Duncan's got a Flea

Hmmm, I was trying to stay out of this thread as I've already said my piece, but now you've confused me, Topaz. Logically, don't all ailerons effectively increase or decrease the local angle of attack of the wing section in front of them? Therefore wouldn't a wing with the ailerons in any position but neutral tend to stall the section in front of the "down" aileron before the section in front of the "up" aileron? I have always understood that the utter lack of ailerons contributed to stall/spin resistance in Mignet types.



EDIT--Added image.

 

Thanks, Topaz, that all tracks with my understanding. By all accounts, the strong downwash effects of the front wing on the rear one are a somewhat tricky aspect of designing any high-low tandem (Mignet, Delanne, etc.) and not really an issue with the low-high tandems (Mauboussin, Rutan Quickie, etc.). That does not change, of course, the fundamental point that you emphasized, that the forward surface should always stall well before the rear one, otherwise things get ugly fast.

Duncan, for some reason I had it in my head that you were thinking of ailerons on the rear wing, not the front one, so basically Cosandey flaps tied to the stick, is that correct? One other point to keep in mind is that, while they might be fine for trimming out some crosswind to reduce the crab angle when landing, ailerons on the rear wing are unlikely to be very responsive. With the rule of thumb that the front surface should always have a wing loading at least 50% greater than the rear one, and the generally larger span and area of the front wing, then ailerons the rear wing have less total lift to play with in order to provide roll control. Ailerons on the variable-incidence front wing, while certainly possible, could introduce their own complexities.

 

Hi Matthew,
Why is it that some errors seem to outlast repeated corrections? They seem to enjoy a perverse imortality. Like the 75 year old fallacy that Pou du Ciels are dangerous. Or that there is a definite slot effect caused by the front/rear wing positions in a Flea. Or this one - that there is a strong downwash over the rear wing caused by the front wing. Yes, there is some downwash - but it is nowhere near as great as is so frequently asserted. Check out the worst case scenario below.

Actually, I had envisaged rear wing ailerons on the outer panels.

Mmmm I'm not so sure about this - well - I'm conflicted about how effective they would be. I don't think there is any danger of them stalling the rear wing (they only decrease the lift on one half of the wing anyway) and there is a healthy extra margin for error in the 6 degree fixed incidence of the rear wing anyway. But I think, overall, it will be simpler to have rear wing ailerons rather than putting them on the front wing. I can certainly see that front wing ailerons would be far more effective, but what we are looking for here is some added roll authority - the rudder is still there, doing its job. The ailerons simply kick the roll off immediately.

Yes, this is my concern also... Although the Watts Bridge Pou (the one built by Byron) has the Cosandy flaps on the front wing, and while Patrick says he seldom or never uses them, he didn't indicate that they introduced any weird effects when he had used them.

Mmmm
Still thinking about this one...

Duncan

 

Ailerons on the outer panels of the rear wings will not give you an aerobatic roll rate but they will give 3 axis control. If the ailerons are rigged for UP only, they will not make the rear wing more prone to stall.

 

I hardly think that I am someone who keeps alive a "75 year old fallacy that Pou du Ciels are dangerous," quite the opposite, in fact. What airfoil is that, under what conditions? The point is that, despite the need to keep the rear wing flying at all time, the rear wing incidence is substantially greater than one might expect from lift calculations alone. As someone who has been around the Mignet community for 20+ years, including corresponding with and speaking by phone and in person with some of the true experts, I'd personally suggest trying the Mignet approach unchanged as a starting point.

 

I think two small ailerons interconnected to a small separate additional rudder might be useful. Limited crosswind cross controling could be managed by cross controlling the ailerons and rudder.
But with these small ailerons interconnected only with the small rudder, stick movement would always provide coordinated roll so no tip stall.

 

I am not sure aileron and rudder is a solved problem on conventional aircraft.
And I am not sure tandem and canard are the same. But with no experience with the latter I guess I will quit speculating.

 

Hi Matthew. I seem to have hit a nerve. My apologies. I know very well that you aren't a member of the "Dangerous Pou" clan. I was merely drawing attention to how folk lore tends to cling to the Poux like **** to a blanket.

