Wing area

[Robert Watkins]

I haven't had time to read the threads yet, so this subject may have been covered before. I want to know if a model aircraft with a reletively large tailplane, designed with an aerofoil section, is included when calculating the total wing area.

I would have thought that in a case like this, there is a significant contribution to total lift. I refer to the Southerner 60, which according to my amateur estimate has a wing area of 454 square inches. The tailplane I have not worked out, but it looks like at least 100 square inches to me.

This model may top out at over 4lbs AUW, so I am concerned about wing loading. I would be grateful for any thoughts.

Rob

[Myron Beaumont]

Robert

Know what you mean .& Have never found a definitive answer .BUT when it says on the box /plan such & such a" wing area "I've never found it to be what I call the effective wing area ie the bits sticking out

[Guest]

the rearward CG of the Southerner probably puts it behind the centre of lift ( or pressure ?) and as a result the tailplane , as well as stabilising the pitching action of the model, will also need to provide some lift to balnce the forward centre of lift - hence it will contribute to lift and therefore reduce the wingloading.

To establish how much the tailplane contributes to lift you need to know where the centre of lift ( or pressue )  is located for the southerner wing and tailplane sections for the speed at which it is flying ( I seem to recall the centre moves according to speed or was it angle of attack ?? )

from that you can work out the moments around the CG knowing that the total lift more or less equals the weight of the model.  that will give you the proportion of tailplane loading compared to the wing .

 having said all that,  comparing the probable small distance the centre is in front of the CG compared to the much larger distance to the tailplane, I doubt that in the case of the Southerner that it will make much difference to the wing loading.  By comparison , in early contest models with CG's almost on the trailing edge , the large tailplane made substantial difference to the lift and wingloading .

[Alistair Taylor]

Hiya Robert

I did some digging on this cos it piqued my curiosity as well. This is what I found;

Normally the tailplane is not lifting, but rather pushing the tail down to counteract the effect of the wing, where center of lift is behind center of gravity, and so the nose always wants to drop (more so as speed - and therefore lift - increases).

Without a tailplane, a plane (with normal wing section - not reflexed) can be set up to fly stably but only at one speed, as the center of lift (but not C of G) moves with changes in speed.

In most model and indeed full size planes the tailplane is not designed to produce much lift - being symmetrical in profile or a flat plate. The C of G is set at a compromise position that balances performance against stability.

If you do move the centre of gravity of the plane backwards, so that the tailplane is no longer pushing down, but either neutral or even lifting slightly, then the plane will become overall more efficient, in terms of lift v drag, but also more unstable in pitch (yaw?) and roll.

In other words, with a rearward CofG, the elevator is producing less drag, and so the wing does not need to produce quite as much lift to counteract this drag (assuming forward speed and weight are constant), and so overall induced drag is less.

In full size planes this would make for a very "nervous" plane that would spin more easily and would tire out (or kill) most human pilots very quickly - so it is not desirable, even if it is more efficient.

In some models this situation is desirable, e.g. high performance gliders, and the disadvantages can be mitigated by creating a very stable design (e.g. free flight gliders with lots of dihedral that will circle gently for hours on end without human interference).

However, the compromise in these designs means that these models are not very manoeuvrable if you do choose to add R/C. I installed R/C gear in a KeilKraft Caprice glider (with rearward CofG and lifting tailplane) and it was extremely relaxing, but you'd never win a spot landing competition - not manoeuvrable enough.

The Southerner 60,  is probably off this ilk, being an ex-free-flight design. So it will probably be designed with a slightly rearward C0fG, and the tailplane will contribute (a little) lift. What do the plans say about recommended weight?

 AlistairT 

[Robert Watkins]

Thank you all for your interest and responses. You are very knowledgeable indeed. I have not heard these facts explained before, and I must say, explained very well.

 The plans are just the drawings really, reproduced from the original Keil Kraft by Ben Buckle. I am referring to the Ben Buckle kit of course. The finished weight is stated as 3 - 3.1/2 lbs with R/C equipment, but I am planning electric power, so there's the battery to consider.

I take it then that the wing area does not include the tailplane. I had overlooked the fact that it provides downward thrust for pitch control, thinking that this is only apparent when the elevator comes into play.

It's just that this tailplane is such a real "build" compared to the run-of-the-mill flat jobs, I thought that there must be a good reason why Bill Dean designed it this way.

Thank you Alistair, John and Myron for your interesting answers. The one other thing that  would boost my confidence on this would be to know that my calculation of the wing area was correct. Over 20oz per sq.ft seems a bit off-putting. I think maybe the wing will have to be strengthened and flying speed quite high.

Rob 

[Alistair Taylor]

Hiya Robert

the tailplane construction may be to save weight, and possibly also to save on balsa.

Bear in mind that for electric power you should be able to lighten the engine mount area of this plane considerably, as electric motors do not vibrate anything like IC engines. If the plan suggests hardwood bearers, leaving these out will save quite a bit of weight straight away.

Also - are the suggested weights based on modern radio gear, or vintage stuff, including the assorted heavyweight gubbins needed to run a spark ignition engine? If so, then even on electric power you should be able to finish this plane well within the suggested weight.

If not - other weight saving measures to consider would be lightweight covering (Litespan or similar), mini or naro servos (micro 5-7gramme jobbies are probably not up to it), and possibly lipos, although you may find that you would then need to add nose weight, negating any weight-savings made.

