DLG tail configuration question

[Marc RC pilot]

Would someone be kind enough to explain why F3K gliders tend to  have part of there vertical stabiliser/rudder set below the fuselage and also why some have there elevator set underneath the fuselage in pic below:

As opposed to the more classic set up:

 

Perhaps more efficiency in the  lateral stability with rudder set this way due to the turbulant air  from wings as it passes over? Slower flying speeds? 

Whilst I appreciate that these gliders can have incredibly slow landing speeds (light weight/size/flaps/modern Rc mixing and ability to catch etc), it just seems odd to have the rudder as a point of contact on ground landings in general. 

Many thanks

 

 

 

 

[mikef]

F3K models experience a very high yaw during the launch at the moment of release on every flight.  The aircraft tends to go on rotating as it is released, tail out of the turn, and the oversized fin is there to damp this out quickly.  Most have a cambered fin section to help with this.  The fin is mounted with a large section below the fuselage to reduce the torque that an all-on-top layout would produce.

Theories may be advanced to explain the, tailplane above or tailplane below, fashions but I think the practical, "when the pull string breaks', situation favours a top mounting. 

[Marc RC pilot]

Thanks for the explanation Mike, that makes absolute sense about the rotation loads/torque on launch...they must be horrendous and all-on-top layout/boom wouldn't stand a chance.

Can you please explain the fin camber?  What type of camber is it (camber profile that increase lift on right side of fin for a right handed launch etc)

 

How about the elevator? 

 

 

 

[mikef]

Here are views looking down on a right-handed thrower's asymmetric fin.   The fin is mounted with a little offset, leading edge to the right, with the aim off producing zero side force in straight flight.  The camber is there to make the aerofoil work best in the large nose-left yaw just after release.   The camber makes the fin more efficient in 'lifting' to the left side.  A left-handed version would be the mirror image laterally.

I photographed the fin between two straight bits of wood.  The second picture is the slot in the back of the boom.  The flat sided back half of the fin is lined up with the  boom - this gives a good result in terms of a straight glide with no rudder offset.

 

Tailplanes: -  Just checked Snipe, Vibe, Twister and Bonus.  All appear to have zero camber.

[Mark Evans]

Hi Mike, this is frying my brain now thinking it over.

I once bought fin for my Snipe from a guy in the USA, it was cambered so had a curved and flat side. The curved side had the hinge gap. 

which way around would this had to have been fitted to ‘lift’ the nose to the left on launch to have suited me?

The more I think about it the more I keep thinking that the horn and hinge was on the correct side but Cambered foil was the wrong way around? 
 

im left handed.

[Marc RC pilot]

Good morning gents.

Mike, thanks again for taking the time to explain. Your pics make it very clear/ answered all my questions (and then some ). The fin/torque offset conundrum was some thing that was puzzling me for some time. 

 Mark you are a lefty right?  Correct me if I am wrong,  but when you launch your glider it will want to yaw  to the left so you will need to introduce some lift on the left side of fin, flatter side of fin profile produces more lift/pressure and in your case it will yaw it back to the right.   I presume also that the control horn of the fin should be on the left side of fin so that the torque is taken up by the servo cord etc rather than relying on the rudder spring system (sry forgot name).

 

[mikef]

1 hour ago, Mark Evans said:

.....it was cambered so had a curved and flat side. The curved side had the hinge gap. 

which way around would this had to have been fitted to ‘lift’ the nose to the left on launch to have suited me?  ...  I'm left handed.

Hand sketch of correct layout for quick reply.  I don't think it matters much which side the hinge gap is but you could get the same rudder travel with a shorter (stiffer and lighter) horn if the gap was the same side as the horn.  The horn always goes on the side opposite the tip peg so that the pull string or pushrod is in tension for that first yaw after release.

[mikef]

29 minutes ago, Marc RC pilot said:

...   Mark you are a lefty right?  Correct me if I am wrong,  but when you launch your glider it will want to yaw  to the left so you will need to introduce some lift on the left side of fin, flatter side of fin profile produces more lift/pressure and in your case it will yaw it back to the right. ...

Marc, no, wrong way round.  Mark (LEFT HANDER) holds the right tip or his glider to launch and rotates to the right.  (Clockwise in the birds-eye view.). On release the model wants to go on yawing nose right.  On that first yaw, the fin needs to provide a tail-to-the-right correcting force.

[Marc RC pilot]

Thanks Mike, sorry Mark. When I said yaw to the left I was referring to the tail not the plane as a whole... My bad  

A pic really does paint a thousand words,  thx.

 

 

 

 

[Mark Evans]

Thanks Mike, that confirms what I always thought, with the hinge gap and horn on the curved side, and that installed on the model opposite my peg, that my Stab on my snipe was always the wrong way around (correct side for horn, incorrect for the camber) This wasn't the supplied Vlad fin, it was a generic one I ordered after I damaged my stock one.

With my current plane and supplied fin, I notice the hinge material is on on the curved side so the gap is cut on the flat. This makes it possible to flip over for either handed. With that old fin I had on the Snipe, the hinge material was on the flat, so although I could have made it work like your pic above, without thinking I put the horn on the gap side also, which then meant I installed the fin the incorrect way around. For years after I always thought I had done it wrong haha. 

