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FAQ

Hovercraft model
- My mistakes -




Don't do like me !

1) How to make smoke with your motor ?

2) How to make smoke with your variator ?

3) Resin is polymerized when it is polymerised, not before !

4) How to pull out everything !

5) How to broke your servo teeth !

Don't do like me !

As already indicated, Im a beginner in radio modelling. But, if I am an experienced do-it-yourselfer, I am sometimes of compulsive impatience. So, you will find herebelow a list (not exhaustive) of errors Ive made.

1) How to have your motor smoking ?

It is relatively difficult to find ferrite motor datas and constructors site are of ashaming indigence on the subject.
Though, you can find herebelow some infos :
http://www.ecalc.ch/motorcalc.php?ecalc&lang=en

So I have done direct testing to see the results.
I have first tested my lifting fan in direct drive on a Graupner Speed 600 BB turbo. Result on fourth test : back bearing burnt, because 'BB' means ball-bearing, but for the front bearing only. Back bearing is only bronze bearing, what stinginess ! After analysis of battery life (a bit too late, I admit), I think the current consumption might have been 60~70A, so a big heat-up and no possibility for the motor to survive.

It must be noted that the 35A variator have not tripped (it is equipped with a breaker)

2) How to have your variator smoking ?

In the intent to be able to plug directly the brushed motor on the battery, I have cabled it similarly to the variator, say male pin on positive, and female on negative.
Big mistake, after a whole set of tests, one second of distraction and I have plugged the battery on the output of the variator, and it have instantly emitted a spark, a noise and smoke. At my great surprise, it still does work, but heat way too much to be still usable.

So, to avoid this sort of costly mistakes, use the more traditional installation for brushed motors :
- Battery + : male pin
Batterie - : female
Both output of the variator : females, to avoid shortcuts

That mounting avoid :
. To plug battery with + and - reversed, it will smoke !
. To plug battery on the output of the variator, it will smoke !
. Shortcuts on battery output: It will heat up and smoke !
. Shortcuts on variator output : It will disjunct ?
Though, it might be possible to reverse + and - on motor, so it will run in reversed direction : So beware when starting up !

3) Resin is polymerised when it is polymerised, not before !

Multiple times, I've removed fixing components (spindles and holders), before complete hardening of glue or resin.
Resin is hard while touching it, you are waiting since hours, everything looks ok. But, while in appearance hard, the resin still have some viscosity, and if your part is under stress, your gluings will began to open very slowly, and after half an hour, you can have millimeters of opening. The problem is that the resin being near end of polymerisation, it will be impossible to retighten the parts. It is needed to cut with a cutter and do the job again.
Polymerisation time to be respected if the mix is exactly to manufacturer recommendations are at least :
. 2 to 4 hours for epoxy '3 minutes' (glue looks hard after ten minutes, but it continues be viscous during hours)
. 12 to 20 hours for ordinary epoxy.

A not perfect mixing of the resin (which is frequent, especially when doing small quantities) can take much more to polymerise (some days). You can accelerate polymerisation by heating with a lamp.

4) How to pull-out everything !

When testing propulsion, I've run the motor in reversed direction (In reality, that was due to a left turning propeller, anyway...). This is not a big problem, but a plastic bag which was not far have gone through propeller, taking in the passage electrical wires, which were not properly fixed, this wires have been pulled by the propeller and have cutted part of the support on 2cm, have pulled-out the whole rudder system and have worn the propeller..

The nice repair part you see on the photo of support base is the result... So are the repairs on the rudders

5) How to break servo gear teeth

After some tests, I wanted to look under the skirt (...). So I've rotated the hull on it's back.

I've just forgot that the rudders were extending largely over the hull, and all weight have gone on rudders. The pinion gear of the servos have not really enjoyed and I had to change them.
This is why there is now on drawings (and on prototype) some bars on rudder support, because it is very useful to rotate the hull vertically.
Note that it is possible to maintain the hovercraft vertically for maintenance, but a weight on propeller cage is needed.

6) Epoxy resin cause skin allergies.

This is written on bottles, all manufacturers advice you, there are full articles on the subject (which I had read), and, moreover, I am of allergic nature.
So, I had take a lot of care during my preliminary tests, but I was not wearing gloves. So, I've washed thoroughly my hands every time after a contact with resin. These energic washing had only resulted spreading out the resin on the back of my hands, and I get a serious allergy on it. That was painful, and some blood points were appearing on my skin. I have efficiently calmed the pain with sun cream (...) That allergy had lasted more than one week.
Since that, I'm using gloves, even if only gluing with fast epoxy, and hopefully I had no further problems.

It is important to note that such allergy can be definitive, and can prevent any further use of epoxy.
All resins have not the same reactivity, but manufacturers are not very precise on the matter. Fluid resins, finition and transparent resins are reputed creating more allergies than the others. While well informed, I've been very imprudent, don't do like me !

To make such a model, at least 15 gloves pairs are needed, maybe more. So, organise your work... And drink something before beginning, after it will be too late, or you will have to throw your gloves in the bin.