Does tailplane need aerofoil section?

[zetland76]

I have just built a 1/10 scale PSS Spitfire. The tailplane (horzontal stabiliser?) is flat 5mm balsa. At this size are there any advantages in giving it an aerofoil section? If so what do you recommend?

Thanks

Mike

[Wendi Smol]

I have just built a 1/10 scale PSS Spitfire. The tailplane (horzontal stabiliser?) is flat 5mm balsa. At this size are there any advantages in giving it an aerofoil section? If so what do you recommend?
Thanks Mike

Non whatsoever, Mike,

however you should round the leading edge and sharpen the trailing edge (to about 1-2mm) for a better efficiency.

W.

[JetPlaneFlyer]

I'm with W.. No airfoil as such necassary but rounding the LE and sanding the elevators to give a roughly triangular cross section should reduce drag a little and looks nicer.. Also any wood you can remove from the rear end will save noseweight..

Steve

[Batmanwpg]

I would shape an airfoil. Try a Drela tail section airfoil. They do make a difference even on small planes. I stopped using flat plates years ago and will never go back!

[Rodney]

The time it may be desirable is if you need to add strength. By making the airfoil thicker at the 30 to 40 % chord area, you can install a thicker spar which will greatly increase the strength. Rounding the leading edge is advisable but you will have best results if you leave the trailing edge square. There is a lot of info available on the forums on the best shapes for trailing edges of which rounding is the worst possible choice.

[zetland76]

Thanks All for this information

Cheers
Mike

[JetPlaneFlyer]

Well Mike there you go...

We have a choice of:
1. 'Approximate' an airfoil by rounding the LE and tapering the TE.
2. Painstaikingly sand an accurate Mark Drela airfoil.
3. Leave it square (at the TE at least)

I think what this conflicting advice, given by people who have found there method 'works for them' tells you one thing.... It really does not make a significant difference whichever way you do it.
I go for the 'approximate' airfoil simply because it's quick and easy to do and for a tail that is not required to produce much in the way of lift it performs 99% as good as a 'proper' airfoil. Also the thicker TE and rounder LE of an 'approximate' airfoil is more robust and 'ding' resistant. A PSS model is not in any case designed to be the pinacle of aerodynamic efficiency.

Take your choice, all will fly.

Steve

[Brandano]

it's all a tradeoff. Square TE's will leave you with a greater drag, but it's a small amount, and might well be offset by the advantage of having a more robust elevator. On the other end it will look worse on a scale ship. Same consideration could be made for the choice of a scale section for the whole tail. The rounded LE is pretty much a must, since it will delay the stall of the tail and make it easier to recover from a stall. if you model the tail with the same shape as the full size spitfire lightening the rear of the elevator and adding some balancing weight to the aerodynamic balance horns will reduce the risk of flutter (which would make a sanded TE more advisable) but might make it easier for the elevator as a whole to twist, which again would make the tail MORE prone to flutter... overall, it's mainly a question of what you find to work better. Many planes fly perfectly fine with a flat plate tail, with rounded LE's and TE's sanded to a wedge. Looks decent enough, but won't win any scale contest. And (most important) it's easy to fix when it breaks.

[HX3D014]

And just to state the Obvious (maybe Obvious if you already knew this ;))

Make sure the Airfoil section you Apply (If you go for a Non Symmetrical one) is Up side down. IE Opposite to the main wing, Rounded smooth part on the bottom.

Have a look at the bottom of page six of this PDF file. (This has got to be the most exaggerated example I have ever seen, This is a relatively slow flyer so the plane needs a High lift foil back there <Inverted>) (http://www.ch701.com/stories/May-June%20proof%201%20of%204.pdf)

Bryce.

[JetPlaneFlyer]

Make sure the Airfoil section you Apply (If you go for a Non Symmetrical one) is Up side down. IE Opposite to the main wing, Rounded smooth part on the bottom.


Bryce,

I'd not advise anything but symmetrical in this case. For most 'standard' RC models a symmetrical tail airfoil is best because in normel flight the tail will be close to neutral trim, neither lifting up nor down.

Aircraft fitted with highly cambered airfoils or heavily 'flapped' wings (like the STOL aircraft you linked to) will need tails that can generate strong down force to counter the negative (nose down) pitching moment of the wing and flaps, so they may benifit from an 'upside down' airfoil on the tail.. But for 'normal' planes this would not be a good design.