[David North]

On 24/01/2020 at 08:34, Marc RC pilot said:

Thanks for the explanation Mike, that makes absolute sense about the rotation loads/torque on launch...they must be horrendous and all-on-top layout/boom wouldn't stand a chance.

Can you please explain the fin camber?  What type of camber is it (camber profile that increase lift on right side of fin for a right handed launch etc)

 

How about the elevator? 

 

 

 

Regarding the question of why the elevator is usually placed underneath the boom, I think it could be because the low-pressure side of an aerofoil is much more sensitive to disturbance than the high-pressure side. So a mounting pylon will cause less flow separation (and hence drag) if it’s on the upper (high pressure) side of the elevator.

[Neil Harrison]

Simple way to remember is that the rudder horn is on the opposite side to the launch peg.

With bottom mounted tail the servo pulls against the spring in the elevator, some say this is better for launching as there is less likely a chance of ‘blow back’ on the elevator.

Blow back is the force applied to the elevator or control surface as it moves through the air, the faster it is flying, the more the force, therefore more power required to operate the control surface.

This is another reason why when you install the servo try to make the servo use its full range of travel to operate the control surfaces.  This gives better servo resolution and applies the correct torque as specified on your servo specs.

And so on, and so on lol

[Dave H]

And just for the full answer, whether it really is another reason would take an aerodynamicist . 
 

theory states the at high aoa the wing is kicking up a lot of turbulence which can affect elevator performance. Mounting under the boom puts it out of the wake of the wing.

at least that’s  my understanding of it.

 

dave

[Martin O]

Putting all theories aside, I have tried both bottom and top mounted tail on the same model type, and honestly could not detect any performance change in launch or flight.

I now stick with tail on the top just because I think it looks better and is possibly a little less susceptible to damage on a missed catch.

[mikef]

On 02/06/2020 at 08:30, David North said:

Regarding the question of why the elevator is usually placed underneath the boom, I think it could be because the low-pressure side of an aerofoil is much more sensitive to disturbance than the high-pressure side. So a mounting pylon will cause less flow separation (and hence drag) if it’s on the upper (high pressure) side of the elevator.

Tail force up or down?  You would need to know if the tailplane had an upward or a downward force at a particular stage in flight to make this a reason for a top vs. bottom choice.    Are you assuming that the tailplane force is always downwards?

Other possible Tail Position choice deciders - some mentioned above...

Wing wake.   I have never seen a flow simulation that would tell us if the wing wake goes over, under or at the tailplane at any particular stage of flight.  If the wing is producing lift, there will be downwash behind it.  The more the lift, the more the downwash.

Blow back.   Given the strength of the springs people tend to use, I think this unlikely but it would help to choose between top and bottom mounting.  I have seen blow back on rudder with wrong sided horn and flexible push rod.  (Can't remember the name of the popular foam DLG that demonstrates this well).

Control disconnect.  Another top vs. bottom decider is, “what happens if the string breaks or comes off the tail horn?”.   I had a crash on launch once - the model went in hard a few metres in front of me.  It had an underslung tail and a broken pull string was a candidate early in the investigation.  To test this theory, I tried a launch rotation with the string unhooked (without release!) and it was obvious from the feel that you wouldn't miss it that had happened.  I guess that would apply to full up as well.

Aesthetics.  Your choice.  I think underslung looks ‘meaner’.

Vulnerability.   Top is best?

Control string fouling.   Not mentioned yet but we have seen a case of ‘kebab ballast’ being delected sideways on launch and putting tension into a pull string.  I think I'd rather have up elevator or right rudder (right handed launch) in that case.  I always put ballast in a tube to guard against this. 

 

[David North]

6 hours ago, mikef said:

Tail force up or down?  You would need to know if the tailplane had an upward or a downward force at a particular stage in flight to make this a reason for a top vs. bottom choice.    Are you assuming that the tailplane force is always downwards?

Other possible Tail Position choice deciders - some mentioned above...

Wing wake.   I have never seen a flow simulation that would tell us if the wing wake goes over, under or at the tailplane at any particular stage of flight.  If the wing is producing lift, there will be downwash behind it.  The more the lift, the more the downwash.

Blow back.   Given the strength of the springs people tend to use, I think this unlikely but it would help to choose between top and bottom mounting.  I have seen blow back on rudder with wrong sided horn and flexible push rod.  (Can't remember the name of the popular foam DLG that demonstrates this well).

Control disconnect.  Another top vs. bottom decider is, “what happens if the string breaks or comes off the tail horn?”.   I had a crash on launch once - the model went in hard a few metres in front of me.  It had an underslung tail and a broken pull string was a candidate early in the investigation.  To test this theory, I tried a launch rotation with the string unhooked (without release!) and it was obvious from the feel that you wouldn't miss it that had happened.  I guess that would apply to full up as well.

Aesthetics.  Your choice.  I think underslung looks ‘meaner’.

Vulnerability.   Top is best?

Control string fouling.   Not mentioned yet but we have seen a case of ‘kebab ballast’ being delected sideways on launch and putting tension into a pull string.  I think I'd rather have up elevator or right rudder (right handed launch) in that case.  I always put ballast in a tube to guard against this. 

 

In steady flight I think the tail always has to be pushing down, in order to give some decalage and hence stability in pitch. And the cg has to be ahead of the wing cp so that the forces balance out. But I don’t know what happens in a near-vertical climb.