G-Force

This post is pretty technical – all about physics and stuff, but hopefully we’ve explained things in a way everyone can understand. Remember if you have any technical questions, or don’t understand anything, you can tweet us @thewhiffletree1 using the hashtag #whiffletreetechquestion and we’ll get back to you.

shutterstock_86879950So here goes……

We’re testing the aerobatic aircraft to +/-10g……. but what does that mean?

Well, let us try and explain……

We’ve all heard of apples falling on Issac Newton’s head and his subsequent questioning which lead to the development of the laws of gravity – he didn’t discover gravity, it’s always been there, he just explained what was going on with this weakest of physical forces.

When an apple falls from a tree, gravity accelerates it downwards at 9.81m/s. We call this 1g as it’s what we have grown up with and are used to. g on the moon is 1.62 meters per second or roughly 1/6th of g on the earth.

Generally on the surface of the earth the amount of matter (mass) is the same as weight. For example, say an object has a mass of 10kg on earth and we multiply the mass by the gravitational attraction of 1g we have a weight (which is a force) of 10kg on earth.

If we measure the weight of a 10kg mass on the moon it would be 10kg x 0.165 (1/6th) = 1.65kg. So on the moon our Show Boss (Phil) would only weigh 13kg, or just over 2 stone, meaning that he could easily eat a few more slices of lemon drizzle cake!

So how does all this relate to us testing this aircraft? Well since weight is a force dependant on the acceleration acting on an object, g is often talked about in the aerospace world.

When in orbit we talk of weightlessness or zero g, but g can also go the other way, for example, if we accelerate at 20m/s we will experience 2g for the whole period that we’re accelerating. This means Phil would suddenly weigh twice as much as he did on the moon! Instead of weighing 13kg, he’d now weigh close to 160kg – definitely time for him to lay off the cake! If he was accelerating at 10g he would weigh over a tonne and a half and a small slice of cake would effectively weigh 2kg!!!

So, back to our plane – if we need to test our plane to +/- 10g how is that force generated and what does it mean?

Well let’s think of the hammer thrower in an athletic stadium, the athlete spins around holding the handle of the hammer, whilst the ball part of the hammer is constantly changing direction as the force in the wire pulls it around in a circle. When the hammer thrower lets go, the hammer flies off at a tangent to the circle.

In the same way, when you are doing a loop in a plane, the force that moves it in a circle is generated by the wings on the air – the tighter and faster the loop is flown the higher the force generated. The reason we get positive and negative forces is because both outside and inside loops can be flown – where the pilot is either on the outside or to the inside of the loop.

Not only does the g force increase the loading on the plane, it has some profound effects on the human body. In a traditional loop with the pilot to the inside of the circle it has the effect of forcing the blood in the body to the outside of the circle i.e. the pilot’s feet. This causes a lack of blood to the pilot’s brain. In an inverted loop (pilot on the outside of the circle) the blood would be forced to the head.

A lack of blood to the brain can very quickly cause a blackout so manoeuvres such as loops are generally short in aerobatic aircrafts. In military aircrafts, pilots wear a ‘g-suit’, which squeezes the midriff in high g manoeuvres to restrict blood flowing away from the head.

+/- 10g generates the largest loads on our little plane so we will be applying loads that are 10 times the mass of the fully fuelled aircraft (with pilot), both upwards and downwards.

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