With the easing of the UK lockdown restrictions I had an opportunity to try and take the Skymaster for its maiden flight. I had some concerns from the outset. I'm not hugely experienced at flying model aircraft (much more used to their full size cousins!), and I haven't flown a model in quite awhile.
Often nowadays we see a carefully curated story of other people lives and projects, the many successes lined up end to end on the internet or social media. It's tempting to hide the failures along the way lest anyone think bad of us. We all know this isn't good for us. It makes us feel like we are the only ones making mistakes. In fact, the only reason I got into this hobby at all, designing and building my own planes was to show my kids that failing is part of life and should be embraced, no more so than in engineering and design. Failing, analysing and learning, and most importantly coming back for more, being resilient, is what life is all about. In engineering it might be called trail and error or the iterative design process but these are euphemisms because the word failure has become tainted. I don't want my kids to be afraid of failing and to not try in the first place. This is why I am sharing this.
I had a technical concern that came about when doing some testing on the power system. I have written previously about how the high speed air from the front prop would affect the rear prop, and how on the real 337 this problem was overcome by having constant speed propellers. During testing I found that when I went to full power there was a noticeable loss of thrust (noticed no more scientifically than by feeling the 'pull' against my hand). I thought that this was perhaps due to the rear prop stalling.
I should point out that the prop I was using for this was a 3 blade 8X4 I had lying around. I knew this prop was inappropriate, thought that this might limit the speed of the model but that it would probably be fine for a quick proving flight...
I set the model up, took my time to make sure everything was tight where it should be, not moving up when it should be moving down, after all, who hasn't been caught out because they have been a bit excited? Just like on the real Airbus I gave myself a take off briefing, rehearsing the course I was going to fly in my mind. Just a circuit or two. In the back of my mind though, nagging away was the thought about the props and that dip in power at max thrust. Should I only use 75%?
I got myself ready, the coast was clear. I set max thrust and launched the model, perhaps a little softer than I would have liked but still expecting to see the aircraft dip but pull away as the props propelled the aircraft forwards. What actually happened was the sound of the props went from steadily increasing to a cavitating whine and the aircraft rolled and crashed into the ground, decelerating rather than speeding up the whole way to the crash site.
As I picked up the pieces I thought about the props and the sound, and it dawned on me. Not only was the rear prop stalling in the high speed flow and not providing thrust, but just like a real propeller, it was beginning to windmill, the air was doing work on the prop not the other way around. Rather than the rear prop pushing the aircraft forward it was causing drag, negating thrust generated by the front prop and slowing the aircraft down.
The diagram below (taken from here) illustrates a prop producing thrust (top) and a drag producing windmilling prop (bottom).
During my airline pilots exams I remember reading an aerodynamics text that a windmilling prop can produce the same amount of drag as a solid circular disc the same diameter depending on airspeed. That is why you have to be quick to feather a failed windmilling engine.
So I failed. And I (re)learnt something. The damage to the airframe is actually minimal, most parts just need sanding and gluing back together, maybe a bit of tape. So what next? My plan is to get two new props. maybe a 9X4.5 and a 9X6 for the rear and I'll try again.
In the meantime, for those that have been following along with the blog, I am going to post the files and assembly instructions for the Cessna Skymaster here, for free a while so you can have a go, at least until I get it flying successfully. All I ask is that you take some photos along the way and send me some video of your maiden flight, wether you fail to learn something new with a successful flight or you discover some new pearls of wisdom in the wreckage. You can post them here, or tag me @rc3dprint on instagram. Good Luck!