Some designs that have quite large tail area (free flight duration models being good examples) actually require the tail to provide positive lift and so would benifit from a 'right way up' airfoil... But for a PSS Spitfire symmetrical is the way to go for sure (just like the real Spit).

steve

[Shedofdread]

In an ideal world, yes use an airfoil because there is a theoretical drag saving to be made BUT the drag of a PSS fuz will outway the advantage so, adding my voice to the others, just radius the LE and taper the control surface, finish model and enjoy.

S

[HX3D014]

Bryce,

I'd not advise anything but symmetrical in this case. For most 'standard' RC models a symmetrical tail airfoil is best because in normel flight the tail will be close to neutral trim, neither lifting up nor down.

Aircraft fitted with highly cambered airfoils or heavily 'flapped' wings (like the STOL aircraft you linked to) will need tails that can generate strong down force to counter the negative (nose down) pitching moment of the wing and flaps, so they may benifit from an 'upside down' airfoil on the tail.. But for 'normal' planes this would not be a good design.

Some designs that have quite large tail area (free flight duration models being good examples) actually require the tail to provide positive lift and so would benifit from a 'right way up' airfoil... But for a PSS Spitfire symmetrical is the way to go for sure (just like the real Spit).

steve
Well.

That put my tail between my legs.

I did not Know that at all. I actually thought that all Aircraft would have an inverted one (Those with a nose heavy CG set up). But come to think of it. RC kits are sometimes a different bread and are not shy of power or in desperate need for safety measures such as real world birds.

I would be interested in seeing any real world birds with a right way up Non Symmetrical (Force Tail Up) H-Stab. Of cause there are plenty with Symmetrical foils such as the Jets and Stabilator jobs. And I can possibly see some canard jobs being Symmetrical.

Would also be interested in seeing any RC jobs (Not that I doubt it) with right way up (Force Tail Up) H-Stab.

I can see that this Discussion is going to educate me some more on a new topic I thought I understood ;) That happens to me now and then :D

I only chose the CH701 as it was an Exaggerated one and the most fattest camber I have ever seen :p .

I must Admit I did jump to an assumption that the Spitfire here in this thread was going to have a Nose heavy CG as (I thought) was mentioned by another poster. but I cant seem to find where I jumped to that Assumption.


Bryce.

For most 'standard' RC models a symmetrical tail airfoil is best because in normal flight the tail will be close to neutral trimcan you explain that to me a bit.

[JetPlaneFlyer]

Bryce,
I was not meaning to put you down...

I guess a good example of a full size aircraft with a positivly cambered 'tail' would be the Pou du Ciel (Flying Flea).. Generally the larger the tail in comparison to the wing the more positive will be it's lift vector.. But wing camber and flaps influence things too.

RC models with 'lifting tails' would generally be restricted to vintage types that had there roots in free flight.. Something like this Mowhawk Buzzard: http://plans.rcmodell.hu/mohbuzz.jpg

For most RC models that have 'normal' proportions the tail in level flight neither lifts up nor down significantly. This is usually considered to be the most efficient set up as it minimised drag from the tail. This is why a thin symmetrical airfoil is usually chosen.

The tail does not need to produce negative lift for the plane to be stable and trimmed.. This is because the CG can be further back than the wing aerodynamic centre. As long as the CG is ahead of the plane's overall neutral point then it will be stable . Ignoring the effect of camber (or considering only uncambered airfoils) then any design that has it's CG aft of 25% wing chord will result in a tail that lifts.

Steve

[HX3D014]

Bryce,
I was not meaning to put you down... (Still Alive ;))

I guess a good example of a full size aircraft with a positivly cambered 'tail' would be the Pou du Ciel (Flying Flea).. Generally the larger the tail in comparison to the wing the more positive will be it's lift vector.. But wing camber and flaps influence things too.

RC models with 'lifting tails' would generally be restricted to vintage types that had there roots in free flight.. Something like this Mowhawk Buzzard: http://plans.rcmodell.hu/mohbuzz.jpg

For most RC models that have 'normal' proportions the tail in level flight neither lifts up nor down significantly. This is usually considered to be the most efficient set up as it minimised drag from the tail. This is why a thin symmetrical airfoil is usually chosen.

