Mixmatching arms

[Ben Kelly]

Hi all,

As you may know from a couple of my previous threads i have a Nighthawk Pro 280 quad that i'm not really getting to grips with and love flying.

Last weekend i had an incident when i was flying around at the local field, i brought the quad into a hover in front of me about 20 feet up when all of a sudden one of the prop nuts flew off and the prop swiftly following it. This resulted in my quad falling straight out of the sky and breaking one of the arms that the motor sits on.

My question to you all follows. Can i replace the broken front arm and the other front arm with a couple of 250 size arms that i have already? or will this affect the quads flying in any way. I know that the props will still clear the main body as it is the same prop size as the 250 size quad. and as far as i'm aware all the pre drilled holes match up with the ones in the main body.

Just thought i'd ask the experts before diving in head first.

Thanks.

[Tony Read 2]

If it was me I'd try it. There are many quads with differing arms fore and aft. The Blade being one of them. Just jiggle around with the battery position to get the CG back to where it was before and give it a hop. If all is well go fly!

[Biggles' Elder Brother - Moderator]

The problem I think is that the shorter front arms will mean that the moment exerted by their props will be smaller than the moment exerted by the rear props. Another way of looking at this is that the centre of lift (ie the point through which all the lift can be considered to act) will move away from the geometric centre of the quad. It will move backwards - towards the longer arms. If you do nothing to correct this then the quad will have all the symptoms of being "nose heavy" (because the centre of lift will be behind the centre of gravity).

You could correct this by moving the CoG backwards - thus restoring the coincidence of the CoL and the CoG. How far back wards? Well I would suggest about 15mm would be a good starting point.

Note that if you only fly in stabilised modes (eg GPS hold or Attitude-Hold) you will not automatically notice the difference as the system will compensate by running the front props faster. But its not a good idea to fly a quad out of balance - even though its perfectly possible - as it means the system is having to work too hard simply to remain level. So if you fit these shorter arms you should make the effort to shift the CoG to compensate. Of course if you fly in manual you will feel the difference instantly!

BEB