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Home>Aircraft Using Stolspeed Vortex Generators>Zenith CH701>Letter to Zenith
I’ve just sent a set of Stolspeed VGs to Zenith Aircraft Company for testing on their CH701. 
Also a copy of the following open letter:
 
Zenith Aircraft Corporation
Mexico, Missouri, USA 
 
Dear Sirs,
 
As you will have heard, our experience here in Australia, of removing the slats from a CH701 and replacing them with VGs, demonstrates a considerable improvement in climb and cruise and glide performance, with little or no real loss in STOL capability.  Hundreds of hours of intensive STOL flying on several aircraft has shown no down-side of the conversion.  Multiple testimonials from other 701 owners in the USA and elsewhere have confirmed these results.  It’s heartening that many of those to try this mod are high-time pilots with considerable experience in different aircraft, and they now like their 701s better without the slats.....  Now we hear that fellas in Brazil have also been flying without slats for years.  There’s no doubt that the improvement is real.
 
Seems to me these discoveries involving VGs instead of slats should be seen not as a threat to the CH701, but an opportunity to broaden the range of an already excellent and popular aircraft.  It doesn’t need any major change in the aircraft construction to implement this alternative.  I reckon customers should be offered the choice of slats or VGs......
 
At the time that Chris Heintz designed the CH701, VGs were not as well tested as today, and even so, no one would have predicted that they could actually replace the effect of leading edge slats so effectively on this wing.  It was a real surprise, but now we know from experience they can be a real advantage.  So why not take that information on board and exploit it???  I realize that those slats were the best innovation at that time, and a great marketing feature, but the facts are now clear that many of us, from experience, prefer the aircraft with VGs rather than with slats.
 
So I’m sending you a set of ‘Feathers’ VGs, free of charge, for testing on your CH701. 
 
Another major improvement we have tested on the 701 is to extend the wings by 400mm (16”) each side.  This has dramatically improved the slow speed performance and considerably lowered the landing speed.  Glide ratio at very low speed is much improved, making for easier, gentler, slower landings.  Now we learn that those same fellas in Brazil have also been building longer wings for years, and already know the benefits....  See ‘Long Wing’ page for more details.
 
Yet another improvement we have tested is to apply VGs to the leading edge of the elevator – not to the horizontal stabilizer, but to the elevator itself.  This way the VGs are hidden at cruise in the slot between the horiz stab and the elevator, but are exposed in the very best position to catch the airflow when they are most needed at flare.  This has sure helped to increase elevator authority at very slow touch-down speeds, making for much improved landing control.  See ‘Tail Feathers’ page for more details.
 
The owner of the 701 with all these modifications is absolutely thrilled with the improvements – no way would he go back to the original configuration!  The two other 701 owners at our airfield have long since replaced their slats with VGs and added VGs to their elevators, and now want to lengthen their wings as well.   Who wouldn’t, after having compared their aircraft’s slow speed performance to the modified one...... 
 
Your competitor, I.C.P. Savannah, had already realized the benefits of the longer wing when they copied your design.  And as soon as I demonstrated the benefit of VGs instead of slats they jumped on the opportunity with their ‘VG’ model, and now are very successful with it.  Yes, they copied your basic CH701 design but made some significant improvements that weren’t and still aren’t available on the CH701  (longer cleaner wing and more powerful horizontal stabilizer/elevator), then they exactly copied the VGs that I was importing at the time, which is ironic in that I wasn’t satisfied with those VGs so felt the need to design and manufacture some better ones, i.e.- Stolspeed VGs.
 
Zenith could easily do the same and even better with the CH701, so why not do it????  We’re happy to have done the amateur experimenting that discovered some of these significant improvements, but I would hope that now that the results are revealed, Z.A.C. would incorporate them, with proper qualified engineering, for all CH701 owners.   It’s just not right that other amateurs without engineering backing should have to make these changes for themselves.  We would rather have the benefit of your engineering experience and stay with Zenith rather than go to some copier.   You have a large and loyal following so please give us the best performing aircraft that you can.
 
All of these changes would be really easy to incorporate – no need to engineer a completely new aircraft.  The original wing profile without the slats is already proven to be an ideal airfoil with VGs.  I’ve heard (but can’t confirm) that the longer wing has already been produced for the French market to comply with their regulations, so Z.A.C. must already have some experience with it.  If us amateurs in remote parts of the world can do it so successfully, then surely Z.A.C. can do it for the benefit of all.
 
There’s an old adage in business, “....If you just stand still, the pace of progress will leave you behind.....”.  I sure hope this doesn’t happen to the 701, ‘cause it’s been standing still for a long time now , while others are exploiting every improvement....  I still love the 701, and I admire all that Chris Heintz and Zenith have done for the homebuilders.  The CH701 is still one of the only designs that the homebuilder can realistically build from plans and scratch materials.  I sure hope that can continue, what with all the move to factory-built, high cost (and I assume higher profit) machines. 
 
If you want to experience any of these improvements that we’re now enjoying then please come to Australia for a holiday/business trip, and fly Hans’s modified 701 and my Savannah – you really do need to know what is possible......  
 
Thanking you for your attention.
 
Fair skies and tailwinds always,
John Gilpin

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