Large Model Association Forum

Hi, My first post.
How does the 20kg limit work for electric powered models? I have done a bit of research and all I can find is that the limit is a dry weight, ie without fuel for IC power. As the main power batteries are the electrical "fuel" is the dry weight 20kg limit with or without the main power batteries for the motor?
(I realise that the receiver batteries are included in the dry weight)

Thanks Dave Osborne

Hi Dave

Having built one over 20kg electric model, this was a weight 'with' batteries......................they are abviously required for 'CG' purposes, so in a 'dry' (no batteries) condition, the model would be unflyable.

The model in question had batteries weighing in the region of 10lb's, so made a VERY significant difference to the overall weight of the model.

Phil

Thanks. I feared that might be the case. Does raise the question what if the batteries were on the CG though!!

Dave osborne

weight of batteries is always there, the weight of them would contibute to the way the plane flew, we take off with loads of fuel on board, and use the weight up, lekky models have to land with this weight still on board

Dave Osborne wrote:Thanks. I feared that might be the case. Does raise the question what if the batteries were on the CG though!!

Dave osborne

Fair point, but at model of 20kg's+, a suitable electric motor will be significantly lighter than a large petrol equivalant, so it's highly likely you'll be needing the batteries in the nose to achieve the correct CG.

However...as Alan has correctly put, petrol weight is used up during a flight whereas the batteries weigh the same fully charged of flat......so they must be included in your total aircraft weight.

Phil

Perhaps it should be thought of as a flying weight. A petrol model has to be flyable (or flyably balanced) with both full and nearly or even empty tanks. Whereas an electric model has to be flyable simply with the battery installed. Just thinking out loud really,

Ian.

Its an interesting topic.
I actually work in the aerospace industry and am very familiar with aircraft certification requirements. Those requirements get opened to interpretation all the time and specific modifications to existing requirements, or new requirements get written all the time for specific projects where the design does not meet the "letter" of the requirement but can be equally as safe. Also many certification requirements have a published set of acceptable means of compliance to add some explanation for why the requirement says what it says, the interpretation of that requirement, and compliance demonstration means that are acceptable.

For a dry weight therefore it could be interpreted as exactly that "dry" so for electric its batteries but for Ic its without fuel. still what stops IC models having a ballast tank for CG trim as per full size aircraft, if the tank is empty it will not be in a safe CG range?

Still it tis what it tis untill it changes!!

Thanks for the feed back

Dave

Think of the batteries as fuel tanks and you will be able to relate the issue to powered models. Battery flat = empty. Battery charged = full but the weight of the "tank " is always included.

John

Maybe a tad off topic, but there is a rumour that the day may come when we fall in line with otheres in that that the limit before needing an inspector may well go up to 25kg. True or not I cant see that beeing anything but a good idea as there are many now building A/C that hover just below 42lbs and need to leave scale detail off just to keep it there. Ken Hart

ken hart wrote:Maybe a tad off topic, but there is a rumour that the day may come when we fall in line with otheres in that that the limit before needing an inspector may well go up to 25kg. True or not I cant see that beeing anything but a good idea as there are many now building A/C that hover just below 42lbs and need to leave scale detail off just to keep it there. Ken Hart

Ken, thats 44lbs = 20kg, 'by ek as like' i was about to go and saw off the last 12" on the Spit.
78 player still working fine, thanks.

Mike.