Pete D

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Thanks John, I will certainly take note of the metal gearing etc. you mention.   I think fom looking one of the Servo ads, that quoted torque, that I can probably over engineer the servos without too much size or weight penalty.  The problem in this respect is that normal shop ads do not quote a torque figure.   I sscrewed up one of the above figures,  the Rudder B figure of 12.43 N cm should read 1.4 N cm.  God knows where that came from and He ain't telling.   Anyway I will be checking the figures again to make sure and probable do some experiments hanging out of the car window or something but the figures so far seem to suggest we grossly overengineer most of the time with consequent weight penalties.

Using John's Electric Flight in Colerado servo calculator I managed to get the following for 2 speeds: A 50 MPH and B 60 MPH using a control throw of 20 deg from neutral. I think I will need to do some more calculations for a range of throws and speeds once I've figured what would be reasonable.   Rudder    A   1.4 oz-in,  1 N cm each,    for both  on one servo   2.7 oz-in, 1.9 N cm.                   B   2 oz-in,  12.43 N cm.                                                 3.8 oz-in,  2.75 N cm.   Elevator    A   3.8 oz-in,  2.7 N cm for one half,  full  7.5 oz-in, 5.4 N cm.                    B   5.4 oz-in, 3.9 N cm.                               10.7 oz-in, 7.7 N cm.   Aileron      A    2.3 oz-in, 1.7 N cm  using a seperate servo on each.                    B   3.3 oz-in,  2.4 N cm.   These figures would be the minimum and I should probably look for a safety margin of +30 to 50 %.   My physics is a bit rusty but I think 1 Newton is 1 Kg/cm/sec so I presume that the N cm figure would equate with the normal servo ratings of grammes or Kg at 1 cm on the output arm.   I will now go and look at the ads. in the mag. to see what is in the range so far. I would like to use the thinnest ones possible at the back to bury them in the tailplane if possible.  The wing is quite deep so not so much of a problem there.   The Aileron would be a mass balanced slotted Friese type to be scale accurate, I don't know if that alters the parameters.  Rudder and elevator would also be balanced.   I told you it was over ambitious.

Advice to Simon B.  (is that Jersey Simon?)                                                                 DUCK!

By the way for the number crunchers I have worked out the control areas as:   Rudder (each) 79 cm sq.  12.25" sq.  total 24.5" sq.  poss. 2 servos.   Elevator (each half) 187 cm sq.  29" sq.  total  582 sq.   Aileron  (each)  179 cm sq.    

I am curious to see what I get from John Gibbs' web site   http://atlas.csd.net/~cgadd//eflight/calcs_servo.htm   but am not sure of the speed component.  I tend to think of models as flying fast or slow in scale not absolute terms so what speed  in MPH would Simon Chaddock's Mossie do for example?   I would guess at 40 - 50 MPH but is that reasonable?

I am now waiting for some bright spark to sue his Local Authority for removing so many of life's risks that as a child he did not learn about risk avoidance or management.    The result being that he was unable to see or avoid a risk that, as children,  we would have learned to all about.     I wonder!!    Remember the school Authority that banned running in the playground because of the risk of falling?  Or the one that banned conkers?  I am so happy that the school that my granddaughter starts next term has recently held a conker bashing event, it was a great success and whenever we pass the school at playtime we can see the kids running and jumping about.  Needless to say it is a happy place which also manages to achieve good academic standards as well.

We could buy all the ingredients for proper black powder from the local chemist.  Powdered charcoal, saltpetre and sulphur as I recall.  We made up some great bangers and fired them with the element from one of those 6 volt gas r4ing lighters.  A couple of long wires then slam them across a 12 volt car battery's terminals.  Good stuff as I recall.  Also at cosford we got some tubing, closed off one end and broke up the ends of Swan Vesta matches.  Stick a small ball bearing down and heat up the end with a lighter.  Managed to put a hole into the billet dustbin and got 7 days jankers. those were the days.

