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vgstol(at)bigpond.net.au
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 Posted: Fri Dec 08, 2006 12:55 pm Post subject: Slats myth busted! |
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Gday, I’m JG, the fella who started all this kafuffle about removing the slats from 701s. It’s sure stirred up a debate, just as I knew it would, and I’ve really enjoyed watching from the side-lines, but now I think it’s time to comment.
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This is understandably a controversial move, since those slats are very much a defining feature of the 701. So I need to explain the history and experience that led to this move – it certainly wasn’t just a wild idea on an impulse.....
I’ve been fascinated by the Zenair 701 ever since a magazine article in1988 entitled “Freight Elevator to the Stars!”. The 701 in that test report was the prototype, powered by a 50hp Rotax 503! It was a revolutionary aircraft, and I was immediately impressed, and ordered a set of plans. In 1990 I went to Sun’Fun and spent most of the week hanging around the Zenair site and helping to assemble the 701 quick-build kit that was the feature of their display in those days. It was at that display that I met the Columbians who had already tried their 701’s without slats and liked them better that way, and had the slats hung up in their hangars. That image has been intriguing me ever since, and lots of book study on aerodynamics and real life observation of different aircraft made it appear truly inevitable. The responsibilities of a young family kept me from building a 701 earlier, but in that time I did fly a single-seat Spectrum Beaver ultralight for 1000 hrs, and modified another Beaver into a twin-engine machine and flew it for 200hrs. Now I finally have the time, and a really good home workshop for aircraft construction, in which we’ve built two Savannahs from kits, and Hans and I have rebuilt three crashed 701’s, two of them with extensive damage, so we know every rivet in these machines! (Each of them had slats at the time of the crashes, but don’t anymore...) Hans with his 701 and I with my <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Savannah each flew about 200hrs in the last year without slats, so we’re certainly not ‘hangar flyers’. In my case that included a 5000km (3000mile) round trip to Tasmania, with all the variable conditions that you encounter on such a trip, and landings into some challenging small strips. Hans did a 2500km (1500mile) trip to North Queensland and back in very wild and windy conditions, with a passenger and full load. We both fly ‘intensively’ and are forever trying STOL tactics and landing into tight spots – we do know our aircraft really well! So, that’s a summary of the history and experience that I use to guide these experiments with VGs instead of slats.
So we’re not guessing how the aircraft will fly without slats – we already know the answer to that! I wouldn’t have published the results if I wasn’t absolutely certain of the facts, checked and double checked. In that article, “The True Story of Leading Edge Slats”, at www.stolspeed.com , I tried to demonstrate all the testing and validation that had gone into it. I won’t go over all that again now, but if there are questions that I didn’t cover in that article then bring them on and we’ll look at them.
Another bit of interesting evidence that has come to mind is to watch those aircraft with retractable slats. I’ve seen a video of one manufacturer doing very slow turns, with the slats on the inner wing popping in and out on their own – no apparent change in handling. Also Carl Bertrand’s experiments with his retractable slat wing, when one slat stuck – “....very little effect, only light aileron and rudder required....”. That just confirms the effect that I would expect from my testing.
You sure would think that the rather bluff leading edge of the wing without the slats would be slower in cruise than a ‘finer’ leading edge. I had thought this as well, and even had already made up new front ribs with the same profile as the wing with slats on, for the new leading edge I thought I was going to need after removing the slats. But when I did the tests with the slats on and the slots covered over, I got a real surprise! Cruise speed was the same as with the bluff leading edge! But the stall was much more abrupt, and the centre of lift had moved forward such that I now had a seriously aft CofG. So now I won’t change that leading edge at all. Those unused ribs still gather dust on a top shelf in my workshop – such are the surprises of experimental work.........
It’s well known that a full-rounded leading edge is good for stall characteristics, and this wing that’s left after you remove the slats from a 701 is just perfect in that regard. I’ve flown it to the limit again and again, and with VGs it’s never ever let me down hard!
