One of the most common questions I hear from readers at shows such as Sun ‘n Fun, AirVenture and the old AOPA conventions has long been, “Why won’t my airplane perform to book specs?”
The majority of folks seem to feel the manufacturers are simply exaggerating their products’ performance, in some cases, wildly so. Perhaps the most allegedly abused parameter is cruise performance, the most important to many pilots. Over 40 years of attending aviation conventions and airshows, I’ve heard that complaint probably 100 times. That may have been a valid protest on older airplanes, but it’s less often true today. Still, any deficiency in cruise often seems glaring, as speed is perhaps the primary reason many pilots fly.
Then and now, it’s difficult to reproduce the conditions the aircraft companies enjoy when they establish the specs, but you can improve your airplane’s performance, no matter what its drag coefficient. Even if you fly something with the aerodynamic sophistication of a boxing glove, it’s possible to make it fly faster.
We’re obviously not talking about speed mods. Some of those work, some don’t, and we won’t attempt to pass judgment on the best and the worst. Opinions on a specific mod vary from ridiculous exaggerations to “a total waste of money.” One owner of a highly modified Centurion commented to me that the only thing his package of speed mods increased was “empty weight.”
Perhaps surprisingly, there are at least a dozen things you can do at minimal or no cost that are virtually guaranteed to make your airplane fly faster. There’s no magic involved here, and I guarantee you won’t see any one trick here that will increase cruise by five knots all by itself. Nothing you’re about to read will let your Super Cub run away from a G36 Bonanza unless both are on water and the Cub is on floats.
The good news is that the increased performance is free or relatively inexpensive. If you do everything right, you may realize a total speed improvement of 5% or more. A 150-knot airplane may suddenly step along at 157 to 160 knots. The aforementioned Bonanza may trip along at an honest 175 knots. There’s no bad news.
Pretty obviously, you’ll need to make certain the engine and prop are operating at maximum efficiency before you start thinking about your cruise performance. If you fly behind fixed-pitch blades where rpm is the only measure of power, have the prop strobed to determine that what you’re seeing is what you’re getting. A constant-speed prop may need balancing to generate book thrust, not normally an expensive process, but one that pays dividends in both performance and smoother operation.
What we’re seeking is incremental speed, typically, small improvements that may be difficult to measure without flight test instrumentation. You’re dreaming if you think you can measure the speed improvement of each technique listed here. What you’re after is a collective result.
If you hope to make a comparison between your airplane’s current performance and the finished product, you’ll need to know your starting point. To that end, fly your bird on a smooth day as you normally would and see how well it performs. Write down the altitude, temperature, power setting, approximate gross weight and GPS speed in two opposite directions. If you don’t have a GPS, buy a local or sectional chart and measure your own short course between two easily recognizable checkpoints. Here in Southern California, we’re fortunate to have two, easily recognizable piers along the coast, the Huntington Beach and Newport Beach piers, that happen to be a numerically convenient 5.67 nm apart.
There are a few fallacies to either technique, GPS or a measured course, but both methods will provide at least a base of information.
The three primary goals of any program to increase speed are to reduce weight within reasonable limits, improve the drag signature, and maximize power and thrust.
Weight reduction has always been a difficult goal to achieve. In fact, it’s a task David Copperfield would find challenging on his best day. Manufacturers sometimes spend huge amounts of money trying to reduce empty weight so they can improve payload. In fact, poor full fuel payload is often the second most common complaint among aircraft owners. Six-place aircraft rarely offer sufficient payload to let you fill all six buckets, and pilots of four-seaters have the same problem carrying a string quartet.
Unnecessary weight is anathema to optimum cruise performance. Any airplane flies faster with less weight to lift, and one of the biggest offenders is excess fuel onboard. Roy LoPresti, father of the Grumman Cheetah and Tiger, the Mooney 201 and 231, and the LoPresti Fury, used to suggest that pilots could sometimes gain a knot or two simply by following the airline practice of not tankering fuel. It takes fuel to carry fuel, and there’s no logical reason to load aboard a five-hour supply of avgas/jet fuel for a one-hour flight. In other words, there’s usually no need to fill the tanks after every flight. A Cessna TTx, for example, can carry 612 pounds (102 gallons) total, but typically burns about 120 pounds (20 gallons) per hour. That means you could fly with only 50 gallons for a 300 nm out-and-back and leave 312 pounds of fuel in the truck.
Aircraft owners don’t make the task easier by carrying excess amounts of miscellaneous stuff in the airplane’s seatback pockets and baggage area. Engineer LoPresti suggested that pilots go through their airplane once every three to six months and eliminate some of the miscellaneous stuff they have accumulated over time and may not need on every flight.
Roy advised you could expect about a 1% cruise improvement for every 100 pounds you offloaded. It’s true you probably won’t find a full 100 pounds of oil, old charts, life vests, tiedown kits, Jepp approach plates, a raft, extra sweaters and gloves, an old camera, an emergency shotgun, flares, a tent and a medical kit. If you’re planning a long cross-country trip over remote terrain, some of those items may become vital, but they’re probably not necessary for local flights. Anything you absolutely must carry should be stored as far aft in the cabin as possible. Once again, this is a game of tiny improvements. If you find 50 pounds to leave behind (and I found 42 pounds 20 years ago), that’s three-quarters of a knot for a 150-knot airplane.
