just how does a "lifting tail" work

[robert chamberlain]

Hello, I can understand how a normal airplane stays stable in pitch,--no problem. Nose goes down, airspeed increases, and the downward force of the tail pushes tail down and nose up.Several A/c advertise a design with a "lifting tail" as part of the specs. (Telemaster). OK,-- how does it stay stable in pitch? Does the center of lift move forward in a dive? Any thoughts out there?--Bob C in Kansas,USA

[riverlandgirl.]

The way I understand it is, as with the old pylon wing free flight gassies, was that the c of g was set well back, and lifting tail plane was used to to conteract the tendancy of the plane to pitch nose up due to the combination of a high wing and a low thrust line. These planes were very high powered for there weight and had avery steep climb.

[Myron Beaumont]

I think there is a destinction to be made regarding what is commonly referred to as C of G and the balance point regarding the horizontal component of the C of G measured on the wing .At high angle of climb on a pylon configuration especially, the balance point will have moved forward in effect which contributes to stability in the very steep climb. CG obviously remains exactly where it always will be being the point within the structure no matter its orientation .Think of the downward vector component at that angle from the socalled balance point on a very high mounted wing with most of the aircrafts weight concentrated well below the wing and almost on the thrust line..On engine cut then the model will gently glide at almost stalling speed but at its designed parameters for max duration .The large aerofoil shaped tailplane helps to give horizontal stability under these conditions .Wow ,Makes me think I know what I.m talking about for once ! I'll wait for one of you experts to shoot me down in flames

Myron YO13 brainstorm dept.

[PatMc]

In some thing like a Telemaster all a "lifting tail" adds is unecessary drag.

[Tangental]

Extremely well explained Myron.Well done.

[Myron Beaumont]

Tangental

Thanks for the compliment .I made it all up based on logic !

Myron YO13 Lateral dept.

[Plummet]

Posted by PatMc on 03/03/2013 13:08:32:

In some thing like a Telemaster all a "lifting tail" adds is unecessary drag.

Is this quite true?

The weight of the model has to be supported by the model's lift. If the tail is providing lift, then the wing needs to provide less lift. This will mean that although the tail will generate more drag, the wings will generate less.

A non-lifting tail at zero incidence will do nothing except generate a bit of drag. If the wings angle of attack increases then the tail will also get positive attack, and so will generate lift and tend to lift the tail, and so reduce the angle. Ditto, in reverse if the angle of attack decreases.

For a lifting tail, the only difference is that in 'normal' flight the tail is producing some (instead of zero) lift, and the changes in angle of attach increase or decrease the tail's lift. The effect will be the same.

I think!

Plummet

[PatMc]

Plummet, if the tailplane was to contribute to the lift the cg would have to be much further rearwards than is normal. The tail would be providing it's share of the lift at a much lower efficiency than the wing resulting in a lower lift to drag ratio for the model as a whole.

In a conventional rc model the only functions of the tailplane are a) to provide a steady force to holds the wing at the correct AoA in level flight & b) to vary that force using elevators or AMT when it's required to change the wing's AoA in order to pitch the nose up or down.

All conventional rc models are balanced so that the tail has a small downward force in level flight. This means that a "lifting section" has a negative AoA which would create more drag than a symetrical section would to provide the same downward force.

[Martin Harris - Moderator]

Posted by PatMc on 03/03/2013 23:08:16:

Plummet, if the tailplane was to contribute to the lift the cg would have to be much further rearwards than is normal.  

That's certainly been my experience in the limited experience I've had of non-symmetrical section tailplane models. A "vintage" style model I was helping someone with called for a C of G at something around 50% of MAC

...and it flew with good stability with it there. TBH the concept of a "lifting" tail is something I've never been comfortable with - a good opportunity for BEB to give one of his plain English explanations?

Edited By Martin Harris on 03/03/2013 23:16:45

[Simon Chaddock]

I believe the answer lies in the differing sections of the wing and tail.

Invariably the main plane had an undercamber section whereas the tail plane was flat bottomed and set at a slightly lower angle of incidence.

