Taming Acro Flying in Betaflight (From Drone May 2018)

Or as Drone renamed it “Aerial Acrobat:  The smart way to tame manual flight” which is another nice title, but doesn’t quite tell you what it’s about on a blog.  This was the first of two articles I wrote for Drone for the May issue, and also included one of my pictures as the front cover – which was part of the GepRC Sparrow review…. which will be coming up next.

 

 

 

 

 

 

 

 

 

 

 

 

This article focuses on another part of flying that’s quite close to my heart, and that’s getting people off of self-levelling modes and into acro flight.  I’ve already made a series of videos on YouTube about how to begin Acro flying and the really big part of it was making sure you rates were set so you felt under control.

It’s a little harder to write about that make a video, but I think what we have here is a reasonably concise snapshot of the main points of taming Acro.  This article didn’t really contain many images other than Betaflight snapshots, so some of my own random quad images were used again – with the actual photo I thought might make a title page ditched.  So for the blog post, the title is back and the library images are out 🙂

If you want a hardcopy, softcopy, or indeed a subscription of the magazine, then you can check out their website here


 

Aerial Acrobat: The smart way to tame manual flight

If your experience of trying Acro mode involves your quad performing impossibly fast flips before crashing into the ground, then Wayne Andrews is on hand to explain how to tame Acro in Betaflight to make it a pleasure to fly, rather than an ordeal.

 

What is Acro mode ?

There’s a common misconception that Acro mode – which is sometimes referred to as rate or manual mode – is just for doing tricks.  Flips, rolls, inversions etc.  Whilst it’s certainly used in freestyle flying, or racing, the way it works it’s much more basic.

In a self-levelling mode, such as Angle, or Horizon if you let go of the pitch/roll stick, then the quad comes back to level.  In Acro, it doesn’t.  If you push your pitch stick forwards to get your quad moving in a forward direction and then let go, Betaflight will hold this forward pitch until you pull the stick back to correct it.

There’s occasionally some confusion about how to activate Acro mode, as it’s not listed in the modes tab at all.  Acro is the default mode, so it’s active if you don’t have another type of flight mode selected

 

Why use Acro ?

So what’s in it for you –  no doubt flying in Acro is harder than any self-level mode, so why do it ?  First and foremost, It’s smoother and allows more fine-grained control.  Self-levelling will always want to return to level flight and so there’s a feeling that you are fighting your quad instead of flying it.  Self-level also requires you to hold the stick in the position constantly, and holding a sprung stick perfectly in position whilst making course corrections makes it much harder to get smooth results.  In Acro you don’t need to hold the stick in position, and instead you can make small and accurate corrections to keep things far smoother.

As mentioned in the introduction, a new pilots introduction to Acro mode can be brutal, and once burned, they can be vary wary to return – instead, often sticking to the relative safety of Horizon mode.  So why is this first flight such a baptism of fire ?  It’s the fault of the default PIDs, or more accurately the rates.

Whilst we might be used to playing with the PID values for Roll, Pitch and Yaw, the three important values that define how you quad reacts to your stick movements are RC Rate, Super Rate, and RC Expo.

The Betaflight default rates may be overly sensitive for new pilots

 

The going rates

Both these concepts are quite familiar in RC terms, and you’ll normally find them on anything but the most basic radio.  Betaflight have added these into their configurator so that you are able to use them even if your radio doesn’t support is, as well as allowing you visualise how you quad should fly when all these values come together

The RC Rate defines how much control input is passed through to the flight controller for a given amount of movement on the stick.  It defaults to 1.00, which means it’s a “full” rate, if you move your stick to the full extent, then the quad get’s the full possible movement.  Similarly though, if you were to set this to 0.50, your full stick movement will only produce half of what it previously did – a simple way of dulling the controls down.  If you are feeling particularly brave,  set it to to 2.00 and watch how fast your quad will spin, even with small stick movements.

RC Expo is a method of making the stick movements around the centre less sensitive and the extremes of the stick more sensitive.  Instead of the stick movement being linear, it introduces a curve which flattens along the middle and increases at the extents.  The reason for using expo is that becomes easier to make small accurate corrections around the centre of the stick without it being overly sensitive, but still having your full rates at the stick extents.  The default setting in Betaflight is 0, giving no expo at all.

The super rate value is easily mistaken for expo at first glance, as it too works on a expo-like curve, but it’s a bit of a mash up of both expo and RC rates.  The more it is increased, the more overall impact your stick movement has – but you’ll still be able to make the small movements you need around the centre stick.  The default Super Rate value in Betaflight is 0.70

At this point, it’s a great idea to jump into Betaflight to look at the visualisation which shows how RC Rate, Expo and Super Rate work together, and how they can be easily tuned to your liking.

If we take a look in the Betaflight configurator under the PID Tuning tab, we can see how the stick responses are visualised through a curve on a graph.  We also get an indication of how fast the quad will move as measure in degrees per second (deg/s).  The default setting for RC Rate, Super Tate and RC Expo produce a rotation of 667 deg/s, which is nearly two complete rotations in 1 second.  This can be overly aggressive if you are just starting out.  A beginner might try Acro mode, panic that the quad is rolling much more than anticipated and so slam the stick over in the opposite direction resulting in more rolling and generally ending up in a crash.

