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 "A Tale of Two Cherokees" by Albert "Doc" Hayes
 "What's That Thing?" by Knud Winkel
 "Met-Co-Aire Tips" by Skip Carden

"A Tale of Two Cherokees"
by Albert "Doc" Hayes, Ph.D. — Engineering
Massachusetts Institute of Technology (MIT)
 
The 77 n.m. run from Bahia Kino to Guaymas was to be a smooth 45 minutes when N5QB and N5985W took off on the next-to-the-last leg of the trip to Los Mochis. Since both planes are Cherokee 140s of late 60s vintage, it would normally appear that they should be pretty closely matched in speed. But both Cherokees had been worked over in weird and exciting manners, with N5QB sporting a 180-horse mill with a constant-speed Hartzell prop, and N5985W now boasted a 160-horse engine, albeit with a fixed-pitch prop. So it was a bit of a surprise when we leveled off at about 7500 feet, set the machines to as close to 75% power as we could, and found that the two were matched in speed within less than 1 knot!

Now every engineer knows that's all wrong. The 180 has a factor of 1.125 horsepower advantage over the 160, thus, at the same percentage power setting, the 180 should have bested the 160 in speed by the cube root of the power ratio-in this case the 180 should have been 4% faster than the 160. But it wasn't. We flew wing-to-wing for almost half an hour, both indicating 120 (m.p.h.) on the airspeed indicators, with nothing but an occasional moment of sloppy flying making any difference in our relative position. Here, indeed, was a mystery.

After landing at Guaymas for fuel and a coke, Bob Cimino and I went over every possible reason for the excellent performance of the 160 conversion, and only one thing surfaced. The 160 sported a set of Hoerner-designed wing tips. It took a close look to spot the difference, but there appeared to be little doubt that this was the only substantive difference in the two airplanes (aside from the h.p. difference). Bob had bought the tips from Met-Co-Aire, in Fullerton, California, and recommended that I contact them for further information.

Since N5QB lives at the Fullerton airport, this presented little problem when we returned to home base a few days later. A visit to the offices of Met-Co-Aire yielded a brochure which promised, among other things, an increase in rate-of-climb by 60 feet per minute, increased cruising speed, shorter takeoff and landing roll, a 4-5 mph lower stall speed, and an increase in the overall stability of the aircraft.

This sounded like quite an order, so we obtained a copy of the original report, by Dr. Sighard Hoerner, which had been prepared and distributed by the USAF Air Materiel Command at Wright-Patterson AFB. (For those interested in the report it is identified as Technical Report No. 5752, "Aerodynamic Shape of the Wing Tips.") A review of the report indicates that the original Cherokee rounded wing tips are just about the least favorable shape to use, while the slight modification provided by the Hoerner contour effectively increases the wing area, increases the aspect ratio, and reduces the parasitic drag. Since the increase in aspect ratio automatically decreases the induced drag, there appeared to be quite an advantage in drag reduction, both induced and parasitic. The Hoerner report also forecast a 1 to 2% increase in range with the new-design tips. The increased lateral stability mentioned by Met-Co-Aire arises from Dr. Hoerner's finding that the improved tip was the equivalent of adding 1.5 degrees of additional wing dihedral.

With promises like this, and the iron-clad proof based on Bob Cimino's 160's performance against N5QB's 180, the price didn't seem too bad, so minutes later we walked out of the Met-Co-Aire shop with a shiny new set of tips and the greatest of expectations.

Since the new tips came undrilled-without holes for the mounting screws-we stopped in at the local parts house and acquired a neat gadget called a "strap duplicator." This is a clever tool which provides a guide when you want to drill a hole to match up with an existing hole under the place where you're drilling. (It's easier to understand if you try to do the job without it, so, be smart, don't approach the job without a strap duplicator. Met-Co-Aire now sells these, also.)

We only had to drill out about five of the roughly 50 screws we had to remove to get the old tips off (rust, corrosion, and lousy screw heads all contributed to the problem) and after the old tips were off it took only about 30 minutes to complete the most workmanlike installation of each of the new tips. (Yep, we used stainless steel screws and molybdenum disulfide lubricant to assure that we'd never have to drill out those particular screws again.)

It was hard to tell, during the test flight, whether the takeoff run was any shorter, but N5QB sure seemed to be a lot more stable in roll during level flight. We tried a few stalls at altitude, and have to admit that the stall speed appeared to be about 5 mph (indicated) lower. But, who's to be certain? N5QB runs around with so many different loads and weight/balance schedules that we can't be sure about that one. The payoff had to be another side-by-side run with N5985W, with its 160-horse mill and the same tips.

A couple of quick phone calls and a week later the Cimino family and the Hayes family met at Camarillo airport with close to the same loading ratio that we had during the original "shoot-out." After a quick lunch we took off and headed for the Santa Monica VORTAC, carefully set power to as close to 75% as we knew how, turned on the DMEs, and sat back to assay the results.

Surprise! The Quiet Bird slowly pulled away from the 160. As close as we could tell, both by comparison of DMEs and airspeed indicators, N5QB was now about 4.5 knots faster when in the 120-130 mph IAS range. This agreement with Dr. Hoerner's prediction made the whole thing worthwhile. After all, a Cherokee 180 should be faster than a Cherokee 160 shouldn't it? This performance ratio was verified at several different power settings, including wide open (to eliminate the inaccuracy of tach and M.P. gauge), and there is no mistake. The Hoerner wing tips do increase the speed about 4%, with absolutely no penalty in any other department. As we said, it also appears to be more stable in slightly-rough air, but that's a subjective thing we can't testify to.

