A question for all Flying Flea aficionados

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rtfm

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Hi,
My question relates to where along the chord of the front wing to attach the pivot.

Some background:
In the plans, the front spar is used to anchor the wing pivots (this is eminently sensible since this is the only solid place in the wing). From the plans, the front spar is situated 260mm from the LE (HM293) and 275mm from the LE (HM360). However, in both cases, the point through which the lift acts is at the 25% chord position, which is (for the 1.2m chord wing used on both the HM360 and the HM293) 300mm from the LE.

It's unclear from the literature or the plans of the HM360 and HM293 if the pivot point *needs* to be ahead of the 25% chord point, or if this is simply a function of the fact that this is the "usual" position of the front spar.

So, to finally get to the question:
I'd be interested to hear from experienced builders/flyers of the Flying Flea (or from any knowledgeable designers) if placing the pivot point of the front wing at 25% chord would be (1) Good (2) Bad (3) Make little difference

Regards,
Duncan
 

Tiger Tim

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I expect it’s a stability of the wing thing and relates to the centre of pressure. Put the pivot too far forward and you’ll need an arm like Popeye to hold the wing at the proper incidence but too far back and you risk the wing showing divergent behaviour which would be at best scary. At worst, very fatal. Shooting from the hip here but I would think that the pivot must always be ahead of the centre of pressure for all achievable angles of attack for the airfoil you choose. Well, unless you had incidence controlled by a jack screw or something.
 

rtfm

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Hi,
If the wing pivoted around the CoP, I'm not sure control would be too sensitive or divergent, since the pitching moment will always be nose-down. Just trying to get my head around this. I might have to build a RC model and test this...
 
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TFF

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I think it needs to be close to the center of pressure to have the leverage to move it. You are moving a whole wing, not just a control surface. It’s also why you can not allow it to go negative incidence. If you hinges it forward my mind says it would pin against the negative incidence stop and not move. If it was behind, you would not be strong enough to stop the loop.
 

rtfm

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Hi TFF,
This is my thinking exactly. Without having delved too deeply into this, it would seem sensible to pivot around the CoP in order to minimise the amount of pressure required to deflect the wing. Placing the pivot point ahead of the CoP simply makes the wing "heavier" with no immediately obvious benefit. I strongly suspect that the pivot point on Fleas is simply because that's where the forward spar generally is situated.
 

TFF

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For most normal designed wings. The center of pressure is going to coincide with the spar. It’s almost always the tallest point of the airfoil for strength. You have to have a real reason to be different as you are giving up free strength if not.
 

Victor Bravo

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But does the CP move during flight, during changes in AoA, during changes in CG, etc??? If it moves even a little, you'd need it to have a margin, for the pivot to be slightly forward of Cp in any condition.
 

rtfm

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That's an interesting observation. I'll check the specs for the airfoil (NACA 23112) and see how far the CoP moves with changing AoA. So then a second question is: what effect (if any) will moving the pivot point *behind* the CoP have?

Thanks,
Duncan
 

Malcolm C

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Barnaby Wainfan wrote several excellent articles about the Flea and it's various problems and solutions in Kitplanes a few years ago. I am building a Flea and am following his advice to the letter. There are lots of armchair experts on Fleas but he is a trustworthy source.
Good luck with your Flea, best, Malcolm.
 

rtfm

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Hi Malcolm,
I'm familiar with Barnaby's articles. But he doesn't address this articular issue. So may I ask - nwhere along the chord have you placed your spar?

Regards,
Duncan
 

tdfsks

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Hi Duncan,

I know a fair bit about Flea's but I don't know the answer to this question off hand. However, I can offer some thoughts from an aero-engineering perspective based on my knowledge of stability, control and aerodynamics.

Consider the following ... and note that the aerodynamics of the Flea are complicated and these are just simplistic explanations.

If the center of gravity is in the right place, a Flea will be statically stable, stick fixed. By statically stable I mean that it will return to its trim speed when disturbed (or in engineering terms Cmo > 0 and dCm/da < 0 for those who understand this stuff). This is just a function of the wing layout, the way in which the lift on the two wings changes as angle of attack changes and the position of the CG relative to those wings. When the angle of attack changes, the change in lift is not in the same on both wings (as a proportion of the initial lift) due to the effect of downwash from the forward wing on the rear wing which changes with angle of attack. So in a statically stable Flea, the change in the lift distribution between the two wings, generates moments about the center of gravity that restore it to the original flight condition.

However flying qualities and stability are also important when the stick is free. For stick free static stability, you need an aircraft to return to the trimmed airspeed when disturbed with the stick free to float. This helps reduce pilot workload. There are really two cases to consider.

