Lifting and non lifting tailplane

[Boots]

I thought I understood a bit about how a plane flies but I cant figure this.

If a plane is tail heavy  can a lifting tailplane in flight support this weight and rebalance c of g.

 

Why do many planes have flat tailplane and only sum a lifting ?

[Former Member]

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[Boots]

so basically a lifting tailing plane more often than not ,increases drag and looks better on a scale model, go for flat then I say

thanks phil

[Former Member]

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[Simon Chaddock]

Yes in free flight gliders is was common to have an under cambered wing section with a flat bottomed lifting tail with little or no incidence between them. Stability was achieved by their different aerodynamics, any increase in speed caused the wing to generate lift faster that the less efficient tail plane so pitching the nose up and vice versa.

 

It certainly allowed the CofG to be set well back (up to 50% chord) but it was rather tricky to get right and only tended to work properly over a relatively small speed range. 

[Evan Pimm]

To answer the original question, no, a tailplane cannot 'support the weight of a tail heavy model and rebalance the cg.' In simple terms the cg (balance) determines the longitudinal stability of the model and is fixed by you, the builder. The tailplane is the 'speed control' of your model, and you vary the speed with elevator trim. The tailplane will then maintain the required main wing AOA by either 'lifting' up or down, hence the preferred use of a zero camber (symmetric) section for this surface. The 'lifting' (cambered) sections referred to are an old specialised free flight method of controlling the flight, with other trim methods, of a 'fixed surface' model and is used to maintain a fixed speed (single AOA), a pretty artificial form of flight. As alluded to in previous postings, if anything upsets this trim, the results can be spectacular, as anyone who has witnessed an FAI power model 'blow up' will agree.

Evan. 

[Braddock, VC]

My southerner major has a lifting tail section and undercambered 84" wing and the c of g is 65% back from the wing leading edge and flies well at all throttle settings including deadstick, my 96" falcon has a symmetrical tail section, modified clark y on the wing and c of g is 35 % back, it flies well, too. Relatively speaking, the former has a much longer tail moment arm.

When I first started flying the southerner major, the c of g was about 30% and there was about 12 ounces of lead up front plus a 61 4 stroke, it used to take a lot of runway to get off and the landing speed was very high. I gradually got rid of all the lead and swapped the 61 fs for an original os 40 fs from 83 which is a further 150 grammes lighter, at this weight the c of g was about 75% and , probably because of the minimal power, she wasn't unduly twitchy but showed thermals well, landing speed was walking pace. I've recently swapped the motor for an OS 52 fs with no other changes and this mix is just fine for me.

My take on it is that the longer the tail moment arm is the more a lifting tail section will affect the c of g. If you look at the old duration power designs you cannot help but notice the long tail, in  fact I had one that flew well with the c of g on the trailing edge believe it or not which was a real pig in any other configuration - this was a pylon job that I sold on and I'm really struggling to remember the name, it was designed by leo schulman but the name escapes me for the moment  I'll post it when I recall it which will probably be 10 seconds after I make this posting.

[David Davis]

If converting a small free flight WW1 scale design to R/C, such as Ken McDonagh's BE2e or David Hughes' RE8, both of which feature lifting tailplanes, would you recommend building the tail as a flat plate or with a lifting section as per the original plans?

[Myron Beaumont]

Aren;t we forgetting one basic fact ? -That the tail surfaces are for STABILISING basically ( like a dart )and to assist in change of trajectory

Simples

[Evan Pimm]

David, if you are converting a F/F to radio you will have to change both the wing/tail angular relationship, balance and possibly the tail section, if only for strength reasons. Fitting an elevator to a model means that you are now capable of varying the airspeed, which means greater loads, and the need for a more forward balance, so stronger and longer wing dihedral braces and spars and less (around +2deg?) wing/tail angle difference. As the tail needs now to 'lift' both ways a zero camber section is desirable.

John, a tailplane cannot affect the cg (balance), only weight added at the front or back can do that . The reason the old models were stable is that the balance, even if at the trailing edge of the wing, is still sufficiently forward of the aerodynamic centre, (the lift cg, if you like) that there is still enough longitudinal stability for the model to be speed stable, although the closer the two get, the narrower the speed range is. The glider guys will tell you about 'tailplane takeover' a condition where the model bunts, no matter how much up elevator the pilot applies. A result of the balance being just a bit far back, and the model being flown just a bit out side the stable speed range. But you guys know all this stuff anyway.

Evan.

[David Davis]

Thank you Evan,

 

Very interesting and useful information. I note with interest that the original full-size BE2es and RE8s had symmetrical tailplanes.

[Tony Banfield]

For an aircraft/model to be longitudinally stable the CG must be forward of the centre of lift (or to be a bit anorakish aerodynamic centre).  This will cause the aircraft to try to pitch down.  This is opposed by the tail plane with a downward force (negative lift you might call it).  A flat plate tailplane does this by being at a negative angle relative to the wing.  This is called longitudinal dihedral in some quarters.  Now, a flat plat aerofoil is less efficient at producing lift (positive or negative) than a cambered one so some aircraft, such as the Jet Provost, actally have the tail aerofoil "upside down" and the F4 Phantom has little slats on the underneath of its tailplane!  With our models, because of their tiny Reynolds numbers, It is fairly irrelevant what section you use on the tailplane but flat plate is so much easier to build that the benefit analysis mods often results in this being the preferred option.

[frank killan]

I have a DB models Bistormer 60 which has a lifting section tail plane. On takeoff you have to hold in a fair amount of up elevator until it reaches flying speed.The tail seems to lift before the wings. In my limited experience a flat tailplane is best as you just fly it off with no worries about nosing over. My other biplanes have flat tails and it is easy to forget and hold in up elevator causing some dodgy takeoff runs until I realise.

                                                                                                         

                                                                                                                  Cheers FK.

[Former Member]

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