It has to be easy and good value.
|Simon Chaddock||08/12/2019 18:29:13|
5733 forum posts
That looks very interesting build and what I call a "development" build where you try out new approaches but in small stages to avoid the dangers of something being a complete flop.
I agree entirely with the principle of putting servos close to where they are needed so the linkage can be mechanically efficient and rigid.
I do suspect the desire to place them in the centre of the fuselage with long control runs in part dates for when servos were both big and seriously heavy. I have several that date from the 60s that were at the time the latest in miniaturisation. About the only feature relevant today is they are seriously powerful.
My only tip about using a gyro is if at all possible have it so it can be switched off in flight. Then you make a direct comparison of its effect under the same flight conditions. The result can be 'interesting'.
Look forward to hearing about the flight tests.
|Geoff S||08/12/2019 22:11:28|
|3700 forum posts|
I bought some 3mm carbon tube from Bucks Composite at the Nats. I can't recall the price but it wasn't a great deal. They make excellent push rods with 2mm threaded rod at one end and a 2mm Z bend at the other (the servo horn). It means fitting the servos in the middle of the fuselage is easy and compromises neither rigidity nor weight by very much.
200 watts/kg is the rule of thumb but I find 150/160 watts is perfectly OK for scale like aeros, so you really need around 300 watts to give enough power with a bit in reserve. Can you fit a bigger prop on 3S without drawing too much current or hitting the ground when the tail lifts on rotation?
11781 forum posts
Simon, when i returned to modeling which must now be some +10 years now, my interests as (as yo know) were gliders and small electrics. Like yourself I had and still have a number of Futaba S128 servos. The issue for me was they were and strangely enough remain, both bulky and heavy.
I became aware of 9g servos, which to me were novel and remarkably low cost.
I was very undecided about a number of aspects, one of which was torque and how much it was necessary to have. With that in mind, I undertook a few ball park calculations based on various speeds, width of control surface, and finally the control linkage geometry (moments really). I used L=0.5*roe*V^2*S*Cl (for a typical section). I came to the conclusion that you had to travel seriously fast, or have a very broad control surface (such as a rudder, or large model) for anything to be a real problem to these small servos. On that basis I concur, for the models that I build and most of your own, the small servos, mounted towards the tail is not much of an issue.
I guess we both observed, where we worked, it was projects that tried to undertake a step change in technology that would more often and not experience multiple challenges to delivering what had been promised. Often the gains proving to be an illusion, and more often than not making a nonsense of any budget or programme.
Incremental innovation, although often sneered at, provides more certain rewards, perhaps just as importantly, development issues are manageable. Avoiding the swamp with the alligators.
Having said all of that I have now started the evaluation process for gyros. Although it looks tight and probably is not as spacious as I planned on my fag packet, I am reasonably confident that all will be OK. It is my intention to create a plinth to mount the gyro on. My reasoning being is that I want the gyro as near the CG in the x,y and z planes as possible, with the hope that the acceleration forces seen, are measured about a singular point. Of course knowing nothing about Gyros this may be in practice not the issue I am concerned about. I know you used at least one on a model.
There are at least two projects that are immediate, I am hoping to see benefit from a gyro. The unfinished Bachem Natter that stares at me and a project I have been considering for a while.
I have been watching the Tony Nijhaus Sabre etc. That using the same fan unit, I could get a semi-scale Douglas X3 of the ground (I really mean thrown). Have you any experience of his units? I am thinking of using Depron and Balsa, as the front is a triangle, the centre pretty much a box, the tail boom is where the balsa comes. The wings would be depron as used on my canard, using a HLG section, which seems to work well.
|Nigel R||14/12/2019 21:02:04|
3982 forum posts
Looking forward to flight report erfolg.
This is one project on here that epitomises scratch building. The reuse of many parts. A delve in the scrap box. Some new things to try. A plan almost drawn straight on the wood.
Re servos and snakes. The ex member who removed all their posts did a weight comparison of various control methods and I seem to remember the lightweight sullivan snake came out quite favorably when compared to closed loop and / or rear mounted servo methods. The extension cable was what added weight for the rear mounted servo. Snakes are very easy. They are not the cheapest of solutions if you get good ones though. But they can look neat and the electromechanical bits are kept well out of harms way.
I like them, they work well for me.
Also, I agree about thick airfoils. They hurt the speed range with little benefit.
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