Improving Drag Signature
If you own a four-seater and occasionally carry four folks, make certain all rear vents are closed when no one is in back. Passengers may tend to open vents and leave them open. Similarly, close your vents up front if you don’t need the air. This reduces parasite drag of cabin air being sucked out into the airstream. You’d be surprised how much drag outflow air from cabin vents can cause.
One myth that needs to be disposed of early on is the viability of a good wax job. A heavy coat of Blue Coral or Simoniz isn’t liable to make any difference in performance, though any of the high-tech car waxes very well may make your airplane look great. Unfortunately, aesthetics is the only benefit you can expect. Automotive waxes aren’t designed for drag reduction since, by definition, most cars don’t normally operate in a speed regime much above 75 to 80 mph except on the autobahn, and drag isn’t significant at that velocity. Subject most automotive waxes to the friction of a 140- to 170-knot airflow, and the wax will be stripped away in short order.
It’s probably true that a good, hard carnauba wax or an aircraft sealant, properly applied in the right places, can improve cruise slightly, but remember, inexpensive speed improvement is a game of fractions of a knot at a time.
You certainly shouldn’t believe everything you read on the bottom of wax cans, but a hard carnauba wax applied to the first 30% to 40% of the wing and tail chord may reduce drag very slightly. (Airflow is usually separated beyond 40% of chord.) I use a sealant called Knot Wax 2, developed by NASA and available from LoPresti Aviation in Sebastian, Florida. I’m convinced it contributes perhaps a ½ to one knot to cruise of my airplane.
Measuring speed improvements of one knot is practically impossible to verify, but Knot Wax 2 seemed to deliver on its promise. Remember that your goal in waxing an airplane isn’t just a shinier, better-looking bird. You’re hoping for fractionally more speed, and that’s what I saw on my airplane. Your results may vary.
Rig can be a major factor in optimizing speed. Most mechanics don’t bother to check this during the annual, but it may not be that expensive to measure and set correctly. An aileron, elevator or flap that’s out of rig can be hanging out in the airflow and costing you several knots. If you fly a retractable, make certain the gear doors are closing tightly in the gear-up position. If there are no gear doors, be sure the wheels are against the up limits in the retracted position.
A prop balance is another speed hedge that usually isn’t checked during the yearly inspection. Again, it’s a relatively inexpensive item to monitor and adjust, and a fringe benefit is smoother (and sometimes quieter) operation. Buy or borrow a strobe gauge to check your tachometer for accurate readouts.
Make certain everything that opens and closes on your airplane does so efficiently. Cowl flaps, cabin or baggage doors, fairings, vents or anything else that interrupts the airstream will cost you speed if they’re not properly secured. On Pipers, assure that both latches on the entry door are engaged before takeoff.
Don’t neglect the surfaces you can’t see conveniently, like the bottom of the wings on a low-wing and the top of the wings on a high-wing. Roll a creeper under your airplane and determine if all belly panels, inspection plates or anything else held down with screws or DZUS fasteners is loose. Anything interrupting the relative wind, whether it’s a flow of air from inside a wing or fuselage or an actual access panel, will cost you speed.
Maximizing Power And Thrust
Any airplane will always fly faster if it’s flown as close to the limit as possible, because an aft CG improves the drag signature by reducing the download on the tail. Again, LoPresti was a stickler about this and used to preach that you should try to keep the weight as far aft as possible, obviously consistent with an allowable CG position.
Manufacturers employ pilots with thousands of hours in type, and they know to slide their seat full back during cruise on autopilot to push the CG aft, again, obviously within limits. They’re also very precise with power and mixture adjustments, and won’t settle for “about 100 degrees rich of peak.” Also, flight-test aircraft fly with very precise instrumentation that can measure every parameter to the finest increment, so make certain your power instruments are reading correctly. If your manifold pressure gauge or tachometer isn’t providing good information, you’ll have no way to set the appropriate power combination.
Be conscientious about cruise altitude. Regardless of any skepticism you may have about book figures, determine the exact altitude that’s listed for optimum cruise and select that height as often as possible. Remember, we’re talking density altitude, not pressure altitude. You’ll need to factor in the temperature to determine the optimum density height. Fly as high as practical with tailwinds, lower with headwinds.
In the old days, the straightest line over a short distance was two points. Today, it seems everyone navigates with GPS, and Great Circle routes are becoming the rule for VFR and often applicable to IFR, as well. That’s a convenient and simple way to operate, but don’t automatically assume it’s the most efficient. Remember, the shortest distance and the shortest time aren’t necessarily synonymous. As counterintuitive as it sounds, you may be able to score a shorter time en route by flying a slightly longer distance, but in more favorable winds.
That’s called pressure pattern flying, and many airlines that employ computerized flight plans use it all the time. The technique is usually reserved for longer distances, but you may find it will work just fine on legs as short as 500 to 700 nm. With the emergence of real-time wind information in the cockpit, you may be able to find significant tailwinds close enough to your planned route to justify a slight diversion.
Keep in mind that if you only realize 10 knots more speed, that’s a significant financial benefit. If you operate a 160- to 180-knot airplane 100 hours a year, and suddenly discover you’re now flying a 170- to 190-knot airplane, you’ve just reduced your operating costs by about 6%. Extrapolated to a 120-knot airplane that now magically flies at 130 knots, and you’ve realized an 8% improvement.
Perhaps even better, you may now be able to fly right by the guy in the hangar down the row with the same model that used to outrun you.