The tail does not need to produce negative lift for the plane to be stable and trimmed.. This is because the CG can be further back than the wing aerodynamic centre. As long as the CG is ahead of the plane's overall neutral point then it will be stable . Ignoring the effect of camber (or considering only uncambered airfoils) then any design that has it's CG aft of 25% wing chord will result in a tail that lifts.

Steve
That MohBuzz has a High wing and a prop in the normal Location. I would imagine the Thrust Drag Couple would be Drag at the top and thrust at the bottom creating a Nose up with thrust. thus the extra need for a better Force up at the tail.

Can you find one where the Thrust line is above the Total Drag reaction point. IE Reverse to the above example.

Spoke to three pilots today. One Guy seemed right on the money. he echoed what you were saying, He flies A340's for Cathay. He said the lift of the tail is a force up but it is unstable. we were talking about incidence and Symmetrical foils and where the Tail lives, In the induced flow region. so a 0 incidence would still have a Negative AoA on most aircraft. but he said they trim the whole H-Stab in flight until it is just producing a slight lift. which is a reduction of drag all around.

He Said he flew C130's and that due to the high thrust line over the Lower positioned Drag reaction line the H-Stab on that bird is an Upside down Non Symmetrical foil. the subject was a bit of a refresher for him, so we did not go to much further in the discussion :)

Bryce.

PS Fear not. I do not feel put down. but rather I feel Enlightened again (This time Truly eh ;))

[JetPlaneFlyer]

Bryce,
Most of the RC models i know with lifting tails are developments of old time free flight models so they tend to be high wing.. I dont think thrust line position has much to do with the selectiopn of the tail airfoil though. It's mainly about tail size and therefore CG location.
Here's one where the thrust line is close to the vertical centre of drag.. http://plans.rcmodell.hu/wedgy.jpg

Going to the extreme with tail size you end up with tandem wings and then canards.. Obviouly in these cases the 'tail' (i.e. rear wing) lifts and will benifit from a positivly cambered airfoil.

Steve

[Brandano]

The tail doesn't need to have negative lift, it only needs to have less lift than the wing. And when recovering from a stall it needs to have positive lift!

[HX3D014]

Bryce,
Most of the RC models i know with lifting tails are developments of old time free flight models so they tend to be high wing.. I dont think thrust line position has much to do with the selectiopn of the tail airfoil though. It's mainly about tail size and therefore CG location.
Here's one where the thrust line is close to the vertical centre of drag.. http://plans.rcmodell.hu/wedgy.jpg

Going to the extreme with tail size you end up with tandem wings and then canards.. Obviouly in these cases the 'tail' (i.e. rear wing) lifts and will benifit from a positivly cambered airfoil.

Steveturn That Tail up side down and see how it flys.

I can not see it flying well (not without Positive elevator being applied.

with all the engine, fuel, tank and that huge lower front fuselage section adding to a nose heavy and lower drag line to trust line component, I can not see how or what stops the tail from lifting up. Of cause there is only one thing left. and that is the induced drag hitting the H-Stab giving the H-Stab a negative AoA. in which case. turning the tail up side down may well result in an ability to reduce area of the H-Stab.

That the way I see it. can you help me to see some thing else there ?

No Wait. I see it. the main wing is the main drag component. its is considerably higher (ok not quite considerably, But I am Considering it ;) )than the thrust line so a lifting H-Stab would be good in this case.

Going to revise Lift Weight Thrust Drag Couples tomorrow. see if there is some thing else I might be missing.

Bryce.

[JetPlaneFlyer]

Bryce,
The thing that 'pushes the tail down' and hence requires the tail to generate positive lift is the centre of gravity. On models that have a large tail the CG will always be behind the Aerodynamic Centre of the main wing, causing, if not for the tail, the nose to pitch up.
The CG does not have to be ahead of the wings aerodynamic centre for the plane to be stable.

Steve

[Flight Engineer]

That MohBuzz has a High wing and a prop in the normal Location. I would imagine the Thrust Drag Couple would be Drag at the top and thrust at the bottom creating a Nose up with thrust. thus the extra need for a better Force up at the tail.

