EDF fitting

[oipigface]

This is almost certainly the wrong forum for this question, but I don’t know which forum would be appropriate.

I am building a watertug so that I can tow watergliders from water. Although not experienced in either waterplanes or towing, the little experience I have had with the former, led me to believe that they should be designed to perform just as well as a boat as they do as an aeroplane. My guess is that this is even more true for a watertug, and in particular that they should be easy to steer on water. The few waterplanes I have built before have, with one exception, had waterrudders all of which were complicated to engineer, and none of which worked very effectively. The exception was a Flite Test Sea Duck, which had twin motors programmed to give differential thrust when rudder was invoked. This was a bit of a monster on the surface, capable of doing something very close to a hand-brake turn!

The watertug I am building I am adapting from John Greenfield’s Ampley design. This has a single engine, and I didn’t want to go to the lengths of fitting nacelles to the wings, which is always a lot of work. So, I decided to retain the single engine design and fit a couple of small EDF’s to the top wing surface. The photograph below shows the fitting that I’m working on, with 25mm EDF in place. The fitting is a liteply and foam sandwich. I intend to bury this in the wing, with the bottom lip of the EDF resting on the upper wingskin, and glued to the fitting.

My question is this: Should I find a way of fitting a strap over the top of the EDF, or will gluing it to the fitting as it is suffice?

I’d like to hear from anyone with more EDF knowledge than I’ve got!

 

Oh No! It’s upside down again!

[mikef]

[John Minchell]

First question I'd be asking John is "are 2x 25mm fans big enough?"  That's indoor lightweight model sizes.  The Ampley is a reasonable size (72" span) airframe and I would have thought you'd need a couple of 50mm or 60mm fans at least.  As they are less efficient at accelerating than a conventional 2 blade prop.  A standard Ampley uses a 20"x10" prop, so might need a 70mm to 90mm pair of fans.  If you haven't got access to Ecalc I am happy to do the calculations for you to ascertain the correct drive package for your airframe.  Just drop me a PM.

[oipigface]

Hi John. Read my original post more carefully! The EDF’s are not the only motors on the plane, they are only providing steering, not power. The power setup is similar to the one described by John Greenfield on the Scale Soaring website. The EDF’s may or may not be sufficient to steer the thing (in tandem with the air rudder) but note that the supports I’ve designed are quite a bit wider than the 25mm ones I’m beginning with. This is so I can fit bigger ones up to 50mm if necessary, while retaining the same hole in the wing, and wire runs.

Anyone any ideas on how best to attach them to the wings?

 

 

[wookman]

I know you are only steering with the fans. Have you thought of installing them in the fuselage crossways like a bow and stern thruster?

At the risk of covering old ground, what about water rudder(s) on the back of the floats with a servo in the float. Perhaps even a receiver and battery too. If you want to take that completely left field put a motor, prop shaft and prop in each float and steer it with them?

[oipigface]

13 hours ago, wookman said:

I know you are only steering with the fans. Have you thought of installing them in the fuselage crossways like a bow and stern thruster?

I haven’t thought of that, no. I believe that the thrusters you suggest are mainly used in full-sized boats when there is little or no forward or reverse momentum. Motors mounted sideways generate thrust that partially negates any forward momentum, which in an aeroplane is unlikely to be helpful unless it’s a Flying Bedstead, Harrier or similar. I already know (from Sea Duck)  that differential thrust from forward-facing motors is perfectly capable of producing more than adequate manoeuverability both in the air and on the water. I don’t think that much power is necessary, which explains my choice of the smallest EDF’s I could find. I’m also keeping my options open, as my last message showed.

13 hours ago, wookman said:

At the risk of covering old ground, what about water rudder(s) on the back of the floats with a servo in the float.
Perhaps even a receiver and battery too.

This is what I did in my Cularis conversion (not the receiver and battery). I can’t claim to know how well they work, because it has only been in the water to check that the floats don’t leak. The problem with the Cularis arrangement is designing a practical way of  getting power to the servos. The wire taped to the undercarriage on the Graecalis is not very elegant, but it does the job. 
Receiver and battery in floats? I’ve never tried multiple receivers. Is this even possible?

14 hours ago, wookman said:

If you want to take that completely left field put a motor, prop shaft and prop in each float and steer it with them?

Motor, prop shaft, prop, receiver, battery? The question of weight springs to mind, as does the question of simplicity, but it would almost certainly do the job, if it can be kept light.

So, for the moment Graham I’m going to stick with my original idea to mount two small EDF’s on the wings. Trouble is, I’m still not sure that my proposed method of fixing them will prove strong enough.Any ideas on that?

 

[wookman]

 

9 hours ago, oipigface said:

Receiver and battery in floats? I’ve never tried multiple receivers. Is this even possible?

It would just be a matter of binding multiple receivers to the same model memory and powering them up symultainiously. 

[wookman]

9 hours ago, oipigface said:

Trouble is, I’m still not sure that my proposed method of fixing them will prove strong enough.Any ideas on that?

If the Damply is using a foam wing I would araldite the mounts into the wing. I might be inclined to run a sub spar though the centre section to join the edf mounts together. Stick two 6mm dowels through the two ply plates at half past four and half past seven next to the edf and run a tie wrap around the edf and dowels. Keep the tie wrap close to ply plate so you are not bending the dowel.

[oipigface]

10 hours ago, wookman said:

 

It would just be a matter of binding multiple receivers to the same model memory and powering them up symultainiously. 

I’ll bear that in mind for the future. Thanks.

[oipigface]

Yes, the wing is foam. If I were sure that the 25mm edf’s were the right size, I would just glue the supports in, but I’m going to add a ply base to them, so they can be bolted onto a ply bearer which will be glued in.

The trouble with straps is that they may distort the duct, which is a not very heavyweight plastic moulding. I take it from what you say that you share my suspicion that gluing the bottom half of the duct to the fitting would be inadequate. 

[wookman]

If you stick them with Sikaflex 221 they shouldn't go anywhere. https://www.uksealants.co.uk/sikaflex-221-adhesive-sealant-300ml.html