 

Try before you fly

One great thing about the Betaflight configurator is it’s Rates Preview window.  It’s all very well trying to imagine how quick your quad would respond based on some pretty graphs and number, but it’s far better to actually see it.

Below the PID settings you’ll find the very useful rates preview window

 

To see your quad respond to your stick movements in the preview window, you’ll need your flight controller connected to Betaflight, your radio on, and your receiver powered up.  In most cases this will require a flight battery being connected, so to be safe don’t forget to remove your props before-hand.

With the battery connected you should be able to move your sticks and watch how quickly the virtual quad spins and flips.  In the default settings you should see that moving the stick around the centre produce quite a small response, but as you get to the stick extents it speeds up fairly dramatically.

It’s often the case that it’s this speeding up as the stick moves that catches a beginner out, so one way of really taming the rates is to remove the super rate altogether.  If you set the super rate to 0 on all the axis and look at the quad in the rates preview window, you will see that response from the stick is linear, there’s no sudden increase in as the sticks move to the extents – it’s smooth and progressive.  The deg/s drops to just 200 which may be too slow for real-world flying.  If you want to keep that linear rate but have more response in your quad, then increase the RC Rate.  If you find it’s still too responsive, then reduce it.

 

Recommendations

Although you can fine tune the RC Rate and RC Expo to make your stick movements feel perfect, it’s quite likely that as you become a more confident flyer and start to build some more finesse on the sticks you’ll want to regularly increase these rates to give you more of a response. 

It’s much easier to keep adding a little bit to the Super Rate instead of re-tuning RC Rate and Expo repeatedly, so rather than turn the super rate off altogether initially, try it first off on a lower figure such as 0.30.  It’ll always just give you that little bit more if you need to make a quick turn.  If you feel the need to increase the RC Rate, increase the super rate first and retry.  If you find you are liking the overall rate, but it’s to sensitive to make small movements, then increase your RC Expo

It’s also worth touching on the Throttle Expo setting at this point.   This is useful if you can’t quite seem to find that throttle position where your quad will hold it’s altitude easily and the slightest of touches seems to make it either decent or ascend.  To give you a little bit less sensitivity at that central stick position, try raising this to 0.20 or 0.30.

 

Adding a small amount of throttle expo can fix issues with finding the point in the throttle stick where it’s easy to maintain your altitude

 

Once you are happy with the feel of you quad in the preview window – don’t forget to click on save – you’ll want to test it out on your quad in a real flight.  Things might not match up perfectly, the preview window doesn’t take into account things like the type of motors, props, size of quad, battery type etc  which can all make a difference to the real world experience.  It should be fairly close though.

If you have only flown in a self-levelling mode before, then some commitment is needed.  Don’t be tempted to take off in self-level and then switch to Acro, the transition if overly jarring.  Try and go from take-off to landing all in Acro.  You don’t need to do anything extreme – fly some basic circuits and get used to how it feels.  I’d advise against trying to hover in FPV in Acro at first, it’s more difficult to get a fix on your position and there’s a tendency to drift.  If you make sure you keep flying in the direction you are facing, then you’ll progress quickly.

If you need to fine tune and have a flight controller equipped with a Betaflight OSD, then it’s fairly easy to tune things on the field.  Entering the OSD menu will allow you modify Super Rates, RC Rate, and RC Expo via the Rate sub-menu – albeit without the graphical visualisation.  But keep the changes small – especially in the super rates.  A change of just 0.05 can feel quite different.

 

A Betaflight OSD makes fine tuning rates in the field easy

 

You should notice fairly quickly that Acro mode feels a lot smoother to fly, and after a few flights, you will hopefully begin to find your muscle memory starts to kick in and the small corrections you previously had to really think about just are just performed by your thumbs subconsciously.


Learning Curves

Linear Rates

By removing the Super Rate setting entirely and reducing the RC Rate to 50% we get a very gentle response all the way through the stick movement.  Whilst a useful example of how simple it is to “tame” Acro mode, the response maybe a little too slow to actually fly with.

 

Gentle Rates

A recommended starting point for a more gentle flight is to reduce the Super Rate and add in a little RC Expo.  This still gives you a fairly large amount of movement around the centre of the sticks for small changes whilst allowing more responsive movement at the stick extents.

 

 

All-Purpose Rates

Once you get to the point where you to making small and subtle stick movements, you’ll want to start increasing your rates for a faster response.  A simple change of increasing gate Super Rates to 0.80 is a popular starting point – and is what I personally fly with.  It will give fast rolls and flips when you need them, but still allow for gentle control at central stick.  When moving your rates upwards though, do so in small increments.

 

 

Mixed Rates

It’s not necessary to keep your rates the same on all the axis. Depending on your model, the roll and pitch response can feel quite different.  This rate was setup for an indoor quad which was flown quite gently in the roll and pitch axis, but needed to be able to yaw very quickly in order escape from tight spaces around the house.

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