Talking of H&S a true story:-   The son of a friend of my wife was serving with the RAF in Afganistan.  As in my day most groundcrew did turns of aircraft marshalling.  Said H&S Goon was insensed that he was not wearing a high visibility jacket or tabard so that the Pilot could see him.  At this point the Very bright RAF lad handed him the bats and told him to go forth and marshal himsel adding, "The snipers are just over there." As you say one size does not fit all except perhaps jobsworth hats.

Actually a Do 17z or 215.  I got some brilliant 1/20 drawings and a stack of factory photographs from Dornierwerke in Friedrichshavn together with an old Flight Magazine Cutaway from 1942 strangely reprinted in Dutch plus a load of other info. I have also aquired a  Classic Airframes 1/48 plastic kit to give surface and cockpit detail information so there will be no excuse for any inaccuracy.  There is so much glazing that interior detail will be a problem and I will need to find 1/10 mannikins (or make them) for the crew who will be visible in their entirety.I know Dragon make some brilliant ones but they are in 1/6 scale which would have meant a model of 3 metres.  Too much to think about. I first experimented with a 1/64 balsa/tissue stressed skin during Cosford days in 1956 when I skinned a KK Spitfire kit this way.  I intended to fit a 1cc diesel instead of the KK rubber power but was unable to afford one on the then RAF pay (or lack of it). All the hard work was lost with other posessions when I was posted to Swinderby and baggage went missing so I took up gliding instead, until I got grounded for doing aerobatics under 200 feet.  I had the CFI in the back seat and did 2 elevator turns instead of a gentle sideslip on finals.  Ah! the madness of youth, I have no illusions about the scale of the project I did say this was an overambitious project and I meant it but if you don't aim high and being retired!! 

Thank you all for your input, particularly John.  I have tried the link again and got in but as it's late will try adding figures tomorrow. The model will (I hope) be 1/10 scale electric with a wingspan of 1.8 Metres, about 70". I hope to use a pair of scale profile 3 blade Vario props of about 13.9" diameter.  I believe the pitch can vary from 4" to 16" and run at 8500 RPM max. so I will eventually be looking for an appropriate power train when I've saved a few more pennies after New Year.  I believe the Vario props give a very good thrust.   So far I've only got as far as drawing the tail feathers in detail but have the rest in outline.  Like Andy I believe that most models are grossly over engineered in the sense of using much too thick timber.  I have been experimenting with a dummy run of fin and rudder using 1/16" balsa for ribs and covered with 1/32 sheet with lightweight tissue pasted oover.  I was surprised just how strong the build was I believe it was stronger than !/4 sheet equivalent at least across grain.  The whole lot was extreemly light, but have now junked that one and have re designed with less ribs but not yet built a test piece.

By the way the proposed model is a twin WWII bomber type hopefully in scale.  Control surfaces would seem to be fairly large but have not measured them yet.

Thanks for the tip John but I tried the sites suggested in my search engine but it couldn't seem to find them.  Any other suggestions? Pete.D.

Having come back to the hobby after over half a century I am keen to progress from rtf/artf to own designs.  It was easy in my control line days as all you had was an elevator  which only needed a handle, lines, bellcrank and control horn.  All the necessary power to move said elevator being provided by the Mk 1 wrist. Now I am trying to draw up an over ambitious project I have come up against the problem of what servos to use. Is there a formula or useful rule of thumb that gives an appropriate value of servo output in grammes for a given control area in square centimetres. There seem to be so many types on the market from micro to standard or even larger.  Most though seem to give an output value in grammes or kilogrammes.  Is this at the outside edge of the output arm or say the mid point? Also would rudder, elevator, flaps or ailerons require a different power input for the same given area? So come on you experts help an old wrinkley to move at least one step nearer to own design heaven.   Pete D.

By the way any ex Cosford Brats out there?  We certainly got up to some brilliant pranks there.  Perhaps that's a subject for another memory lane thread.   P.D.