Another surprise was the dramatic effect of the VGs. I had read that they reduced the stall speed a bit and improved low speed handling, but I was amazed at how much difference they made! They allow pretty much the same STOL performance as the slats did, but give much better cruise efficiency, and the landings are easier.
Slats are known as a high-lift device, but it should also be noted that they are also a high drag device. The high lift only comes in at the very high angles of attack, but the drag is there at all speeds, and goes from high at cruise to extremely high at stall aoa. The only time I’ve found a use for all that drag is for power-on spot-landings. Nose way high, hanging on the slats and the prop, 'dragging' the aircraft in below flying speed, with power controlling the descent. Can't see where you're going with the nose so high, but easy to do a spot landing that way - just reduce the power and it'll drop down right now, no floating on. But a real serious down-side of all that drag is getting caught behind the power curve, or getting caught in an increasing stall condition without power. Just imagine all that drag at the highest point of the aircraft, not only slowing you down really quickly but also tending to pull the nose up – makes it increasingly difficult to recover...... A few 701’s been bent that way.........
At cruise, all that the slats give is d-r-a-g-g-g..... It takes power to overcome that drag. I saved $250 on fuel on a 50hr trip (4 litres/hr less burn (at) $1.30/litre) alongside an identical aircraft with slats, so now I’ve probably already saved $1000 over the 200hrs without slats!
There is another benefit of slats (not worth considering), is that in the event of a collision with trees or such hard objects they absorb lots of impact and protect the main structure of the wing...... After repairing those crashed machines we had a pile of mangled slats out behind the workshop, until I sent them away for recycling..... We haven’t found any real use either for the good slats we’ve taken off, except to hang them up in the roof of the hangar – makes a good ornament and conversation starter......
So the slats myth is well and truly ‘busted’. To me it’s just amazing that someone, including Zenair, hadn’t worked this out this long ago......
JG
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n801bh(at)netzero.com
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| Posted: Fri Dec 08, 2006 4:51 pm Post subject: Slats myth busted! |
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Ben Haas
N801BH
www.haaspowerair.com
-- "John Gilpin" <vgstol(at)bigpond.net.au> wrote:
Gday, I’m JG, the fella who started all this kafuffle about removing the slats from 701s. It’s sure stirred up a debate, just as I knew it would, and I’ve really enjoyed watching from the side-lines, but now I think it’s time to comment.
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
This is understandably a controversial move, since those slats are very much a defining feature of the 701. So I need to explain the history and experience that led to this move – it certainly wasn’t just a wild idea on an impulse.....
I’ve been fascinated by the Zenair 701 ever since a magazine article in1988 entitled “Freight Elevator to the Stars!”. The 701 in that test report was the prototype, powered by a 50hp Rotax 503! It was a revolutionary aircraft, and I was immediately impressed, and ordered a set of plans. In 1990 I went to Sun’Fun and spent most of the week hanging around the Zenair site and helping to assemble the 701 quick-build kit that was the feature of their display in those days. It was at that display that I met the Columbians who had already tried their 701’s without slats and liked them better that way, and had the slats hung up in their hangars. That image has been intriguing me ever since, and lots of book study on aerodynamics and real life observation of different aircraft made it appear truly inevitable. The responsibilities of a young family kept me from building a 701 earlier, but in that time I did fly a single-seat Spectrum Beaver ultralight for 1000 hrs, and modified another Beaver into a twin-engine machine and flew it for 200hrs. Now I finally have the time, and a really good home workshop for aircraft construction, in which we’ve built two Savannahs from kits, and Hans and I have rebuilt three crashed 701’s, two of them with extensive damage, so we know every rivet in these machines! (Each of them had slats at the time of the crashes, but don’t anymore...) Hans with his 701 and I with my <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Savannah each flew about 200hrs in the last year without slats, so we’re certainly not ‘hangar flyers’. In my case that included a 5000km (3000mile) round trip to Tasmania, with all the variable conditions that you encounter on such a trip, and landings into some challenging small strips. Hans did a 2500km (1500mile) trip to North Queensland and back in very wild and windy conditions, with a passenger and full load. We both fly ‘intensively’ and are forever trying STOL tactics and landing into tight spots – we do know our aircraft really well! So, that’s a summary of the history and experience that I use to guide these experiments with VGs instead of slats.