Can you find one where the Thrust line is above the Total Drag reaction point. IE Reverse to the above example.

Spoke to three pilots today. One Guy seemed right on the money. he echoed what you were saying, He flies A340's for Cathay. He said the lift of the tail is a force up but it is unstable. we were talking about incidence and Symmetrical foils and where the Tail lives, In the induced flow region. so a 0 incidence would still have a Negative AoA on most aircraft. but he said they trim the whole H-Stab in flight until it is just producing a slight lift. which is a reduction of drag all around.

He Said he flew C130's and that due to the high thrust line over the Lower positioned Drag reaction line the H-Stab on that bird is an Upside down Non Symmetrical foil. the subject was a bit of a refresher for him, so we did not go to much further in the discussion :)

Bryce.

PS Fear not. I do not feel put down. but rather I feel Enlightened again (This time Truly eh ;))


According to Airbus the cg is always ahead of the CP and the tail always produces down force of some magnitude.

This was on my Flight Ops Eng course at Airbus. I heard the same from Embraer on their course. I haven't done the Boeing course yet.

AB make a publication called Getting to Grips with Aircraft Weight and Balance. You can find the quote on page 76.

[HX3D014]

According to Airbus the cg is always ahead of the CP and the tail always produces down force of some magnitude.

This was on my Flight Ops Eng course at Airbus. I heard the same from Embraer on their course. I haven't done the Boeing course yet.

AB make a publication called Getting to Grips with Aircraft Weight and Balance. You can find the quote on page 76.
I have heard since that post that Boeing do it by moving fuel to the H-Stab even. this is to trim it up .

I "NOW" wonder if Cathay Pacific fly AB or Boeing to Sydney (From Hongkong) (Airline not Cargo. the cargo flight is a Boeing 747 variant).

what you suggest is what I originally thought. and it is a slow topic with most pilots I speak to.

I will ask a few Pilots today about the Embraer (Virgin Blue Fly those here both the small one and the Smaller one, they also fly 737s)

Will ask any of the pilots flying AB if they have heard of that book and if they have it at hand or what?

You are ground crew? what do you do ? loading management ? I am clueless here :)

Bryce.
maybe post a question at the prune ? see what feedback you get there ? PM me link if you do.

[vintage1]

According to Airbus the cg is always ahead of the CP and the tail always produces down force of some magnitude.



Big wing. small tail.
(relatively)

[HX3D014]

Big wing. small tail.
(relatively)
Rather short post from you ;)

it is confusing me :confused:
relative to what. other planes or the big wing.

I am also confused as to why the relatively small tail would necessitate or eliminate the up or down force at the rear. Am I missing the message ?

Bryce.

[JetPlaneFlyer]

I am also confused as to why the relatively small tail would necessitate or eliminate the up or down force at the rear. Am I missing the message ?

Bryce.

Bryce,
It's because a large tail causes the aircraft's neutral point to move back. For instance if there is no tail then the neutral point of the aircraft will be at the Aerodynamic Centre of the wing (25% chord).. If you increase the 'tail' to be the same size as the wing then the Neutral point will be exactly half way between the aerodynamic centres of wing and tail... For a normal tail that is between these two extremes then the neutral pont will also be somewhere between the two extreme locations.

An aircraft is stable if the CG is ahead of the Neutral Point. Generally the CG will be 5-15% ahead of the neutral point. If the tail is quite large (and hence the neutral point is well aft) then the CG will end up behind the wings centre of pressure which results in the tail carrying some of the aircraft's weight.

Steve

[HX3D014]

So the Spitfire would need a CG forward of the CP?

[vintage1]

More or less, yes.

Spitties balance out quite far forwards.

As do most warbirds.
I suspect it makes them more manoeuvrable..less moment of inertia.

Its only a few types of plane that have net positive lift on the tail in cruise.

[kcaldwel]

Bryce,
It's because a large tail causes the aircraft's neutral point to move back. For instance if there is no tail then the neutral point of the aircraft will be at the Aerodynamic Centre of the wing (25% chord).. If you increase the 'tail' to be the same size as the wing then the Neutral point will be exactly half way between the aerodynamic centres of wing and tail... For a normal tail that is between these two extremes then the neutral pont will also be somewhere between the two extreme locations.

