rtfm
Well-Known Member
Hi,
If you are a member or one or more of the Flying Flea forums, please forgive this cross-post, but those forums seem to be frequented by enthusiasts rather than people interested in the aerodynamics of the Flea. I would really like to initiate a discussion about the wing-gap on Fleas. So here goes...
It seems that there is a persistent conviction that the Flea's two wings act like a venturi or "slot", with the airflow from below the front wing energising the airflow off the top of the front wing. This has been cited ad nauseum as one of the reasons the 1934 HM14's would enter an unrecoverable dive. This, despite the fact that neither the French nor British wind tunnel tests found this to be the case.
So I was intrigued and started looking for anything which might corroborate this widely-held belief. The closest I was able to come to an analogous scenario was Fowler flaps, but for the Fowler slot effect to work, the gap needs to be between 0.2% and 0.3% of the chord of the front wing. In most Fleas, this gap is of the order of half a metre! Similarly with Junkers flaps. We're talking about gaps of millimeters, not half-meters.
Then I came across the "Pulga" variant of the Flea designed by Jean de la Farge in Argentina. He seems to have been well aware of the requisite magnitude of the gap between the wings and did everything to narrow this distance. His solution was very clever. First, he placed the rear wing far enough below the front wing so that in cruise it flew mostly in undeflected air. Second, he allowed the rear wing to pivot also. Now when the front wing was deflected (take-off and landing) its trailing edge came within centimeters of the LE of the rear wing which was deflected about 4x that of the front wing (about 48 deg). What he now had was a giant Fowler/Junkers flap running the entire length of the wing, and a genuine slot. His third design feature was to counter the strong pitching moment created by this giant Fowler/Junkers slot. He added a horizontal stabilizer at the top of the tail. He did this because when the wings were creating this genuine slot, the plane would strongly pitch forward. The horizontal stabilizer added sufficient downforce to keep the plane level.
He was able to land at 27kph (about 14kts), and take off in under 20m (65ft)
Sure, the Pulga wasn't as "simple" as the Pou, but from what I've read, it was a far superior plane. Landed slower, flew faster, more stable at high speeds.
I'd love to hear your comments.
Duncan
If you are a member or one or more of the Flying Flea forums, please forgive this cross-post, but those forums seem to be frequented by enthusiasts rather than people interested in the aerodynamics of the Flea. I would really like to initiate a discussion about the wing-gap on Fleas. So here goes...
It seems that there is a persistent conviction that the Flea's two wings act like a venturi or "slot", with the airflow from below the front wing energising the airflow off the top of the front wing. This has been cited ad nauseum as one of the reasons the 1934 HM14's would enter an unrecoverable dive. This, despite the fact that neither the French nor British wind tunnel tests found this to be the case.
So I was intrigued and started looking for anything which might corroborate this widely-held belief. The closest I was able to come to an analogous scenario was Fowler flaps, but for the Fowler slot effect to work, the gap needs to be between 0.2% and 0.3% of the chord of the front wing. In most Fleas, this gap is of the order of half a metre! Similarly with Junkers flaps. We're talking about gaps of millimeters, not half-meters.
Then I came across the "Pulga" variant of the Flea designed by Jean de la Farge in Argentina. He seems to have been well aware of the requisite magnitude of the gap between the wings and did everything to narrow this distance. His solution was very clever. First, he placed the rear wing far enough below the front wing so that in cruise it flew mostly in undeflected air. Second, he allowed the rear wing to pivot also. Now when the front wing was deflected (take-off and landing) its trailing edge came within centimeters of the LE of the rear wing which was deflected about 4x that of the front wing (about 48 deg). What he now had was a giant Fowler/Junkers flap running the entire length of the wing, and a genuine slot. His third design feature was to counter the strong pitching moment created by this giant Fowler/Junkers slot. He added a horizontal stabilizer at the top of the tail. He did this because when the wings were creating this genuine slot, the plane would strongly pitch forward. The horizontal stabilizer added sufficient downforce to keep the plane level.
He was able to land at 27kph (about 14kts), and take off in under 20m (65ft)
Sure, the Pulga wasn't as "simple" as the Pou, but from what I've read, it was a far superior plane. Landed slower, flew faster, more stable at high speeds.
I'd love to hear your comments.
Duncan