The first is when there is a sudden change in angle of attack such as when the flea flies through a gust. In this case the lift increases on the wing and you want that increase in lift to drive the wing to a lower angle of attack to alleviate the effect of the gust and to help maintain the trim speed. If the wing pivot is in front of the quarter chord point, increasing lift on the wing will cause it to rotate to a lower angle of attack (remember it is floating free), reducing lift on the main wing and having the desired effect.

The second is when the speed increases or decreases - speed stability. For stability you want the aircraft to return to the trim speed when the speed suddenly increases or decreases. In the case of the Flea, if speed suddenly increases the lift on the wing increases and, since the pivot is in front of the quarter chord point of the wing, the extra lift on the wing pitches the wing nose down, reducing lift and increasing speed which is destabilizing.

However .... The airfoil on the Flea is nominally a NACA 23112. This airfoil has a slightly positive pitching moment but the moment is quite small (approx +0.006). I suspect that this airfoil has been chosen to ensure that the wing does not generate any pitching moment as speed changes which would generate forces through the stick. However thinking about this in the context of stick free speed stability it is possible that the positive pitching moment does enough to neutralize the destabilizing effect described in the previous paragraph. Only some analysis would shown whether this is, in fact, the case.

Now, one of the most critical aspects of airplane stability is the stick force gradient. This is the slope of the stick force vs speed graph. If the pivot was at the aerodynamic center of the wing (and knowing that the wing has almost zero pitching moment), the lift would always act through the hinge point and the pilot would have zero stick force for any position of the wing. There would be no feedback through the stick as speed changed from the trim speed. This is not a good thing and an aircraft that has this characteristic is difficult and tiring to fly. However, if the wing is pivoted in front of the aerodynamic center, the pilot has to apply load to the stick to increase then angle of attack (i.e. lift is increasing generating a nose down moment about the pivot that the pilot needs to counter with additional stick force). So the aircraft as a positive stick force gradient. However unlike a conventional aircraft, where you feel increasing elevator force to make the aircraft fly at speeds both above and below the trim speed (stick force is zero at the trim speed) I don't think a Flea will exhibit that behavior. The stick force will reduce for speeds below the trim speed (i.e. the stick forces get lighter) .. in fact I realize now I don't fully understand this and should need to go and look at the plans.

Also there is one other aspect of stability, that is not so relevant to the Flea, but which could exhibit some interesting behavior. This is stick force per 'g'. What I am not sure about is whether the center of gravity of the wing is at the hinge point. If it were, there would be no inertial coupling between the wing mass, load factor and stick force. However, if the center of gravity of the wing is behind the pivot, the inertial coupling would be such that the stick force per g would be reduced with increasing load factor ... not a desirable characteristic.

The above is just off the cuff, thinking aloud, without any detailed analysis .... really just some points to illustrate that there is no easy answer. What is clear here is that this is not a question that can be answered easily. A proper answer requires the development of a mathematical stability model. This is not a difficult engineering problem, but it does need some knowledge of aircraft stability theory and aerodynamics, and there is a bit of work in it. I would leave the pivot where it is if you do not have the ability to make such an analysis ......

Also, someone mentioned some Barnaby Wainfain articles on Fleas ... I don't suppose anyone has scanned copy of those that they could send me do they ?
 
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rtfm

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Hi tdfsks,
Thank you for your detailed analysis of the issues - it is quite late here, so I have only read it through quickly. I'll get back into it tomorrow morning over a coffee.

And I have a copy of Barnaby's articles - I'll send you a copy.

Regards,
Duncan
 

TFF

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The mission of the Flea is important to remember. It is designed for the non pilot. The Ercoupe although more conventional is the same type of idea, move a group of people into flying without going all the way with training. The plane will cover up mistakes. Part of the design. A key part of the design. They work well for what they are intended to do. They don’t do well past that.
 

wsimpso1

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I can provide some insights here. Look at the tail on a Cherokee. Symmetric foil, pivoted at 0.25c, used for pitch control and stability. Sound familiar so far?

Anyway, with the foil pivoted at 0.25c and symmetric, it will have close to zero feel, which can make for all sorts of unintentional speed and pitch deviations. Technically this is described as having zero stick force per g and zero stick force per speed change. So, on the Cherokee line, Piper added an anti-servo tab. This is rigged so when trailing edge up, the tab moves even more trailing edge up, same sort of thing moving down. The effect at the foil is to make it have some pitching moment back toward equilibrium position. This ends up driving stick force per g and stick force per speed change in the right direction. The Piper folks selected a combination of trim tab size and gear ratio to have about the right feel.

Now the foil we are talking about has close to zero moment and rotates for changing its lift. Since most foils do not appreciably change pitching moment with AOA, you would have to get restoring moment from somewhere to give decent stick force per g and stick force per speed change. Looks like the pivot was shifted forward a bit to get it.

Now if there was an antiservo tab in the system, pivot could have been closer to 0.25c.

