so can not be used) as pitot input.

mpxv7002 pitot tube for INAV

More than that, INAV 1.8 will (or at least should) bring PID scaling according to airspeed for fixed wings. This should result in better handling on both low and high speed.

mpxv7002 pitot tube for INAV

As you can see on the picture above, APM Airspeed Sensor is already installed on my small flying wing and is waiting for first flight tests this weekend. Logging only for now…

INAV 1.8: Automated landing for fixed wings

It’s official: next release of INAV (1.8 or maybe 1.7.2) will incorporate an automated landing procedure for fixed wings. I was already writing about it 2 weeks ago, but now new code has been merged back and will be released.

Bear in mind, that this is not “state of the art” landing yet. It’s rather a simple solution that can be used in emergency situations that will not crash an airplane, but rather put it on the ground without crashing. Procedure is quite simple:

  1. When Return-To-Home or Failsafe with RTH is engaged, go to Home position
  2. When Home is reached, start to loiter with defined radius and descend. Descent speed is limited to nav_landing_speed when altitude is above nav_land_slowdown_maxalt. When altitude is below nav_land_slowdown_maxalt, vertical speed is scaled down to one fourth of nav_landing_speed at nav_land_slowdown_minalt. So, on using default values, vertical speed is between 2m/s and 0.5m/s
  3. During descend, airplane is not allowed to raise throttle above nav_fw_cruise_thr when nose is up. This is to prevent airplane from gaining horizontal speed
  4. When nav_land_slowdown_minalt is reached, ROLL axis is locked to 0 degrees, PITCH axis is locked to nav_fw_land_dive_angle (default is 2 degrees) and motor is stopped when MOTOR_STOP is used or put to IDLE when MOTOR_STOP is not used. This puts airplane into a shallow dive to the ground

fixed wing landing in inav

That is all. Airplane should glide last few meters to the ground. Most designs should be able to do it without a problem. My testing platform did it like that:

Since there is no auto-disarm procedure yet, MOTOR_STOP is recommended to prevent propeller from breaking and motors/ESC from burning.

INAV can do automated landing on fixed wing?

Those of you how subscribed to my YouTube channel should have noticed, that I got an interest in automated landing of fixed wings after RTH in INAV. And the sad truth was that, well, INAV up to 1.7 was unable to do it right. When landing after RTH was enabled (nav_rth_allow_landing = ON) and it was enabled by default, airplane usually started a 20 degrees dive to the ground. One does not has to be a prophet to figure out how it ended.

For example like this:

If not manual override, that would end up in a beautiful crash and probable full rebuild of an airplane.

Luckily, that motivated DigitalEntity enough to something about that, and yesterday I was able to perform (probably the first one ever) a controlled descend after RTH that ended up with an airplane on the ground without any damage. With enough optimism one can call it even a landing. This is how it looked like:

Current implementation is still far from perfect. Although it does not crash, it has a few small problems:

  • No disarm. Throttle is open all the time
  • It happily ignores speed. Both ground and airspeed
  • It also ignores wind, heading and so on

But, to be honest, this is a nice progress. Stay tuned for more changes here, since I’m planning to work on it in the near future.

Mini Flying Wing: MiniWing v2

Miniquads are fun, right? After all, there is a reasons most uf us flies 210-250 quadcopters. If so, small flying wings should be fun too! I tried that already in late 2015 and failed miserably! My design did not survived maiden flight. Well, things like that happens from time to time, so few months ago, after learning few new things, I’ve made a second attempt. And this time I’ve succeeded.

Continue reading “Mini Flying Wing: MiniWing v2” »

Fixed Wing 101 – Lift, Angle Of Attack and Stall

With this short article I would like to initiate new series: Fixed Wing 101 where I will describe some basic concepts connected with fixed wing airplanes that should help beginners to enter the hobby. Today: why airplanes fly and why, from time to time, they fall down from the sky…


Airplanes fly thanks to the lift. It is a force generated by wings (by the way, propeller thrust and wing lift are the same force. After all, propeller is a rotating wing) thanks to pressure difference. When air pressure below the wing is higher than above it, lift appears. To archive level flight, lift has to be big enough to counteract mass and gravity.

The most important condition for a wing to generate lift is: wing has to move though the air (or air has to move around the wing). If there is no movement, there is no lift.

When there is air movement, there are two factors responsible for lift:

Bernoulli’s principle

Bernoulli’s principle states:

when gas or liquid is moving faster, it has lower pressure.

Simple. To obtain lower pressure above the wing, we have to make air move faster over there. This is why wing has a shape (airfoil) it has: top side (above chord) of a wing is longer than the lower side. Wing splits air into 2 stream. Upper one has longer way to travel than lower one, so it has to move faster. If it moves faster, it has lower pressure. If it has lower pressure, lift appears.

Continue reading “Fixed Wing 101 – Lift, Angle Of Attack and Stall” »

Using PCA9685 with INAV

One of the hardware limitations of flight controllers that usually multirotor users ignores is a number of PWM outputs. To fly a quadcopter you need “only” 4 PWM outputs. Since most FCs have 6 outputs and 90% of multirotors are quadcopters, there is no problem.

In case of airplanes, this is not that simple. 6 PWM outputs is an absolute minimum to fly a classic airplane using MultiWii and derivatives (Baseflight, Cleanflight, INAV): 2 outputs reserved for motors, 2 ailerons, elevator, rudded. Suddenly, 6 outputs barely meets the requirements. If you want flaps, gas engine, pan & tilt or anything else, you are missing some outputs.

For some time INAV tries to address this issue by supporting external PWM driver: PCA9685.

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INAV: Launch mode video tutorial

One of the best new features of INAV 1.4 was Launch assistant mode (NAV LAUNCH). It greatly simplified the process of hand launching a fixed wing. All you had to do was to throw it into the air. INAV detected the throw, engaged motor(s) and stabilized flight and kept constant climb rate in the initial flight phase. INAV 1.5 will make it even better: it will also allow swing launch!

Since INAV 1.5 should be release in next 2 days, and there is very little info on INAV Launch mode, I’ve decided to create a short video showing how to do it.