Greetings, friends!
So you remember that last post? That long, complex one from last time?
Yeah, I think I went a little overboard on that one. So today, instead of fancy graphs, we’ll be looking at what we want our aircraft to do, and how it will affect its design. Coming from an engineering perspective, graphs are a great way of converting raw data into a format our brains can interpret. However, there’s no substitute for pretty pictures, and we’ll be studying plenty of those today. Think of this as a fun break. Or try. I’m doing my best here!
Lesson of the day: there’s no right or wrong to aircraft design. As long as you’re designing it to fly, you’re on the right track.
Don’t get me wrong – It’s quite possible design something that’s rubbish for your chosen role. And it’s perfectly possible to design an aircraft which isn’t really good at anything. Let’s say you want to fly 15 people about 120 miles over a mountain range, and you strap a pair of wings to a horse. Congratulations! Your aircraft can now mow the lawn for several years. Problem is, unless you’re somehow able to accelerate it to several dozen metres per second, it won’t fly. You would also have some pretty brutal stability problems, and a very unruly horse which would probably not survive the ordeal. You would also be a horrible person for trying. But if it could fly, then you would have designed an aircraft. Happy now?
In all seriousness though, there are plenty of wierd and wonderful designs out there; from jet-powered biplanes to aircraft with no wings to “flying pancakes”. Many of these designs have useful attributes – the flying pancake’s structure could withstand apocolyptic events, and it had the potential for excellent manoeuvrability and slow-speed performance. The jet-powered biplane could (probably) achieve high altitude flight while maintaining a reasonably small wingspan, which could have been handy for storage and manoeuvrability.
Understanding how to make an aircraft ‘good’ tends to require some level of knowledge on how flights work. Ideally, we’d like a manned aircraft to have:
– High maximum speed – allows us to go places fast
– Low stall speed – allows us to operate from smaller airports
– Low fuel consumption – fuel is heavy and ruinously expensive
– Low maintenance requirements – cheaply and rapidly fixed, tweaked and fuelled
– Low capital (buying) and running costs – no explanation needed
– Sound construction – nobody wants their aircraft to disintegrate on landing
– Positive stability and good handling qualities – aircraft won’t want to fly off on its own, fight the pilot, and spin out just above the ground
– The ability to carry a payload – people, cargo, etc.
These requirements are often contradictory. You want top speed? Add a bigger engine. You’ll also need to cough up more dosh. And more fuel, because that thing isn’t exactly going to be sipping it. Oh, and you’ll probably want to beef up your forward fuselage, because the extra torque from the prop is going to want to twist you as if wringing out a cloth. While trying to pull your nose off. And vibrating like a million ringing mobile phones. And because all this weight must be counteracted by lift, you can’t fly particularly slowly anymore, so suddenly you can’t fly to that one airport which would have been perfect.
I have been told that an aircraft is a thousand flying compromises. The Europa is a good example of a thousand compromises done properly. It’s designed to have a very small cross-section, which reduces fuel burn and increases top speed, and uses a specialised wing section which produces very low drag in the cruise. It’s also a homebuild, so is WAY cheaper than a production aircraft would be. Downsides include a sharp stall, the requirement to build it yourself, and various legal restrictions (no low-light flying, no flying through or over clouds, etc). Arguably the two-crew limit is also a compromise, as it can’t carry as much as a good four-seater.
Now, we’re only going to design a simple model aircraft, so factors such as maximum speed and fuel consumption are not necessarily priorities. We want a simple, fun aircraft that flies well. We don’t need to go anywhere fast, and if our aircraft runs out of juice, we can just recharge. However, landing at slower speeds is much easier, and also mitigates landing damages, such as scratches or dents from rough grass or gravel. Something is bound to break eventually, so we’ll need to be able to access parts to replace them. We don’t want to spend too much money on this – ideally, we don’t want to spend anything. And we certainly don’t want to take our aircraft home in a binbag.
So, we start to get the picture that an aircraft must balance its compromises on a delicate set of scales, depending on its intended function. An airliner uses (proportionately) small wings to achieve high cruise speeds and fuel efficiency at the cost of short field performance. A bush plane sacrifices fuel efficiency, high cruise speeds and payload capacity for a rugged structure, simplicity and the ability to take off and land on a dime. If aircraft are designed for very specific purposes or functions, they tend have more biting compromises. The Harrier, while a brilliant feat of engineering, compromises fuel efficiency, range, payload capacity and top speed to gain its vertical capabilities. An aerobatic aircraft such as the Extra 300 is obscenely manoeuvrable and can accelerate like the cat you just trod on, but won’t have anything resembling fuel efficiency, range or cargo capacity.
So: what do we want our model to do?
Well, ideally it would have a low stall speed for easy, calm flying. This will require large wings to provide the extra lift, thereby compromising our top speed – but that’s not really a concern for this type of model. Of greater concern would be that the model would suffer gusts and turbulence with much less… elegance. Annoyingly, this could make landings trickier in windy situations, but it’s a necessary sacrifice. We’ll just have to be careful on windy days. Since this is going to be a model that (ideally) anyone can fly, we’ll need a good deal of positive stability and benign handling characteristics. Now, for reasons I should get round to explaining later, this will sacrifice some manoeuvrability and lift. The latter point will sacrifice some low-speed performance, or we could increase the wing size a tad to compensate. Structurally speaking, we’ll want a rugged airframe to compensate for rookie antics and Sod’s law. It will be simple, which will have the double impact of reducing complexity (so less stuff can break) and generally making maintenance easier. Bear in mind that we’ll need to access the electronics regularly for charging. Let’s see… we won’t need it to take a payload, and fuel won’t be a problem. Our aircraft will be powered with electrickery, as some aeromodellers like to refer to it. This is partially because I’ve never dealt with glow engines before, but largely because electric power trains are comparatively safer (improper care still results in fire and explosions), easier to manage, and often much cheaper.
Grand! We’ve established that we want fairly big wings to provide our lift, a simple and easily-accessed structure, and a good lick of stability with quality handling. We have established what we want. Further down the line, we’ll establish how to achieve it. In the meantime, have a picture of a flying lawnmower.
And remember: don’t be a bad person to horses.
And the result? Imagine you’ve got one hand on each surface, both pushing the on wing with equal force. That wing isn’t going anywhere. Now reduce the force being exerted by the upper hand (the one pushing the wing down). Since the hand pushing the wing up is now exerting more force than the one pushing the wing down, the wing rises.
Basic Model Aircraft Design 