Also - you will not need 100watts per pound to fly this plane (i.e. 350 - 400W). I would guess at 250 - 300W, using a low Kv outrunner and monster prop. In which case a 9.6V CP1700 pack, running at 30 ish amps, would give you reasonably long flights, not weigh too much, an be fastchargeable in between flights while you're waiting for your pulse to speed up again   You could even go down to 8.4V, saving further weight.

Hope this helps a bit

AlistairT

[Guest]

Robert,

i didn't pick up on your weight - apologies.

I am about to build a southerner 72" and expect to get a weight of 3 lbs so I think your estimate of 4 lbs seems a bit high unless you are using old brushless motors with heavy nicad/nimh batteries.

 there is a guy called "vintage1"  usually active on RCgroups vintage forum who redesigned th 60 for electric and has the plan available on the forum. He also did a build thread so his all up weight will be there - if not just post on the thread asking for weight.  it is a long detailed thread to work thru.  the wing on his design which I think will be very similsr to the original will not like 20 oz loading 

do a search on "vintage1 +southerner"

he also arranged for laser cutting of the parts - again should be in forum but he also has a link with belair so might have arrangement with them as well - they might have a partial kit on offer.

there are plenty of Southerners around - all sizes and power types and I doubt any of them come anywhere near 20 ozs  - I would hope for about 12 ozs /sq ft

also in case you are already building to vintage1 design - the nose I think has been shortened making the model look a bit snub nosed and detracts from its looks.  for both aesthetics and balance extend the nose by abour 1.5 ins

[Mike Rolls]

As has been said, the Southerner was originally designed for free flight bcak in (IIRC) 1949. The structure, including the tailplane was designed to cope with a lower weight than a radio conversion would have - for F/F it would have had an all up weight of around 30 ounces. Powerplants quoted by KK were anything from a Mills 249 up to a Frog 500.

Somethnig to bear in mind when considering the CG position. The Sutherner, in common with all F/F power models of relatively low power, was a two speed design - one speed under power, another, slower, on the glide. This being the case it was possible to trim it out with a CG which, on a radio model, would prpbably be too far back for comfortable control in pitch, leading to a very twitchy flight characteristic. I (and many others) have flown F/F competition power models which balanced on the wing TE (some even balanced a tad behind the TE) - but this is only possible because of the 'two speed'character of the models.

I don't know, never having seen the plan, but I hope that the Buckle modifications include substantially stronger spars than the original had.

HTH

Mike

[Robert Watkins]

Your contributions, gentlemen, have been priceless! Makes me realise that I am a heathen talking to boffins!

I have heard of Vintage 1, a remarkably knowledgeable and experienced gent from East Anglia, I believe. I did read some of the threads a year or so ago, on my employers' computer. His build of the Southerner I must look up. I seem to remember he was not a big fan of Ben Buckle kits, but we won't go into that, will we?

Having taken all this on board (Thank you!), I now believe the path to be taken must be:- strengthened wing, possibly spruce spars or webbed spars; lipoly battery; brushless out-runner, lengthened snout, although I do like the design of the original, no beech bearers. The wing joiner should maybe be aluminium or ply, I have heard of one story of a complete snap, each wing half floating earthwards!

As you say, the weight I had in mind was with a GP 2200 nimh, and 600 with gearbox - now terribly out-dated. I have two 600's with gearboxes, they will end up in the deleted drawer, I think.

I had planned a nice long build, but I may have to get the laser cut parts. I think they are available from Replikits, but I'll give it a go first.

I hope you don't mind if I chip in on this forum again, some time in the future, as I shall probably get stuck!

Rob

[Mike Rolls]

Rob

I have just remembered reading a few years ago of a Southerner being converted to electric, I can't remember what mag it was in I'm afraid, but what I do remember was what I thought to be a very neat battery mounting. The electric motor and ESC was about the same weight as the original IC, which made battery location potentially difficult, so the builder took advantage of the Southerner's deep fus to mount the battery (with a suitable bulkhead for retention), standing vertically at the CG - so he could also use different batteries without having to worry about shifting the CG.

Mike

[Robert Watkins]

Thank you Mike for your response.

 I did get that article by purchasing the back issue, in fact, there were two articles in different magazines, I still have them somewhere. I remember they were from 2000 and 2002. I have thought of following these, but they are now pretty dated, of course. Nevertheless, I may still take that road, but I would prefer the battery in a different position. The items required are still current, and a cheap option.

I have just registered again on RCGroups, my second attempt in 5 months, but the result is the same, - cannot log in!

rob

[Guest]

I have a geared 600 complete with ESC on the back of it - never used it - far too heavy for poor power output- gone brushless and lipo - the lighter weight is easier on the model and it should last longer subject to pilot error  

and I wont chuck it out - paid too much for it - must be good for something ??

 vintage1 design has battery box on bottom of fuz

try emailing rcgroups to sort out your registration

john

[Robert Watkins]

Yes, John, I can't throw stuff away either, I mean they worked then, and they will work now, but today's stuff is better. I love my nine year old Passat, but I wouldn't go back to the old '59 Minx.

I can't seem to find Vintage 1's Southerner stuff. I have emailed RCGroups, but I've tried that before.

Rob

[Guest]

try here http://www.rcgroups.com/forums/showthread.php?t=626645&highlight=southerner

he calls it the   "new southerner 60"  and the above should take you to the build log - all 12 pages of it

john

[Robert Watkins]

Thank you John.

I seem to remember it was put under a different thread, perhaps. But I'm not too clever finding my way around this lot anyway. Cheers

Rob