However, having said that, I must go on to say (and again, this is not directed at you, but at all the good old boys in a thousand hangars on both sides of the Atlantic) that I find it easier to believe Barnaby Wainfan's wind tunnel tests which concluded that there was no observable slot effect operating with the Pou configuration - than the received wisdom regarding the slot effect being the culprit in the unrecoverable nose-dives of the HM-14. As he goes on to explain, those dives can be explained in terms of airfoil choice, amount of front wing pivot angle and CG location. Barnaby does not give reasons for his conclusion, but I suspect it is because even at 16 deg AoA on the front wing, the airflow deflection experienced by the rear wing is only 3 deg.

Friends?

Duncan

 

Hi,
I'm not sure I follow. What do you mean by a "small separate additional rudder"? Do you mean splitting the rudder into two parts, the smaller of which is linked to the ailerons, and controlled by the stick, and the larger part controlled by foot pedals?

Duncan

 

Yes. That would work.
Or two separated rudders. I haven't thought the idea through yet, but think it might have merit for conventional aircraft as well.
It seems to me the ailerons should go on the larger wing nearest to CG. Rutan tried ailerons (elevons actually) on the small forward surface on the VariEze prototype and found marginal and later moved the ailerons to the large wing. Makes sense to me that ailerons should be on large wing as a rule of thumb. But that invites aileron tip stall if the ailerons are not inter-connected like a Ercoupe. So that's why I suggested interconnection for the ailerons.
But since the Flea is set up for rudder actuated roll control with big rudder and large dihedral, it makes sense to keep the roll control mostly with rudder.
For an Ercoupe, an auxilliary small third rudder could be fitted and with rudder pedals installed for this small rudder. So some crossed control slip could be done but not enough to cause loss of lateral control from crossed controls.

Edit. Not sure if the stick should control rudder or ailerons. Depends on pilot, I guess. An old Flea pilot might want set up like an old Flea. If you fit ailerons then that's a choice you must make also.

 

Oops... Double post [deleted]

 

I'll strongly recommend you stick with conventional controls on an airplane with conventional control surfaces.

Nothing to unlearn. Other pilots need only type training checkout, vs. Conditioned reflex reprogramming. ( insert backward bicycle video here ) And there aren't enough Pou pilots that can't deal with conventional controls easily to make that a factor.

The original Quicksilver hang glider controlled no differently than the company's Rogallo gliders. Despite the rudder.
The modern ATOS, ditto, despite the spoilerons.
The Pterodactyl ultralight deleted the rudder pedals as does a Pou, both 2 axis birds.

A aileron equipped Pou with the pedals controlling the ailerons just seems a bad idea ergonomically.

But if you prefer a Wright early set up with one fore & aft lever controlling elevator/front wing and another fore & aft lever controlling rudder, and shoulder yoke for ailerons. .... your plane. No one else can successfully steal it. Crash it on takeoff? Every time.

 

Ha ha. :ban:

 

Just remember Henry Mignet was no Kelly Johnson, he was a well meaning enthusiastic amateur who wasn't able to pilot an aircraft with conventional 3 axis controls. I'll bet he was as surprised as anyone when his first Flea flew and was controllable.
Henry never lost his dream that anyone could build an aircraft and fly it, even after there were some accidents, and continued refining his original concept - but because of his own lack of coordination all of his design have been 2 axis.
The basic idea of Henry's Fleas is sound, and if you feel comfortable with the lack of roll control, build it as Henry designed it.
If someone can build a safe aircraft using the Mignet formula of simple construction and stall resistance, compact and foldable, but with conventional controls it will be attractive to many more amateurs, I know I would reconsider the flea.

 

"The proof's in the pudding" - as they say in NZ and Aussie. I have no idea why they say this, because it doesn't even make sense! The expression is: "The proof of the pudding is in the eating", but when I mention this to the locals, they look at me as though I'm from another planet.

Anyway. A three axis Pou is on my workbench as we speak. Let's see how it turns out. Sacre Bleau!