Zenith 601HDS Performance, and why Vx and Vy are important.


Larry Nelson sent in these rate of climb performance numbers for his Zenith 601HDS: 


Outside air temperature at 175′ MSL …100F

Density Altitude…..3,000′

Unbroken climb from 175′ MSL to 4,000′ MSL…800′ per min. 

Airspeed for climb…..75 kts.

Engine sustained temps……….Oil-235F (max is 265F) CHT-330F (max is 420F)



709 pounds, empty (HDS’s are the lightest of the 601 family, and this is an exceptionally light one, notice it has no paint nor interior) Test was flown at 1,100 pounds.

Powered by 2,700cc, 100HP Corvair, Warp Drive 2 blade prop, 66″ diameter, set at 8 degrees at tips, turning 3,000 rpm in climb.




Note: Larry is an engineer who works for the Department of Defense at Yuma Proving Grounds, testing vehicles and weapons systems. When he provides data, it is real. He does not put ego ahead of the truth. Our industry is unfortunately dominated by “Brochure Performance.” All tests I have ever done, dyno runs, weight and balance measurements, performance numbers, have all been done in public, where people can watch. Larry is flying his plane to the September Zenith Homecoming, where anyone can verify the numbers above.



Above, Larry with his bird. This plane has my new nose bowl. Read this: STOL Bowl #4201-B, flying in video.  If his name sounds familiar, it is because he is also our selected oil analysis expert. Check out this story: Corvair Oil Analysis. This program will shortly be the subject of another video.  Larry is also remembered as the guy who paid for  the parts in his engine with ammunition: Acceptable methods of payment for Corvair parts. Quite a character.






Two months ago, we had a builder who had a 601-HDS like Larry’s, with the exception of it having tricycle gear and a 3,000cc Corvair tuned for 115HP.  He reported to me that his plane “barely climbed.” He told me he decided to remove the engine from the plane, and sell it for whatever he could get for it.


After going over a lot of details, I finally asked him what airspeed he was using to climb, and he told me 95-100kts. I pointed out that this was way too high an airspeed to be climbing at on a plane with a top speed of perhaps 8-10 mph more. I pointed out that as a rule, all aircraft in level flight at their top speed have a rate of climb of exactly zero feet per minute. That light planes climb best at their L/D speed. (The only exception to this is aircraft with a very high power to weight ratio and fixed pitch props)


The builder adamantly refused to climb the plane slower, claiming that it would overheat. I suggested that he revise his cooling, because I had just flown in Ken Pavlou 601XL / 3.3 Corvair, and we were a lot heavier, and had still had plenty of cooling for a 125HP Corvair engine at a normal climb speed. I pointed out that the phase one testing of the airplane required him to test the plane to determine Vx and Vy, and he had to enter these in the logs. If his plane ran a bit hot, it could be corrected, as proven by Larry’s data showing the performance at 100F.  Last we spoke, builder was not interested.


These are not new concepts, I have written about them extensively, such as this story: Critical Understanding #3, Rate of Climb, the critical prop evaluation. If you had reasonably good flight instruction, or any kind of a rigorous biennial flight review, the instructor would cover these types of information. If you are new to flying, and you would like to understand more of these issues from the undisputed source, buy a copy of this book and read it: Greatest Book on Flying Ever Written,







About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

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