For Safe Landings On Two Planets
The 2013 National Air and Space Museum Trophy Winners.
- By The Editors
- Air & Space magazine, April 2013
NASA / JPL-CalTech
(Page 4 of 5)
In an effort to slim the 747 down, I gave my project engineers a weight budget and asked them to pursue lower target weights than they had been coming in with. I thought we were doing pretty well when weight again became a focus of concern on the part of upper management. This time it wasn’t just Boeing Commercial Airplanes that was worried; it was T [Thornton] Wilson, president of all Boeing. [Bill Allen was now chairman of the board]. He assembled a team of top engineers and told them to perform a weight audit on the 747. Managing them was Charles Brewster, a hard-nosed old-timer. A confidant of Wilson’s, Brewster had the president’s ear and didn’t hesitate to use the power this gave him.
Resisting the temptation to feel threatened (“Hey, these guys are undercutting my autonomy”), I chose instead to see this management-imposed review as an opportunity (“My team can certainly use the scrutiny of more engineers, so let’s welcome them”). I ordered my people to support this audit to the best of their ability even as they continued their own efforts to slim down the 747.
The inputs we received from the audit team were pretty much the same ideas my team was coming up with. There was only one audit team proposal that I disagreed with and refused out of hand.
We had given the 747 a triple-slotted flap to keep approach speeds low. We knew we had to have high-lift capabilities for the freighter version of the airplane, which would be landing with heavier payloads than passenger 747s. This requirement led us to do some innovative thinking.
The challenge is that jetliner wings are optimized to high-speed flight. This is great when you’re cruising near the speed of sound, but not during takeoffs and landings when you instead want a wing that’s geared to lower speeds. In general, the way to convert a high-speed wing into one suited to lower speeds is to increase its area and camber.
Increasing the wing’s area reduces its wing loading, which is how much weight each square foot of wing supports. This has the effect of reducing the airplane’s stall speed, the speed at which the airplane can no longer fly and instead begins to fall. Lower stall speeds are particularly desirable during landing, because they mean a slower touchdown, shorter stopping distance, and generally safer operations.
Changing the wing’s camber (that is, how arched or curved it is) also helps reduce landing speeds. So how do you change a wing in flight? If you’ve looked out the window during a jetliner flight, you may have seen the answer. Jets have flaps that extend aft and downward from the rear of the wing. In addition, they have “leading-edge devices” that extend from the front of the wing. These two modifications together significantly increase the wing’s area and camber.
The enormous three-segment flap system did the trick, yielding low approach and landing speeds as well as excellent flight characteristics throughout this critical phase of flight.