The result is that with increasing speed the wing generates proportionally more lift than the tail so causes a pitch up and vice versa.

It does not have the same degree of stability as a not lifting tail and with an elevator in the picture it can easily become unstable so tends not to be used in conventional power layout but as Myron pointed out the pylon designs have their own stability effects in a steep climb and once the motor shuts down they then fly as a single speed glider.

[PatMc]

"Lifting" tailplanes in models have nothing to do with mainplane aerofoil section or pylon mounted wings. They came about for two reasons, a) competition rules b) lack of understanding the tail's function.

A number of different discipline free flight comp rules included max horizontal surface area. This included the area projected across the fuselage and the tailplane area.
In order to make the most what would otherwise have been fuselage area & for other reasons pylon mounted wings became fashionable.
To make the most of the tailplane area it became common for it to be made to contribute to the total lift. This meant increasing the tail area at the expense of wing area & moving the cg a long way rearwards, sometimes behind the wing's TE. Compared to the wings the induced drag of the tailplane is high in proportion to the lift it can contribute. A lifting tailplane therefore has a negative effect on the lift/drag of a model.

The "lifting" tail section was simply copied into many FF sports models then into early RC models because the reasoning behind it was not questioned or if it was the justification didn't stand investigation.

Edited By PatMc on 04/03/2013 11:50:24

[Myron Beaumont]

PatMc

Good point ,had forgotten the "rules" but why weren 't the tailplanes made with a similar section to the wing high lift one I wonder? By the way I lost two high pylon models many tears ago 'cos the dethermalisers didn't fire up to their job and because I put too much fuel in the tank ! That was in the days when I couldn't afford more than a few aeroplanes or engines a year based on xmas/birthday pressys/2s -6d a week pocket money in the early 50's.Still doing modelling though .Nothing like learning from experience

[PatMc]

Posted by Myron Beaumont on 04/03/2013 12:09:51:

but why weren 't the tailplanes made with a similar section to the wing high lift one I wonder?

Sometimes they were. Probably trial & error led to the use of less cambered sections.

BTW remember that some old (& not so old) glider models also used lifting tails, so I'm afraid I don't think that the pylon model climb theory holds up.

[Simon Chaddock]

The original poster did ask 'how' lifting tailplanes worked rather than 'why' they were used.

[Myron Beaumont]

PatMc

Do you mean my theory i exponented (? word) above ? You could argue I suppose that any lifting surface should produce as much lift as possible, ie wing & tailplane, if rules dictated a timed engine run and a long as possible glide .When confronted with a problem I always take extreme values/parameters to seek a solution .In this case I'm thinking of a pylon model with an outrageously tall pylon . If you think about it ,then the centre of balance on the wing (not the CG) is verging on the irrelevant. Bit like a parasol type aircraft in the extreme .In fact more like a para glider comes to mind .Only power or wind/thermals will keep you going upwards

Edited By Myron Beaumont on 04/03/2013 12:37:16

[Martin Harris - Moderator]

Simon said:

The original poster did ask 'how' lifting tailplanes worked rather than 'why' they were used.

Don't let that distract anyone from this discussion! I'm finding it very illuminating.

Edited By Martin Harris on 04/03/2013 12:36:26

[Frank Skilbeck]

Posted by Myron Beaumont on 03/03/2013 15:03:59:

Tangental

Thanks for the compliment .I made it all up based on logic !

Myron YO13 Lateral dept.

Doesn't that break forum rules

[Myron Beaumont]

Isn't cross posting annoying at times,but just shows how interesting this hobby is now we can pass info back & forth on the forum ?

Edited By Myron Beaumont on 04/03/2013 12:43:25

[Hugh Coleman]

What about in the situation of something like an SE5a or a BE2c with a fully simetrical horizontal stab but an angle of incidence of around 3 to 4 degrees (coupled with a main plane angle of incidence or between 6 and 9 degrees!).

Is that considered a lifting tail?

Cheers,

Hugh