useful load

[bsam33]

I just joined this forum because I am interested in a Kitfox 3 on floats, but concerned about its useful load. Can someone explain what makes the Kitfox 4 have a higher gross than the 3 - and is it possible to increase the useful load of the 3 somehow? I have been told that the 3 can carry significantly more than its legal maximum gross, but I prefer to stay legal. So what is it that limits the max gross of the version 3. I have also been told that the gross can be increase by 20% if the airplane is on floats. Is this correct, and why?

[DesertFox4]

Welcome bsam33.
The model 3 has a 1050 lbs. gross weight limit. The differences between the 3 & 4 are stronger carry throughs on the wing spars (where the wings spars attach to the fuselage both front and rear) and larger diameter wing struts. There may be more but that seems to be the ones that changed between the two models affecting gross weights. The control mixers are completely different also but of course has no effect on gross weight. Other differences are in the wing design. The 4 has a higher speed wing than the 1-3's and different design of the flapperons. Fuselages are somewhat different throughout also. If I've overlooked something someone will fill in the missing blanks.

The guys that fly floats will have to speak up as to the rules concerning gross weights increasing when attaching floats. That's above my pay grade.
I know I've heard the reasons before but will let others with more knowledge address the float issues so you get accurate info.

As far as carrying more weight than the model design limits specifies, not going there. As you said ,the best idea is to always keep it legal.

[akflyer]

There is NO LEGAL gross weight in a KF or Avid, or any other home built plane..

The standard is to allow an upgross of 100# for floats.

[bsam33]

thankyou both for your replies. I am just a simple engineer, and I'd like a technical explanation of why adding floats means that the airplane gross can increase by 100 pounds (or whatever other number one might choose)

[akflyer]

The thought is that the floats support their own weight aerodynamically at ~ 70 MPH and therefore, the FAA gives a 100# allowance for upgross on when you switch to floats, hence 1320 max take off weight on wheels and 1420 on floats for LSA.

Most float setups are going to set you back around a 150# (for straight floats) so you will be getting roughly 25-30# weight penalty once you trade out the weight of the wheels n gear for the floats and you get the 100# gimme.

[jdmcbean]

The thought is that the floats support their own weight aerodynamically at ~ 70 MPH and therefore, the FAA gives a 100# allowance for upgross on when you switch to floats, hence 1320 max take off weight on wheels and 1420 on floats for LSA.

Please show me a reference that the FAA allows this. To date no one has been able to provide evidence. FAA puts everything in writing.. a simple FAR will suffice.

The LSA allowance was a set allowance from the begining to accomodate the extra weight needed for the addition of floats and still maintain a decent useful load.... not because the floats carry their own weight. The structrual limits still need to be met by the manufacture.. therefore, if taking advantage of the higher gross weight allowance of 1430 (not 1420) the structure must be tested to that limit. It is not automatic when floats are added. Although some use the system as if that is so.

[rogerh12]

On many planes, increasing the gross weight also requires upgrading the landing gear and possibly the fuse attachments/structure. I heard the model 4-1200 has thicker vertical tubes behind the seat (as compared to the model 3). Does anyone know if this is true or if any other landing gear changes were needed to accommodate the greater landing weights of the 4-1200?

Note: my plane has Grove spring gear attachments and grove asked me about the gross weight of the plane (now 1250 lbs) when I talked to them about choosing the right metal thickness, width and heat treatment for a 1250 lbs gross weight model-4.

Roger