An aircraft is stable if the CG is ahead of the Neutral Point. Generally the CG will be 5-15% ahead of the neutral point. If the tail is quite large (and hence the neutral point is well aft) then the CG will end up behind the wings centre of pressure which results in the tail carrying some of the aircraft's weight.

Steve

JPF,

These statements are a bit general, and not necessarily true. The height of the CG, wing planform, height of the fuselage drag centre, height of the tailplane/rear wing, Cm of the airfoils, flap deflections, and even the dihedral raising the MAC of the wing, will all effect the neutral point location.

And of course the thrust line and drag centre of the prop, will mean there is likely a power-on and power off neutral point location, and even make the location dependent on the angle of attack.

An airfoiled stabilizer can eliminate elevator hysteresis effects that flat plate stabilizers usually have, especially at low Re.

Kevin

[JetPlaneFlyer]

JPF,

These statements are a bit general, and not necessarily true.

Yes my statement was rather an over simplification but to bring in all the complexities, some of which you hinted at, would tend to make grasping the concept i was trying to convey rather difficult.

Steve

[Flight Engineer]

I have heard since that post that Boeing do it by moving fuel to the H-Stab even. this is to trim it up .

I "NOW" wonder if Cathay Pacific fly AB or Boeing to Sydney (From Hongkong) (Airline not Cargo. the cargo flight is a Boeing 747 variant).

what you suggest is what I originally thought. and it is a slow topic with most pilots I speak to.

I will ask a few Pilots today about the Embraer (Virgin Blue Fly those here both the small one and the Smaller one, they also fly 737s)

Will ask any of the pilots flying AB if they have heard of that book and if they have it at hand or what?

You are ground crew? what do you do ? loading management ? I am clueless here :)

Bryce.
maybe post a question at the prune ? see what feedback you get there ? PM me link if you do.

I trained the first few courses of Embraer pilots at VOZ and Air North and the now defunct Sky Air world. Along with a large percentage of all the worlds Embraer 170/190 pilots. To be honest, the level we go into this and the info supplied by the manufacturers is pretty low. There is a much higher theoretical knowledge of PoF on these boards than the average airline crew room. But there aren't so many people capable of flying a single engine circling approach on here.

[ciurpita]

i thought you have to consider at least three elements contributing to the total moment of the aircraft: the lift produced by the wing around the CG, the lift produced by the tail around the CG, and the moment (Cm) of the wing itself. the total has to equal zero. the lift is a constant. that leaves the tail to compensate for the change in the moment of the wing which varies with airspeed. the moment of the wing will typically increase (nose downward) with speed which means the tail needs to produce more downward force to compensate. at lower speeds, the tail may produce lift because the wing moment is so low.

center of pressure is the combination of the wing's lift and moment (Cm). it represents the position a force would be needed to balance the wing's lift as well as the moment, meaning to prevent the wing from rotating. (and since the lift can be zero but the moment is not zero, the Center of pressure could be infinitely behind the wing).

keeping the CG ahead of the neutral point (not CP?) helps the plane recover from a stall, when it's not producing lift.

consider a flying wing: the lift around the CG must balance the moment (Cm) of the wing which can be changed using trailing edge changes. (see http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm).

[HX3D014]

i thought you have to consider at least three elements contributing to the total moment of the aircraft: the lift produced by the wing around the CG, the lift produced by the tail around the CG, and the moment (Cm) of the wing itself. the total has to equal zero. the lift is a constant. that leaves the tail to compensate for the change in the moment of the wing which varies with airspeed. the moment of the wing will typically increase (nose downward) with speed which means the tail needs to produce more downward force to compensate. at lower speeds, the tail may produce lift because the wing moment is so low. (if the moment becomes 0 what is the force required from the tail ? can the moment become -1?) (if the tail is inverted then the force will be greater due to its higher AoA <Given it was an inverted Non Symetrical foli>)
center of pressure is the combination of the wing's lift and moment (Cm). it represents the position a force would be needed to balance the wing's lift as well as the moment, meaning to prevent the wing from rotating. (and since the lift can be zero but the moment is not zero, the Center of pressure could be infinitely behind the wing). (Is that Wings Lift location and the CG location distance ? explain more for me please :))

keeping the CG ahead of the neutral point (not CP?) helps the plane recover from a stall, when it's not producing lift. (is the Neutral Point just a point where <a mass, The CG, A force> can be placed/"applied normal to the line of flight" without causing a moment ?)

