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The ‘Original’ Wing v/s the New Savannah Leading Edge
 This is an excellent wing for both STOL and cruise.
Original Wing
My Savannah aircraft still has the ‘Original’ main wing with the slats removed and replaced by VGs. I did 50 hrs with the slats, and now have a further 350 hrs flying and testing with VGs. It’s outstanding in its performance, both fast and slow - surprisingly so!
I had expected these excellent stall characteristics with this somewhat ‘bluff’ profile, but figured that it would be slower at 80 kts cruise than a ‘finer’ leading edge. As a matter of fact I had already made up new ‘finer’ leading edge ribs, not unlike those that Savannah now supplies. But testing soon showed that the original ‘bluff’ leading edge went just as fast as a ‘finer’ one, and had better stall characteristics.
This was not a complete surprise, since I have had considerable experience in the old tube and fabric ultralight days, when it only took an hour to change the airfoil by removing the battens and bending a new profile in them. In those days we were trying to get more speed by easing the bluff leading edge, but it didn’t work... The speed remained the same, only the stall characteristics changed with the new profiles. But that was all at the 50 kt speed range, and I had figured it would be different at 80 kts. Well it turns out that it isn’t necessarily so - I still have those new Savannah ribs gathering dust in the workshop.....
From aerodynamic theory and experience you would expect that the stall characteristic of that ‘bluff’ leading edge would be very good. It’s long been known that a fully rounded leading edge, with a large L/E radius gives the best stall characteristics. This ‘Original’ leading edge is about optimum in that regard – if you were to start off to design the very best leading edge for stall characteristics, it would end up pretty much like this one. Even without VGs the stall is very gentle and predictable due to that fully rounded profile. With VGs the stall is pretty much non-existent – it just becomes a progressively increasing ‘mush’ that can be reduced instantly by easing the angle of attack.
I.C.P. Savannah labels this airfoil as ‘non-conventional’, and so it is.
But too bad they didn’t try it before going with their new one.......
It’s proving to be exceptionally good!!! It’s one of those discoveries that come along once in awhile that surprises conventional thinking and opens new opportunities.
To have such superior STOL performance without sacrificing cruise speed is a real winner! I just love it!!!
I would far rather have this ‘Original’ airfoil on my wing than the new Savannah profile.....
The New Savannah Leading Edge
Very soon after seeing the results from the work I had done on removing the slats from my Savannah, the I.C.P. factory brought out a new leading edge for retrofitting to slatted Savannahs. They claim this new leading edge to be a NACA 650-18, and it was selected because, “....it is a standard aerofoil on which they have summary data and general information....” But you can readily see from comparing it to a true NACA 650-18 that it’s not quite the same at all. The under-camber has been eliminated and the bottom flattened, and the top is somewhat flattened to fit the right-angled spar caps top and bottom.
.jpg) Not that it matters all that much, since those same modifications of flattening the top and bottom of the NACA 650-18 were incorporated in the original 701 wing from which the Savannah copied in the first place. And we know the excellent flying characteristics of that wing from extensive history.
So all they’ve really done is to extend the leading edge to a smaller radius to give it a ‘finer’ profile. You’d think it would go faster, but it doesn’t...... And that extension of the leading edge moves the centre of lift forward, so that the CofG as a %m.a.c. moves aft 3%....... not so good......
I think it was more of a clever marketing exercise by I.C.P., followed by some high-pressure selling, with scaremongering claims here in Australia, “....the original wing with VGs will flip inverted....”. What a lot of nonsense! With my ‘Original’ wing with VGs I can ‘hang on the prop’ at full power, at such an angle that the pitot is totally ineffective and the ASI shows zero, perfectly stable, and still with enough aileron control to do slow turns. The only way to get it to do anything dramatic is to haul it up into a full-power accelerated stall, but then so will the new Savannah VG, and so did the old Savannah with slats, or any aircraft for that matter...... That’s not a manoeuvre that has any validity in real life flight testing. But just in Australia alone they sold 50 of those conversions at $1000 each – go figure......
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Flight Test Comparison:
New Savannah ‘VG’ v/s ‘Original’ wing with VGs
Bill G. has a new Savannah ‘VG’ so we swapped aircraft and did some flying. Bill previously had a slatted Savannah, and flies out of a 145 metre strip, so has good experience for comparison testing.
Not much difference between them at all......
Glide Ratio at 45 kts with prop stopped is equal at 8:1.
There is no actual ‘stall’ with either of them – just a progressive and easily controllable ‘mush’, at speeds so low that the ASI is inaccurate.
Bill reckoned that mine gave more of a buffeting warning at ‘mush’.
Bill reckoned that his was better at full flap, but only did one landing in mine to test that....
Cruise at 5000 rpm, same engine, same prop, same pitch, the Savannah VG was slower - 77 kts compared to 81 kts for my Original VG – that was a bit of a surprise....... This really doesn’t figure, could be a tacho or ASI error – more testing needs to be done......
but the new leading edge certainly doesn’t appear to be any faster.....
(My flight testing of VGs indicated slightly more drag with VGs spaced in close pairs as on the new ‘VG’, compared to the even spacing that I use, but certainly not 4 kts worth.....)
The biggest difference I noted with the ‘Savannah VG’ model was that on no-flaps take-off, if I rotated to a very high AoA too early it would trundle down the runway with nose way high for a long way before getting off – sort like a heavy chicken trying to fly...... It would seem that the wing was stalled on the ground. This could well happen when the wing stalls right behind that finer leading edge, making the VGs totally ineffective..... The ‘Original VG’ wing, with it’s larger leading edge radius, just leaps off the ground in that condition. At first stage flaps the ‘Savannah VG’ didn’t have this problem, and take-off was excellent.
The test flight was done solo, so the CofG was nowhere near the aft limit, but I could certainly feel the difference with the extended leading edge and the battery in the tail. After discussions and further consideration, Bill has decided to move his battery from the tail to behind the seat.......
- Bill G. really loves his new ‘VG’ wing compared with the slats....
- I just love my ‘Original’ wing with VGs compared with the slats....
So, those of you who got sold that new leading edge now have an aircraft that’s much better than with the old slats, but it’s certainly no better than the ‘Original’ wing with VGs, it just cost a lot more and lots of hassle to fit......
Those of you who still have the ‘Original’ wing, be glad you do.
Just add VGs and stay with it, I reckon....... |