So we’re not guessing how the aircraft will fly without slats – we already know the answer to that! I wouldn’t have published the results if I wasn’t absolutely certain of the facts, checked and double checked. In that article, “The True Story of Leading Edge Slats”, at www.stolspeed.com , I tried to demonstrate all the testing and validation that had gone into it. I won’t go over all that again now, but if there are questions that I didn’t cover in that article then bring them on and we’ll look at them.
Another bit of interesting evidence that has come to mind is to watch those aircraft with retractable slats. I’ve seen a video of one manufacturer doing very slow turns, with the slats on the inner wing popping in and out on their own – no apparent change in handling. Also Carl Bertrand’s experiments with his retractable slat wing, when one slat stuck – “....very little effect, only light aileron and rudder required....”. That just confirms the effect that I would expect from my testing.
You sure would think that the rather bluff leading edge of the wing without the slats would be slower in cruise than a ‘finer’ leading edge. I had thought this as well, and even had already made up new front ribs with the same profile as the wing with slats on, for the new leading edge I thought I was going to need after removing the slats. But when I did the tests with the slats on and the slots covered over, I got a real surprise! Cruise speed was the same as with the bluff leading edge! But the stall was much more abrupt, and the centre of lift had moved forward such that I now had a seriously aft CofG. So now I won’t change that leading edge at all. Those unused ribs still gather dust on a top shelf in my workshop – such are the surprises of experimental work.........
It’s well known that a full-rounded leading edge is good for stall characteristics, and this wing that’s left after you remove the slats from a 701 is just perfect in that regard. I’ve flown it to the limit again and again, and with VGs it’s never ever let me down hard!
Another surprise was the dramatic effect of the VGs. I had read that they reduced the stall speed a bit and improved low speed handling, but I was amazed at how much difference they made! They allow pretty much the same STOL performance as the slats did, but give much better cruise efficiency, and the landings are easier.
Slats are known as a high-lift device, but it should also be noted that they are also a high drag device. The high lift only comes in at the very high angles of attack, but the drag is there at all speeds, and goes from high at cruise to extremely high at stall aoa. The only time I’ve found a use for all that drag is for power-on spot-landings. Nose way high, hanging on the slats and the prop, 'dragging' the aircraft in below flying speed, with power controlling the descent. Can't see where you're going with the nose so high, but easy to do a spot landing that way - just reduce the power and it'll drop down right now, no floating on. But a real serious down-side of all that drag is getting caught behind the power curve, or getting caught in an increasing stall condition without power. Just imagine all that drag at the highest point of the aircraft, not only slowing you down really quickly but also tending to pull the nose up – makes it increasingly difficult to recover...... A few 701’s been bent that way.........
At cruise, all that the slats give is d-r-a-g-g-g..... It takes power to overcome that drag. I saved $250 on fuel on a 50hr trip (4 litres/hr less burn (at) $1.30/litre) alongside an identical aircraft with slats, so now I’ve probably already saved $1000 over the 200hrs without slats!
There is another benefit of slats (not worth considering), is that in the event of a collision with trees or such hard objects they absorb lots of impact and protect the main structure of the wing...... After repairing those crashed machines we had a pile of mangled slats out behind the workshop, until I sent them away for recycling..... We haven’t found any real use either for the good slats we’ve taken off, except to hang them up in the roof of the hangar – makes a good ornament and conversation starter.....
So the slats myth is well and truly ‘busted’. To me it’s just amazing that someone, including Zenair, hadn’t worked this out this long ago.....
JG
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