consider a flying wing: the lift around the CG must balance the moment (Cm) of the wing which can be changed using trailing edge changes. (Don’t trailing edge changes just change the CP? <Of cause also change the Cl and Cd values, but focusing in CP and its relation to Cm>) (see http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm).
can you help me understand the pictures I have attached. they are from RealFligh G3.5. (Looking at the Moment line in particular)

Bryce.

maybe I need a clearer description of what is
CP
neutral point
Cm

[kcaldwel]

i thought you have to consider at least three elements contributing to the total moment of the aircraft: the lift produced by the wing around the CG, the lift produced by the tail around the CG, and the moment (Cm) of the wing itself. the total has to equal zero. the lift is a constant. that leaves the tail to compensate for the change in the moment of the wing which varies with airspeed. the moment of the wing will typically increase (nose downward) with speed which means the tail needs to produce more downward force to compensate. at lower speeds, the tail may produce lift because the wing moment is so low.

center of pressure is the combination of the wing's lift and moment (Cm). it represents the position a force would be needed to balance the wing's lift as well as the moment, meaning to prevent the wing from rotating. (and since the lift can be zero but the moment is not zero, the Center of pressure could be infinitely behind the wing).

keeping the CG ahead of the neutral point (not CP?) helps the plane recover from a stall, when it's not producing lift.

consider a flying wing: the lift around the CG must balance the moment (Cm) of the wing which can be changed using trailing edge changes. (see http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm).

You have to consider all moments about the CG. Fuselage drag, height of MAC of the wing above the CG, thrust, etc. In order for the aircraft to pitch stable (the plane will try to return to trim speed if disturbed), the CG must be ahead of the aircraft neutral point. Without electronic stability augmentation, the aircraft must be pitch stable to be flown, or at worse, neutrally stable (CG at or close to the neutral point).

The Wright Flyer was pitch unstable, but due to its slow rate of divergence, it was sort of flyable.

An aircraft with a high wing MAC (high wing and or lots of dihedral like a DLG) will have the MAC shift relative to the CG with changes in AoA. This means the stability margin will change with AoA. Th MAC will move aft with respect to the CG at high angles of Attack, so the aircraft gets more pitch stable. At low AoA, it will get less pitch stable, and may even become pitch unstable if the static margin was low to start with.

And then there is dynamic pitch stability...

Kevin

[ciurpita]

HX3D014

moment is a rotational force like that produced by a screwdriver. it doesn't cause the object to change location, translate, just to twist. a wing generates 3 forces: lift (Cl) perpendicular to flow, drag (Cd) along the flow, and moment (Cm). (see your diagrams). the aerodynamic center or neutral point of the wing is where a single point of lift and drag produce only a translational and no rotational force.

center of pressure (Cp) comes from sailing. it characterizes a wing using lift and position where there is no rotational force, instead of using Cl and Cm. (see simons). you calculate Cp by dividing the moment force by the lift to determine the moment arm. if the lift is zero, the moment arm becomes infinite. (this has nothing to do with CG). http://www.rcsoaringdigest.com/pdfs/RCSD-2002/RCSD-2002-11.pdf

but the wing is only part of the aircraft. the lift and tail forces produce moments around the CG (as well as drag and thrust). when you combine all the moments together, they must equal zero if you want the plane to maintain its' pitch. as your diagrams show, the lift curve goes through zero, and can produce either positive or negative lift (even for a cambered inverted airfoil). the tail is used to balance the moment under various conditions in order to maintain or change pitch. http://ciurpita.tripod.com/rc/rcsd/lowSpeedCm.pdf

[Texas Buzzard]

How many full-scale Light Planes and Ultralights, RC Trainers and all Depron built electrics have an airfoiled hor. stab? The answer is well over 90%.

Why? It's because it really doesn't make enough difference to measure in a small model RC. A flat-plate hor. stab will be just fine and easier to fabricate.

The difference in drag between an airfoiled stab and a flat-plate stab is so close to being the same you won't notice any difference unless you go to a sensitive wind tunnel........... fly it with the flat-plate stab. :)