BMFA Frog

Or, by the sound of it, in Sarah's case - delerium tremens!
De-thermaliser (or zer, depending on the spellchecker) is a way of making the tailplane tilt up to a huge angle, so stopping the aeroplane from flying, and it should 'parachute' down.

The tailplane is only attached to the fuselage at the leading (front) edge, by a hinge, and the back is held down by a release device of some kind. This can be a bit of fishing line, or an elastic band.
There is a timer device used, which can be clockwork, and releases the fishing line, so releasing the tail, when the time runs out -
or it can be a slow fuse, which burns away, and eventually burns through the rubber band, allowing the tailplane to tilt up (and sometimes sets the model on fire, too!).
The tailplane is tilted up by the tension of another elastic band, or a spring.
There are probably other ways of doing it, but it's not my field of partial expertise.

This little thing flys very well on just the top surface covering, to cover the nottom would make it a tad heavier, enough to spoil its flying duration, and i thought you would, by now, know what the DTs were, :whistle:

YOU ARE SHORT OF BALSA? you wont be when you get here,

The answers so far are absolutely right, Sarah, but I'm going to go a bit more basic.

Dethermaliser: Thermals are rising currents of air. In the free flight world, we try to put our models into a thermal to get them to fly for longer. Unfortunately, when this happens, we often loose them (the mode), so as a means to bring the model down we fit a de-thermaliser or DT. Also, in competitions, a maximum flight time is often set that is a little above what we call the 'still air time' of the model. The still air time is the maximum time the model would fly for without thermal aid. So, if a model achieves the competition declared maximum time, it has caught a thermal and we don't need it to fly for longer to prove it's capabilities. Competitions often have several rounds and at the end of the event a fly-off is arranged for all the modellers that have achieved a full-house of maxes. The fly-off is often anything goes, but sometimes it has a maximum time as well.

SIngle surface covering: The short answer is yes, you can double cover the wingt, but it will reduce the aerodynamic efficiency of the wing and it won't fly as well. It will still fly, however, so if it pleases you, go ahead and do it. The longer answer is: The aerodynamics of a wing changes with Reynolds Number. Reynolds Number can be thought of as the number of molecules passing between the leading edge of the wing and the trailing edge in unit time. So a slow flying narrow chord wing such as on the Frog flies at a very low Reynolds Number. As Reynolds Number reduces, to be aerodynamically efficient, wings must get thinner. In the limit and in the case of the Frog, the most aerodynamically efficient configuration is single surface covering and for different reasons, the more wrinkles it has the better! Double covering the Frog will increase the thickness of the wing and make it less efficient.

If you want a real odd ball how about the Mercury Marauder?


Maybe not for Sarah just yet but with its fuselage 'on the diagonal' it has virtually none of the features on Alan's list so its fin was on top and ahead of the tailplane.

Still have the plan somewhere.

I built one of those in the 60s as well. B thing clapped hands on it's first launch up the line, probably because the balsa I used for the spars wasn't strong enough........ I seem to remember it has an underfin as well as the one on top of the fus, but I may be wrong.

Cheers guys, I had a basic idea that dethermalising meant bringing it back down so its not flying forever, but didnt realise DT was the abbreviation or further details.

Completely stupid question but what is the DT then on the frog? Cos am not at that bit yet, or can find any info in the instructions regarding it.

Another stupid question, haveing read up in more detail today regarding lift and molecules of air, having said wrinkles in the tissue on the wing does this not create any pockets of larger molecules of air that may disturb the balance of the air flow?

Andy, i can see where you are coming from with the wing being thicker, but would the air not break up in the normal way on contact with the leading edge, and the crossbraces? therefore automatically being a thick wing?

i can remember, as an aside, a friend of mine flying on the nats display line, he was flying a glens Cap, with a zenoha 62 petrol engine, so, quite a size, his tow surface covering came off at the leading edge, and blew back words, the entire structure was on show, after the initial shock, and after a few test manouveres, he flew it round--carefully, and landed as normal, would the leading edge, in this case, be breaking the air up as normal? enabling it to fly on the underside covering, or was he just a lucky git :blink:

All tissuing done, didnt bother so much with the odd wrinkles as to reiterate what Andy says, it does specifically say in the instructions not to try to water shrink or dope the tissue.
Tissue applied using 1:1 PVA glue and cut with a 2mm edge so as to wrap around the balsa slightly, overlapping 5mm at each dihedral point.



tail covered with end plates glued in place


I wasnt very happy with the wing mount, it looked a little wonky so I pulled it apart and using a new strip of boom remade it.





fin attached and glued to the boom and underfin.

There's no DT system on the Frog Sarah. DTs are normally reserved for outdoor models, the Frog is designed mainly for indoors.

Good question Alan, I'll answer in a new thread rather than clutter Sarah's build thread with info non-relevant to the Frog - www.modelflying.org.uk/forum/free-flight...t-aerodynamics#48814

Put the wings onto the motorstick and attached rubber bands and checked the boom when placed on the motor stick is set so the fin looks vertical when the wings are horizontal.
Had to slightly sand some of the cut away at the end of the motor stick to ensure this was level. It does not state where the wings should be placed along the motor stick? So any ideas guys on that please?




Smally problem now, I have to attach the rear hook to the motor stick but first I have to bend a slight bit of the end in a right angle......and I cant, pliers and me= not strong enough and it aint bending! Was wondering if by slightly heating the metal that would make it more pliable?

AH, i thought it was a simple flat plate section, now i see its an uncercambered jobby, even nearing a jedelsky section, nice airfoil, great for slow model flight, wonder how it works? :whistle:

Andy Sephton wrote: There's no DT system on the Frog Sarah. DTs are normally reserved for outdoor models, the Frog is designed mainly for indoors.

Good question Alan, I'll answer in a new thread rather than clutter Sarah's build thread with info non-relevant to the Frog - www.modelflying.org.uk/forum/free-flight...t-aerodynamics#48814

Well decided Andy. Very good decision

Garry

alan cantwell wrote: AH, i thought it was a simple flat plate section, now i see its an uncercambered jobby, even nearing a jedelsky section, nice airfoil, great for slow model flight, wonder how it works? :whistle:

Doesnt it add drag being undercambered?

I think an undercamber needs about 4 degrees incidence to work, so, i suppose that yes, it will be a high drag sort of foil,

C of G - I suggest you fit a motor and balance it half way between the LE and TE by moving the wing backwards and forwards. Then give it a chuck and see what happens. If it dives move the wing forward, if it pitches up, move the wing back.

Drag of undercambered sections - yes Sarah, the drag does increase but so does the lift. The efficiency of a wing is measured by the ratio of lift to drag and in this case, as the lift increases more than the drag, the ratio increases making the wing more efficient.