Building to fly or building to crash?

I'm going to put something a bit controversial out there today. For the past 6 or 7 months as I have been building and flying aircraft I have noticed a phenomenon in how people approach building and modifying their aircraft that I think is flawed. In fact, extreme application of the philosophy turns good flying aircraft into bricks.

What am I talking about? Reinforcing aircraft to improve their crash survivability. For some it seems to become an obsession as carbon fibre rods are inserted here, there and everywhere, pieces of wood are glued to foam to reinforce weak spots, and the model is covered in heavy tapes (like duck tape).

Now it would be folly to suggest there aren't any good crash survivability mods out there, but when I see people that have tried to turn a Parkzone Stryker C into an M1A1 Abrams, or a foam warbird into an armoured car I can't help thinking that they haven't been told what makes aircraft fly well, and what makes them fly like rubbish. Perhaps they don't realise this very important fact about model flying (and crashing). It is this:

In a serious crash the energy has to go somewhere. Often all reinforcing does is transfer the energy of the crash to some unreinforced part of the airframe.

Further to the above, if your strengthening mods have substantially increased the weight of the model then this increases the inertia which the model is using at the moment of impact to destroy itself. And of course, increasing weight also increases the wing loading and leads to worse flight performance (which in extreme cases can actually lead to the crashes).

As an example I have seen many a pilot reinforce the PZ Stryker particularly around the nose region. In most serious crashes the Stryker's nose pops off, or busts off. It's a five minute fix with foam safe CA or maybe 15 minutes with some fast setting epoxy. Re-inforcing of the nose has led to crashes where the load of the impact is transfered further down the fuselage leading to breaks much deeper into the fuse which are harder to repair and require reinforcing themselves. Also, it often seems motor mounts are much more likely to come away when the energy isn't dissipated through the breaking of the nose.

Another example is a balsa Cessna 180 someone I fly with owns. After a serious prang the plane was "re-inforced" to prevent breakage. My estimate is that something like 20-30% was added to the weight of the aircraft by way of carbon fibre, tape, extra balsa, extra covering etc. The plane became a flying brick, needing more than 75% throttle to "fly". The next time it went in (which was hardly surprising due to the weight of the plane) it destroyed itself in the crash so completely that it was really beyond repair.

No amount of reinforcing will prevent model planes from becoming damaged in a serious crash. There is too much energy involved.

When it comes to reinforcing model aircraft I think you need to set yourself a level of "load" that you may reinforce up to, but not beyond. The benchmark I would say is "routine" things (and no - you aren't allowed to count crashing as routine). So your model needs to be strong enough to handle air loads while flying and landing loads, so reinforcing/redesigning for issues there is fair enough. You might even want to stretch it to "bad landings" like nose overs etc if they are common with the aircraft.

The other time you might want to do it is where the reinforcement costs you almost nothing in terms of weight, and should be effective at preventing damage, and ideally distributing the load. For example, using clear packing tape to protect the trailing edge of a trainer so that the pusher prop doesn't cut the wing, and also using packing tape on the leading edge of a wing to prevent it from being cut if the plane happens to end up in a tree. In both these cases the tape distributes a realistic amount of load over a larger area meaning the whole thing survives.

However, trying to reinforce a fuse so that it doesn't break in half when the plane comes in hard is folly. All that will happen is that the plane will break elsewhere and you have added weight to it which will make the crash all the worse.

Now of course, it is slightly more complicated than that. For example, EPP foam is clearly stronger, but heavier than balsa. So am I saying don't use EPP even though it is stronger - not at all. I am talking about reinforcing an aircraft made of any sort out of any material. For example, if you have an EPP foam warbird that breaks around the avionics compartment when it crashes do you reinforce the area with carbon fibre, or just epoxy it back together. Whilst I wouldn't object to some tape around the area, I think all carbon fibre would do is transfer the load somewhere else to break something else.

If you have a crash, and the repair takes you 15 minutes that's a great result - don't reinforce anything. Chances are the plane broke exactly where the designers planned knowing that it would be easy to repair.

So in closing:

• Models need to be strong enough to survive air and landing loads. If a wing folds in the air then yes it is worth reinforcing. If it breaks because she touched a wing tip and cartwheeled then no it doesn't.
  
• Reinforcing against crash damage is fine provided it doesn't add much weight (for example clear packing, or fibreglass tape) and you have some appreciation of where you may be transferring the load.
• If your reinforcing is adding more than 5% of the total weight, it is too much.
• When a model has a serious crash you are going to bust it. The more weight you have added to it through reinforcing the more damage that will be done to it.

So, in short, build your models to fly, not to crash.