What do pilots have to say about control feel in pitch of the airplanes in question? If they really like it, I will suggest that maybe the balance is properly chosen...

Billski
 

Malcolm C

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I am still building the fuselage of my Flea , I am waiting for Aircraft Spruce to get my large order of materials together before finishing the wings, the ribs are all there, the pandemic helped me finish them. As my Flea is built from Falconar plans I have had to adapt a lot of the free French plans dimensions and details to fill in the many gaps in the Falconar ones. My spar is exactly where the Falconar plans show it, I have not modified it's position at all, I am hardly qualified to do that......
 

rtfm

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Hi all,
First, thank you for your many considered replies. And a BIG thank you all for staying on topic. It usually only takes two or three posts before someone hives off into some obscurely related topic, and the original question/comment gets forgotten entirely.

Now - tdfsks (what do these letters stand for, BTW?) - you certainly got me thinking, and backed up by Billski, I think I finally have the basic principles sorted in my head.

This is the configuration of the HM293:
1594073855906.png
Interestingly, the pivot point is 40mm ahead of the c/4, and the Incidence Control Arm is set back 400mm from the pivot. With this arrangement, although a low CM airfoil is advantageous, the pitching moment of the airfoil doesn't matter nearly as much as the lift of the wing itself. Any lift at all will raise the rear of the wing. This is experienced by the pilot as a pull on the stick, and he/she will have to pull back against it. The so-called "living wing" for which Fleas are famous.

The extreme case (the pivot right on the LE), would see the entire wing pivot upwards, and with the increase in lever arm (i.e. 300mm instead of only 40mm), the pilot would in all likelihood not be able to exert sufficient force to keep the wing from pivoting onto its head, as it were.

If the pivot point is *behind* the CoP, then any lift would swivel the wing downwards.
1594074675618.png

As it is, the Mignet "Formula" (as verified by the plans) seems to place the pivot point slightly *ahead* of the CoP, and to place the Incidence Control point rearward of this creating at least a 10:1 moment arm. This now makes a lot of sense.

My own design also uses a 1200mm chord, but places the Incidence Control point on the trailing edge. This means, to keep to the same "formula" (and hence, "feel"), the pivot point needs to be 10% of the Pivot-Incidence distance ahead of the CoP. Like this:
1594077254818.png

This would require a bracket affixed to the spar extending some 160mm or so forwards, with possibly a number of holes to fine tune the pivot point. I *think* my reasoning is sound. Comments welcome, of course.

Regards
 
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BrianW

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Hi,
My question relates to where along the chord of the front wing to attach the pivot.

Some background:
In the plans, the front spar is used to anchor the wing pivots (this is eminently sensible since this is the only solid place in the wing). From the plans, the front spar is situated 260mm from the LE (HM293) and 275mm from the LE (HM360). However, in both cases, the point through which the lift acts is at the 25% chord position, which is (for the 1.2m chord wing used on both the HM360 and the HM293) 300mm from the LE.

It's unclear from the literature or the plans of the HM360 and HM293 if the pivot point *needs* to be ahead of the 25% chord point, or if this is simply a function of the fact that this is the "usual" position of the front spar.

So, to finally get to the question:
I'd be interested to hear from experienced builders/flyers of the Flying Flea (or from any knowledgeable designers) if placing the pivot point of the front wing at 25% chord would be (1) Good (2) Bad (3) Make little difference

Regards,
Duncan
~Disclaimer~ Not experienced FF builder/flyer.

Swept back wings are not uncommon - swept forward wings ARE uncommon.
Why? In a gust sweep back reduces wing load - but sweep forward INCREASES wing load.
In the same way, center of lift is likely near 25% chord - so if pivoted there, little unloading available.
If pivoted FORWARD of 25% chord, the wing is likely to decrease its angle of attack if it can - so this would appear to be a useful insurance.
 

tdfsks

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Comments welcome, of course.
Hi Duncan,

I sent you a private message which I am not sure if you saw.

I think what you are proposing is reasonable. That is similar to what trike pilots did in the early days ... they had a row of holes to experiment with and adjust the handling and bar forces.

What I am not clear on (I need to look at the HM-290/360 plans) is how the wing is trimmed for zero stick force in cruising flight. My recollection is that some Pou du Ciel aircraft have a trim tab on the wing. Otherwise I suppose it would be possible to use a spring. However there has to be something preventing the wing from floating to full nose down .... this is where the issue of the airfoil positive pitching moment might become significant.
 

Victor Bravo

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There is a balance spring on at least some of the Mignet versions. I believe this spring pulls rearward on the stick, which would balance the slight nose-down force put on the upper wing because of the pivot being forward of Cp.

So if the spring were sized correctly, it would result in little or no force being required on the stick for normal flight, but the pivot loation would still allow the wing to shed gusts and un-load the wing.
 
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