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How to Avoid Crashing

Part 2 - Building for Survival: The Power System

Article by Andrew Gibbs

Crashed RC electric Lancaster

The Power System
The safest way to bring a model back to earth is under power, and with sufficient energy remaining in the battery to carry out an unhurried go-around and another landing, should this be necessary. Any failure of the power system will result in an emergency landing, which is unlikely to improve the model's odds of survival!

Consequently, a reliable electric power system is an essential requirement for all powered models. To achieve this, we need to select suitable power system components and install them properly. Also, the system must of course be operated within its limits, and be properly maintained.

Motor Mounting
The motor must of course be securely mounted in the model. Considering that the propeller pulls the model along, it may appear that the forces exerted by the motor on the airframe are relatively low, and all in a forwards direction. However, this is absolutely not so; the propeller is capable of generating surprisingly high gyroscopic forces especially during tight turns or aerobatics, and these forces are transmitted directly to the motor mounting.

As an illustration of how severe these forces can be, I actually had a motor rip itself out of the nose of a model, taking the motor mount with it as a result of gyroscopic forces generated during ordinary aerobatics! Some additional reinforcement may therefore be a very good idea if your motor mounting arrangements look at all doubtful.

Motor cooling
The motor will of course need cooling, and it's well worth making sure the cooling arrangements are well thought out. A cool motor will provide more power than a hot one, and will be more reliable.

 


Motor mount damage

10. Gyroscopic forces can be surprisingly high. It was gyroscopic forces that caused this motor mount to detach in flight (click image to return to original size).

Gyroscopic forces can be surprisingly high (click small images to enlarge and get more information).


11. Motors and ESCs must be well cooled (click image to return to original size).

Motors and ESCs must be well cooled (click small images to enlarge and get more information).

 

ESC matters
It is important that the ESC is rated for at least a little more than the current being drawn by the motor. A generously rated ESC will run cooler, and will provide more 'headroom' for any propeller changes.

Like the motor, the ESC must also be adequately cooled. This is extremely important if the RC system is fed by a BEC which is part of the ESC - if the ESC overheats, the BEC may no longer work, causing a loss of control. It is not possible to over cool an ESC.

ESC installation
The ESC should not be free to flap about; make sure it’s restrained in the model and that its associated flight battery and RC system connectors cannot become undone in flight.

Battery installation
The battery must be properly restrained - you really don't want it falling out of the model in flight, endangering people below. The battery must not be able to move inside the model either, otherwise it may shift in flight causing a disastrous change in the CG of the model. This would render the model un flyable.

Battery cooling
The battery should be adequately cooled. High power installations will generate much more heat than lower powered ones. Don't over cool or under cool the battery - LiPo batteries must not be allowed to overheat, and work best when slightly warm.

 


Electric RC model battery retaining strap

12. The battery must be firmly held within the model. The battery of this Interceptor by Chris Golds is retained by two straps made from hook & loop fabric. This is a good system.

The battery must be firmly held within the model. The battery of this Interceptor by Chris Golds is retained by two straps.


Electric RC model airplane prop

13. Props should be balanced - any vibration may lead to equipment failure. Spinners may also need balancing, especially if they are large.

Props should be balanced - any vibration may lead to equipment failure.

 

Propellers
The propeller should not be overlooked. It's important to make sure the prop is balanced, otherwise it will create vibration which can cause a high rate of wear on the airframe and electronics, which can potentially lead to equipment failures. Also, any energy used to produce vibration won't be available for propelling the model, so a balanced prop will bring a performance benefit too. Spinners may also need balancing.

Motor failures & multi-motor models
Motor failures with multi motor models are potentially much more serious than for single motor models, as an asymmetric power condition can quickly cause a pilot to lose control.

The damaged model in the heading picture was a Lancaster, which unfortunately suffered a failed gearbox, leading to an asymmetric power condition. Moments after the failure, the model entered a spin and crashed. At least the builder had not spent time painting the model! Happily, the airplane was rebuilt and finished to a high standard, rewarding its builder with satisfying flights.

 

Click here to read part 3

 

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