I think there are two entirely different things going on here and it doesn't help to get the two mixed up:
- reinforcing the tongue of the HF trailer;
- lengthening the tongue of the HF trailer.
Most of those examples show lengthening and in one case weakening (not reinforcing). Adding steel in the wrong place does not make the tongue stronger.
What matters is the strength at the point of maximum bending, which is at the point where the tongue leaves the front of the main frame. To reinforce a tongue, this point and a good length in front
and behind that point need to be strengthened.
So this photo is a good example of really bad practice. The tongue has been lengthened by adding the central single tongue, but it hasn't been reinforced at all at the critical point where it leaves the main frame. The longer tongue makes more leverage but the strength is the same - the result is that the carrying capacity is reduced. If the trailer is only going to ever carry a small load, that might be OK (and RC planes are unlikely to be very heavy) but it's still an dangerous idea to modify a trailer so it can't carry its rated capacity/GVWR.
On the other hand, this looks like a sensible lengthening
and reinforcing (though the lengthening requires reinforcing, so the capacity may be no greater than the original trailer). The extra central single tongue is adding to the tongue strength at the critical point and it continues well back into the main frame - as far as the third cross member. Picking up more than two cross members doesn't itself give much benefit (in this example, the second one will carry very little load), but the important thing is the good distance the first and last cross member picked up. As a rule of thumb this should be at least as much as the width of the main frame.
And making this mistake is not just for amateurs - some Carry-On trailers sold by Tractor Supply and Northern Tool have exactly the same problem and forum members have reported problems with the second cross-member bending under the tongue load (for one guy, it happened while he was still building his teardrop body!). They fit a perfectly good enough single tongue, but it is attached to fairly weak angle cross members that are positioned way too close together. Long before the tongue reaches its maximum load, the second cross member fails in bending.
PS I am working on a new, simpler tongue strength page - but it may not be able to go into this sort of detail and yet remain 'simpler'. Looking at it now, the current page is a disaster!
