Lifting Tails!

[Edgeflyer]

Can someone with experience of vintage or free flight aerodynamics please advise... 

I have a KK Southerner modified from the Ben Buckle kit for electric power. 60 inch wingspan. I wanted to make the tailplane removeable like the original and to have its leafing edge top sheeted as per the KK plan. 

The increased weight at the tail has brought the cg back. 

As the tailplane has an undercambered section and a large surface area by modern standards am I correct to think a lifting tail needs a further aft cg thsn woul normally be the case? 

Where might a good starting point be say 33 % back from le? 

[John Wagg]

My Southerner has a flat bottom section tailplane but it is a cambered lifting tailplane. The KK plan also shows a flat bottom cambered. The KK plan shows the CG to be about 37% back from the leading edge if that's of any help.

My tailplane is also removable and held on with rubber bands

[Edgeflyer]

Did you need weight up front to get the cg at 37%?

[J D 8]

 You are correct Phil, these vintage design's were made that way so the the lifting surface's included the horizontal tail to enable flight on the limited power to weight ratio of the engines of that time so cg ends up further back.

 I recon it became a bit of a fashion to make them that way even when better engines became available .[ Early full size aircraft are much the same ] The short nose is also a result of heavy engines. Over powering these types should be avoided.  John.

 Pic. Veron Cardinal gliding in.

[John Wagg]

28 minutes ago, Phil B said:

Did you need weight up front to get the cg at 37%?

No but mine is IC and uses a 20 size engine. Tell the truth off hand I don't know where my CG is. I will assemble it later and let you know.

[Edgeflyer]

Mine needs 7 oz when the 2200 lipo is right up beh8nd the motor. 

[Edgeflyer]

Tempting to just pair up 2 batteries 

[John Wagg]

Checked my CG and 3.5 inches back from the leading edge. My calc' gives that at 36.8% so near enough to plan.  

John

[Simon Chaddock]

With elevators a lifting tail plane is a bit of misnomer as the actual net force produced will be almost entirely dependent on the elevator angle.

True lifting tails only made sense for a 'single speed' plane and relies on the tail plane producing less lift than the wing as the plane speeded up and vice versa.

Such a characteristic usually only works well when there is a significant difference between the sections of the wing and tail. This was usually the case as wings back then tended to use an under cambered section whereas the tail plane was flat bottomed.

The nearer the wing and tail sections are to each other the harder it becomes to achieve natural longitudinal stability without using significant decalage and a forward CofG but in this state it is unlikely the tail plane will actually be producing positive lift anyway.

[J D 8]

 A example of what Simon describes above, very much a one speed aeroplane, at least not much difference from slow to full speed in normal flight. 

However give to much power [ something the full size did not have ] and it rears the nose up even with down elevator.  At lower power if given down elevator it dips the nose and as the speed comes up and is then slow to pull out  [ best just to shut the throttle.]  I tend to fly it mostly with just the throttle, rudder.  Which is why I say not to overpower these vintage model types.

[Edgeflyer]

Thanks chaps thats fascinating stuff. Ill try and fly mine effectively rudder assist only. 

[john stones 1 - Moderator]

When I wor a lad, a mate of mine bought a big Flair ? Taube, neither of us wanted to test fly it, we were both just learning, so he asked the local topgun to test fly it, meh it's way tail heavy he says as it's lurching around the sky, lump of lead in try again, nah still tail heavy says he, sat there dejected thinking he's built it badly, when an old lad who we used to watch tootling n free flighting turns up, giz yer box he says, ooh that flys nice, aye, he says, take the lead out n slow down son, happy days.