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Goblin/plank stability

Hai-Lee

Old and Bold RC PILOT
#3
Pitch stability on a plank, (flying wing), is maintained by creating a second, (negative), centre of lift on the wing using the upturning of the wing trailing edge, (reflex).

The Balance point of the wing is shifted forward and the reflex supplies a counterbalancing force against the wings tendency to want to pitch downwards with the CG forward of the point of maximum lift.

The amount of reflex required is dependant upon the wing loading and the balance point/CG point selection.

Whilst stability can easily be obtained, the short coupled nature of the plank means that a degree of precision is required in setup and in addition the wing will be very sensitive in pitch to control inputs. Do not forget the Expo! One other thing to remember is that planks, unlike other flying wing types, really benefit from adjusting the control mix ratios in the elevon mixing.

The plank will fly with a large range of setups though there is an optimum setting which can be determined during the initial test flights.

I hope that helps

Have fun!
 
#5
Thanks. Some followup questions!

Pitch stability on a plank, (flying wing), is maintained by creating a second, (negative), centre of lift on the wing using the upturning of the wing trailing edge, (reflex).
Have fun!
This is the airfoil iforce2d uses in his plank build. What is it about its polars that make it good for a wing? MH64 vs Clark Y.

On a swept flying wing you'd want the reflex primarily on the tips, right? What about a straight plank?

How far apart are the tail fins supposed to be?
 

Piotrsko

Well-known member
#6
Thanks. Some followup questions!
this is the airfoil iforce2d uses in his plank build. What is it about its polars that make it good for a wing? MH64 vs Clark Y.
On a swept flying wing you'd want the reflex primarily on the tips, right? What about a straight plank?
How far apart are the tail fins supposed to be?
I believe the COP doesnt wander around much as a function of AOA. STALL is interesting. Does do limited inverted, thinner than a typical "Y". Hard to say since I don't believe we approach reynolds of 10,000.
Typically on a plank the entire elevon is reflexed as a matter of easy build. No other reason not to.
On a swept wing, the advantage is more stability in pitch, and faster roll response. Elevon placement generally just for looks. They can also be full span.

Tail fins AKA rudders: 1 in the center, more generally equally spaced symmetrical. No hard reason for symmetrical other than looks. With a crow system or split surfaces, rudders are unnecessary.
 

BATTLEAXE

Well-known member
#7
I believe the COP doesnt wander around much as a function of AOA.
I disagree, COP does move around as a result of AOA, and not only move but also varies in intensity as it moves further away from COG. The COG is the more consistent of the two, unless you are moving the battery around mid flight. The higher the AOA the more forward the COP moves to the LE and decreases in strength, more dependent on Newtonian Principle for lift, yet still affected by airspeed. This unbalance between COP and COG and lack of airspeed all work together to cause a stall. The Hstab on the tail is meant to maintain this balance through attempting to equalize the pressure in airflow by way of leverage in the tail moment. This is the same reason why flying wings need reflex and the most effective areas of the elevons tend to be on the outer surfaces being furthest back from the COG/COP balance point. As I understand it
 

Hai-Lee

Old and Bold RC PILOT
#8
Thanks. Some followup questions!



This is the airfoil iforce2d uses in his plank build. What is it about its polars that make it good for a wing? MH64 vs Clark Y.

On a swept flying wing you'd want the reflex primarily on the tips, right? What about a straight plank?

How far apart are the tail fins supposed to be?
You can actually use almost any wing profile that generates lift on a plank or flying wing. The use of the wing TE or elevons to provide the reflex is almost as good as a purpose designed airfoil.

I build and fly wings with full span reflex and they fly very well. With full span reflex the actual amount of reflex, (upwards deflection of the wing TE), can be extremely small on a very light build.

Almost any wing planform can be used and flown as a flying wing. The real issue is the distance between the centres of pressure or lift. The closer they are together the more sensitive the wing will be in pitch.

As for the vertical fins they can be almost anywhere as long as they are behind the centre of mass and provide somewhat equal drag on each side of the wing. a single centreline fin and wing tip fins are the easiest to fit.

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
#9
I disagree, COP does move around as a result of AOA, and not only move but also varies in intensity as it moves further away from COG. The COG is the more consistent of the two, unless you are moving the battery around mid flight. The higher the AOA the more forward the COP moves to the LE and decreases in strength, more dependent on Newtonian Principle for lift, yet still affected by airspeed. This unbalance between COP and COG and lack of airspeed all work together to cause a stall. The Hstab on the tail is meant to maintain this balance through attempting to equalize the pressure in airflow by way of leverage in the tail moment. This is the same reason why flying wings need reflex and the most effective areas of the elevons tend to be on the outer surfaces being furthest back from the COG/COP balance point. As I understand it
A properly setup wing, (including plank types), do not actually get to stall. As the speed decreases the lift also decreases and the wing tends to descend. If the elevons are used to maintain level flight as the speed decreases the wing will slow until it reaches the point where maximum elevon input does not provide sufficient balance force to maintain level flight and the nose starts to lower.

As the nose lowers the forward airspeed and the generated lift both increase and balance is resumed.

The only wings that stall are those whose setup or balance is poor, (normally too far rearwards in their balance point). With such a poor balance the balance pressure is too far behind the required balance point and as the angle of incidence is increased the angle will rise far higher than on a balanced wing and the wing will actually stall, (often with a lot of directional instability prior to the stall being reached).

At stall the wing will behave as any stalled wing and the bird will drop a wing rapidly and will tend to fall to one side and dive.

A wing will fly with a large variety of setup balance points but for each wing there is only one proper setup or balance point where stalls cannot occur and the lift centres are in balance across the entire range of flying speed.

Have fun!
 

Merv

Well-known member
#10
I agree that CG and throws can be set to make a wing stall difficult if not impossible. And if you want to avoid those conditions, great. I prefer a slightly aft CG and more aggressive throws. To the point that I can induce a wing stall at will. In my opinion, a plane set this way, is just more fun. Having fun is the goal, isn't it?
 
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Hai-Lee

Old and Bold RC PILOT
#11
I agree that CG and throws can be set to make a wing stall difficult if not impossible. And if you want to avoid those conditions, great. I prefer a slightly aft CG and more aggressive throws. To the point that I can induce a wing stall at will. In my opinion, a plane set this way, is just more fun. Having fun is the goal, isn't it?
Effectively a balance point further aft than optimal enhances its agility but at the cost of slow speed stability Too far forward and it also loses low speed abilities but it is far more docile in handling.

If you like enhanced manoeuvrability try combining aft balance with thrust vectoring,:eek:.

Have fun!