Met-Co-Aire has a stack of brochures on the Hoerner tips that they'll be glad to provide in response to any inquiries. Their address is P.O. Box 2216, Fullerton, California 92837, and their phone is (714) 870-4610. You might also ask them about the Cherokee tip tanks they are presently in the process of developing. Imagine, the Hoerner tips with 12 gallons of extra fuel in each tip. Lovely!

 
 
"What's That Thing?"
by Knud Winkel
East Coast President — International Comance Society
 
We were just turning final. I reduced the power to 15 inches and my Comanche 180 slowed down to 65-70 mph indicated. Over the fence, I pulled the power and pulled back on the yoke. Suddenly my left seat passenger cried out, "Watch that airspeed-we're below 60!" At 50-52 indicated, we sank to the runway to the accompaniment of the familiar squeak of a perfect landing.

Marc, my trusted radio man, flying left seat to check on a radio problem, could not believe we had not simply fallen out of the sky 50 feet above the runway. The answer-Met-Co, Hoerner design wing tips.

It all began two years ago when I met George Flynn at the Northeast tribe Fly-In at Mt. Snow, Vermont. George and his wife, Lucille, were very enthusiastic about their new Hoerner tips. Listen to what George told me:

"We purchased our Hoerner wing tips strictly on the basis of the Met-Co-Aire advertisements and also because we observed that Beech used that configuration on their Bonanza. Beech dropped it on the "P" and some of the "S" models, and returned to it again. Cessna has switched, also.

"We flew our Comanche 260B for about 150 hours before converting. Frankly, I did not like the sloppy aileron response of the Comanche tips at slow speeds below 85 mph. 90 mph over the fence was OK, but nothing to brag about, and they were not sharply responsive in cross winds. The tips cost about half what Piper was getting for their conventional tips, so it struck me that it was a pretty good gamble to buy the new tips, even if they did one-half of what the ad claimed.

"I was chomping at the bit to try them, and the paint wasn't even dry, when I took off about 400 pounds under gross with full tanks and two aboard. I whipped it into a sharp stall and all it did was hang on the prop with 12 inches and nice aileron response. Indicated speed was around 60 mph with gear and flaps up.

"I have spun Comanches with regular wing tips, but the two times I have attempted to spin my bird with the Hoerner tips produced only a solid stall. Jumping on the rudder only produced a slow roll and a nose drop.

"Landings are a lot more comfortable. I now come over the fence at about 80 mph indicated and touch down in a nice controlled stall at something around 60 mph. Short field landings are duck soup. Comanche owners on the field ask how I can turn off so soon. I tell them I never use brakes and they walk away talking to themselves.

"Speed was not a factor when I bought the tips. However, I am getting something more than 4 knots additional speed. I have flown along side a 1965 Comanche 260B at the same power settings (23 square) and went by him at a pretty decent clip.

"All of which proves nothing to the purist. I am no physicist nor have I run definitive tests to establish absolutes under varying conditions. Nor do I intend to. And if it's all in my head, as some claim, that's OK, too, because the tips please me and perhaps add something to my 5000+ hours of flying and observation."

So after that salvo, I couldn't get hold of those wing tips soon enough. The minute they arrived they were installed. That took about two hours, by myself, and after they were signed off by the AI, four of us took to the air to find out what the wing tips had done to my 180 Comanche. Well, in a nutshell-everything short of a miracle! Stall speed is now around 52-54 mph indicated, versus previously 60-62. There is plenty of stick shake for prior warning before the stall. All during the stall we have complete aileron control. That wing just refuses to completely stop flying.

Departure, or power on stalls are a piece of cake. You will have to stand the shake for several seconds before the plane finally stops flying. Any tendency for a mushy, over the top roll is quickly controlled and stopped with combined rudder and aileron response-the way it comes naturally. Just imagine what that means in heavy turbulence.

Simply stated, I would hate to fly a Comanche that did not have Hoerner wing tips!

This is a reprint of an article written by Mr. Winkel-East Coast Vice President of the International Comanche Society, for inclusion in the "Comanche Flyer," the club's official publication. All opinions and data are solely Mr. Winkel's.

 
 
"Met-Co-Aire Tips"
by Skip Carden
Editor, Cessna 150-152 News
 
Several months ago I had a set of the Met-Co-Aire tips put on my airplane. I had investigated the claims made about all of the supposed super tips and found that the Met-Co-Aire tip had the most realistic claims, and they were also backed up by government test. Not only that, but they looked good also and the price was also reasonable. But do they work? Well, I believe that mine perform as advertised. The rate of climb is better with a shorter takeoff run. The top speed is increased by about 4 of 5 miles an hour and the stall speed is also lower. Add to this more lateral stability and a better control. I can't prove it but I think that the rate of roll is better.

Well, let's get down to the bottom line. They will only fit a 150. They will definitely give the performance increase that they claim and I think that they are a good buy. I would recommend that if you are thinking about wing tips that you definitely consider the Met-Co-Aire tips. You'll be glad you did.

 
 
 


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