The Power of Modifying Your Prop and Wings

The prevailing winds, the winds at surrounding airports, and the weather briefing I received prior to the flight all indicated that Runway 13 was the one to use. During the 15 to 20 minutes leading up to my departure, however, the windsock lazily swung around and slightly rose to show a light wind from the opposite direction. It stayed there, and so, I chose Runway 31 for the takeoff.

The takeoff performance of my 1953 Cessna 170 is easily the weakest point of the airplane. With only 145 hp on tap, a well-worn propeller churning the air, and 10 psi in the 26-inch Alaskan Bushwheels creating additional rolling resistance through the grass, my typical takeoff rolls and climbouts are, as a British friend of mine says, quite luxurious. Departure-end obstacles always seem to loom large, and I always find myself wishing for a more powerful engine. 

Lacking such an engine, and left to depend on my entirely mediocre skill and judgment, I utilize the 50/70 rule for my takeoffs. This rule states that if you achieve 70 percent of your takeoff speed at or before you’ve used 50 percent of the runway, the takeoff may be continued safely. Conversely, if you have not achieved 70 percent of your takeoff speed before reaching the halfway point of the runway, you should abort the takeoff, and you should have sufficient distance remaining to safely come to a stop.

I used the 50/70 rule during last week’s uncomfortable takeoff, and I did confirm that it was safe to continue. As I was flying with a light tailwind, however, I did not have my usual cushion of extra energy and altitude. Although the takeoff went smoothly, I once again found myself wishing for a more powerful engine. 

As I gradually learn my airplane, this is something I think about regularly. It doesn’t help that I have multiple friends who have 170s with more powerful engines. It certainly doesn’t help that I’ve experienced the sublime feeling of being firmly pressed back into the seat as they advance their throttles for takeoff.

Unfortunately, such engine and propeller upgrades don’t come cheap. By the time you select an engine, a parts kit, a propeller, and the necessary STCs, you're looking at about $80,000. And then there’s about 60 hours of labor to install it all. Lacking a trust fund, an inheritance, or some illicit arrangement with an open-minded cartel, I am left to explore other options for improving my airplane’s takeoff performance.

The quickest and easiest means of increasing the thrust of a modestly-powered engine is via the propeller. The selection and variety varies quite a bit by type; some have a wide range of options available via STC, while others have only one or two. In the latter case, owners can opt to have their prop repitched for relatively little money, and are simply required to ensure the static rpm falls within limits prescribed by the aircraft and engine manufacturers. 

If you own a Cessna 170B, you have the option of installing an 80/42 seaplane propeller via STC. Compared with my stock 76/53 prop, this is like shifting into first gear. Gone is any semblance of a decent cruise speed, but so too are most of the departure-end obstacles that have consistently been the focus of my attention during takeoffs.

I had the opportunity to fly a friend’s 170B with one of these props. Cruise speed is reduced from about 115 mph down to around 95 to 100 mph, and one must closely watch the tachometer to ensure redline isn’t exceeded. In exchange, the airplane leaps off the ground and quickly accelerates to a safe climb speed. 

I ordered one of these props recently, and expect delivery in about 6 to 8 weeks. I’ll plan to have my existing prop overhauled and will keep it on hand to use for particularly long trips. For the price of an hour or so of labor, it will be a quick and easy transition back and forth. 

The second item I put on order is a Sportsman STOL kit. Consisting of a cuff that permanently extends and droops the leading edge of the wing, the kit drastically improves slow flight characteristics, increases the glide ratio, and shortens both takeoff and landing distances. It is something of a legend among the bush pilot and STOL crowd. 

Stene Aviation, the manufacturer of the Sportsman kit, doesn’t presently publish specific performance numbers one can expect with it installed. But an exploration through old, archived versions of its website reveals that data. According to previous versions of the site, the takeoff ground roll is reduced from 618 feet to 420 feet, and the distance to clear a 50-foot obstacle drops from 1,625 feet to only 780 feet. These numbers echo reports from owners, and they are substantial improvements. 

Landing distances are said to be similarly reduced, with the ground roll improving from 458 feet to 380 feet, and the landing distance over a 50-foot obstacle improving from 1,145 feet to 760 feet. No stall speeds are listed, but Vx with the kit is listed as only 48 mph…which is 4 mph lower than the full-flap stall speed of the stock wing. Cruise speed at 7,500 feet msl is said to increase from 121 mph to 126 mph. 

Similarly impressive is the improvement in glide ratio with the kit. Per the data, it nearly doubles from 7:1 to 13:1. In my case, this would enable me to cross Lake Michigan…without supplemental oxygen…while remaining within glide distance to shore the entire way across. The stock wing leaves a window of about 20 minutes in the center of the lake during which neither shore is within glide range. I’m still not certain I’d cross it when the water is dangerously cold, but it’s an option that doesn’t exist now.

Between the prop and the STOL kit, I expect my takeoffs and departure-end obstacle clearance to be significantly less concerning, even when the windsock isn’t telling the complete story of local winds. Yes, a far less expensive alternative to purchasing performance mods is to simply fly more and develop my skills. This is something I plan to do regardless…but the allure and fun of cool performance mods is significant, and I look forward to seeing how the new ones work out.