STOLSPEED
 Vortex Generators    Performance Enhancement for Light Aircraft
 
 
  
 
Home>Aircraft Using Stolspeed Vortex Generators>Zenith CH701>Longer Wing
 
 
Longer Wing for the 701
 
When Hans started flying his 701 it was the first of type in the area, and he was fairly satisfied with its STOL performance compared to other aircraft around.  But when I started flying my Savannah it was noticeably better at STOL.  That was a bit frustrating for Hans because he especially bought the 701 for its STOL reputation.  The 701 was a bit faster at cruise, but that was when we both still had slats.  When I pulled the slats off the Savannah and replaced them with VGs the STOL performance improved even more, and cruise improved dramatically.  Now Hans was really frustrated, since his 701 couldn’t perform nearly as well, both in STOL and cruise.  So he pulled his slats and added VGs, and then the 701 passed the Savannah in cruise.  But he was still frustrated that he couldn’t match the STOL performance of the Savannah, no matter what techniques he tried.  Then he flew the Savannah and realized that it wasn’t his ability, it was the aircraft that made the difference.  The obvious difference between the aircraft was the wing span, so we decided to extend the 701 wings to match the Savannah.  That was the key, and now he can pretty much match the Savannah – the improvement in slow flight performance of this longer wing is very satisfying, especially at full load.  His modified 701, with longer wing and VGs instead of slats, flies a heck of a lot better than the original!  Such is the true story of how these discoveries evolved.
 
I note that the original design specs for the 701 were for a MTOW of 880 lbs and wing loading 7.2lb/ ft2.  That was for a wing span of 27ft and area of 122 ft2 (including the area over the cabin top).   I’m not quite sure how to calculate the true wing area of the 701.  In most aircraft the carry-through over the cabin is counted as part of the wing area, but with that large cutaway (5 ft) on the 701, and no profile at all in that centre section (3.5ft), can it really be included??  At any high angle of attack that area over the cabin is totally stalled and giving no lift at all, only drag.  Subtracting just that flat portion on top of the cabin (16 ft2) leaves an effective area of only about 107 ft2.  With a present MTOW of 1100 lbs, that’s 10.3 lbs/ ft2 wing loading, and many on floats are over 1200 lbs so that would be 11.2 lbs/ ft2.  That’s a pretty high wing loading for a STOL aircraft....  So the weight and thus wing loading has now gone up considerably while the wing area has remained the same.......
 
The Savannah wing is 29.6 ft span, with full airfoil over the cabin, giving an area of 141 ft2 = 7.8 lb/ ft2 wing loading for 1100 lbs.  That’s much closer to the original design of the 701......
 
Subtracting just that cutaway (5 ft) and adding the extra span (2.6 ft), the Savannah wing has 7.6 ft more full profile ‘working’  wing – that’s a heck of a big difference!  Those stubby little wings on the 701 sure do work hard, and do a remarkable job, but they  just can’t match the extra span and area on the Savannah.  We added 400mm (16”) each side of the 701 to make it the same span as the Savannah.  That increases the overall span by 2.6ft and the area by 11ft2.  You wouldn’t think that such an increase would make such a big difference in performance, but it certainly did.
 
The Lift to Drag Ratio (as measured by the Glide Ratio) for the Long Wing 701 is now much improved over the short wing.  The short wing wouldn’t really glide at all at 40kts with prop-stopped (descent +800fpm and poor control), now it’s a good glide at 40 and still flyable at 35kts. 
 
 Lift to Drag Ratios  (Engine off, prop stopped)
 
Slow landings are now much slower, and much better control – much better ground effect cushioning.  The tendency now is to float on – the standard 701 never did that....  The effect of the 15º flaps improved considerably when the slats were removed, but seems to be even better now with the longer wing.  Now he can round out and then hold off with a trickle of power, getting slower and slower and higher AoA, until it settles on gently at a very slow speed, with the nose way high – it’s just beautiful to watch! 
 
Take-off roll is much shorter than before – almost immediate rotation and lift-off into ground effect.
Climb rate is 100fpm better at 40kts, so better angle of climb out of tight spots.  This would figure in the lower induced drag of the longer wing at slower speed.
Best rate of climb speed is unchanged at 55-60kts.
Fast cruise at 5000rpm is 82 kts, which is only one knot less than the short wing.
 
 
 

Of course, lengthening wings like this has serious structural implications.  We’re not qualified engineers but have considerable practical experience with the structures of several aircraft.  Having assembled two Savannahs and repaired three extensively damaged 701’s we were able to do direct comparisons.  There are differences between early spars and the SP model, so caution is advised.  We believe that the modification we have made is as least as strong as a Savannah wing, which has a considerable flying record already.  We’re not qualified to advise others how to do this mod, but one method that's been used very successfully is to get 750 plans, and build a 750 wing for the 701 - that works really well!

JG,
I have the scratch built 750 “style” wing installed on my 701 with your VG’s on wing and elevator and it performs great. 
Stall (that's if you can call it a stall) with just me is so low it's difficult to measure, but seems to be around 30mph with a 200lb copilot!  

Ken, Oregon 

This larger wing would be really great all for those float flyers carrying 1200lb!!!

(Return to 701 page)