The need for speed
Kirsten, a private pilot from Georgia, writes: “What tricks do air racers use to make their planes go faster?” We have got a plane load of tricks for plying the sky at ever greater speeds, but they can be broken down broadly into four categories: Adding power, reducing drag, changing aerodynamics and reducing weight.
Option one add more
power: The first thing that most people think of when they want to go faster is to simply add more power. Whilst this works, more horsepower is not the most efficient option. In his excellent book The Art of
Cross-Country Air Racing, Sport Air Racing League
Chairman Mike Thompson says, “increasing power is the least effective way to improve speed. The increase in horsepower necessary to increase your speed by one mph is amazing and the closer you get to the wing’s limiting speed, the harder it is to power your way to faster speeds.”
So, do not get to thinking you can just double your horsepower and double your speed. It does not work like that. Not that there is anything wrong with more power just for the sheer joy of it, but not only does it take a herd of additional horses to increase airspeed, but each extra horse lightens the wallet whilst adding to the weight of the plane and as we will see later, weight is an enemy of speed.
Option two reduce drag: Of course, there are two flavours of drag: Parasitic drag and induced drag. Induced drag is a side effect of lift and is largely inherent in the
design of the plane. Parasitic drag, on the other hand comes from what Thompson calls “various lumps and bumps on the outside of the aircraft.” Smoothing out those various lumps and bumps or getting rid of them completely is the essence of most ‘speed modifications.’ Some of these mods cost virtually nothing, whilst others are expensive. Starting on the cheap side, a clean plane is a fast plane because parasitic drag can come from dirt, grime and oil on the belly of the plane, as well as squashed bug carcasses on the leading edges of the wings and tail. If you do not think that some crunched up bug bits could make a difference, consider that some competition sailplanes have ‘bug wiper’ devices to keep their wings clean from insect remains in flight. That is because smashed insect remains on your leading edges act like miniature speed brakes.
The next step above clean is waxed. Wax works on a wing like ice works on a sidewalk. So, nothing beats a good wax job and waxing the wings, belly, nose and tail reduces drag enough to cause a significant increase in speed. The Comparative Aircraft Flight Efficiency Foundation actually undertook a scientific study of waxing and found it added a whopping 2.8 mph to its Mooney M20E! Just from the reduced drag of increased air flow, thanks to smoother surfaces.
Of course, parasitic drag also comes from objects and accessories on the airframe, such as antennas. It is often said the only thing you can put on an airplane that is worse than a brick is a cylinder and most antennas are cylinders. How big a difference could that possibly make? My Race 53 has a ‘blade’ com antenna that has a 250 mph drag force of only 0.85 pounds. Compare that to a standard whip antenna that has a drag force of 2.70 pounds at the same speed! Of course, Race 53’s wings would shear right off if I somehow managed to go that fast and the blade is more than double the weight at 20 ounces instead of eight, so it might not be doing me as much good as I would like to believe. Meanwhile, if you don’t mind spending some money and if you’re in aviation, you probably don’t, or like me, you have become de-sensitised to it, there is no end of fairings that can smooth the flow of air over the body of the plane, reducing drag and adding speed. The company Laminar Flow Systems, for instance, makes a landing gear, brake and strut faring system called Fancy Pants, that it claims ups the speed of older Piper Cherokees by a whopping 10 miles per hour, all simply due to reduced drag. That is a sizable percentage increase in speed and a guaranteed trophy when racing the guy in a similar plane without Fancy Pants.
Other drag reducers include aileron and flap gap seals, hinge fairings, exhaust pipe ‘wrap fairings,’ wing root fairings, and dorsal fin seals. Some airplanes can pick up yet more speed by installing re-designed windshields or nose bowls. A budget version of the fairing family is ‘gap tape,’ commonly used by the sailplane crowd and also an air racer favourite. It is used to tape up any cracks or less than perfectly smooth seams that might create even the smallest amount of drag. Taping up the various joints of the cowl to prevent ‘plume drag’ is commonplace amongst racers, as is taping the joints of fairings. I used it for a time, but found that when I stripped it off, my paint came with it!
One part-time racer had advice from more knowledgeable friends literally written on his plane, showing him where to place sail plane gap tape right before the race. Many racers place gap tape over seams, joints and access panels right before the race. Small reductions in drag add up quickly when it comes to improving speed. However, Race 53’s paint is too old and all attempts at using various gap tapes have resulted in chips of paint coming off the plane when the tape is removed. So, by simply reducing parasite drag, even without boosting power, we are flying faster.
Option three play with aerodynamics
Now, whilst boosting engine power is not the most effective way to boost speed, an important component of raw airplane power is the propeller that is lashed to the front of the engine. The pitch of that prop makes a bigger difference than you would think when it comes to speed. After my engine was rebuilt, the same propeller gave me crazy-fast cruise speed. Or it did once I could get the plane into the air, which took way too much runway. I had to have the propeller re-pitched, sacrificing some of that beautiful speed for the ability to get into the air.
As with all things aerodynamic, there are always trade-offs. Naturally, a constant speed propeller is the best choice, as you can adapt it for changing needs from the comfort of the cockpit. The next best option is a ground adjustable prop, with the fixed pitch compromise propellers like mine being the last choice in the speed game. However, it is more than just pitch, as propeller shape matters when it comes to speed. For instance, re-equipping a Cessna 182’s stock prop with a three-blade Hartzell Scimitar can add four mph.
I added what my family calls Voodoo Propeller Tape (technically a vortelator kit) to my propeller that increases RPM by modifying airflow over inefficient areas of the propeller, keeping the air attached longer. Think of it as a vortex generator for the propeller’s airfoil.
Another aerodynamic trick is to shift the centre of gravity (CG) as far aft as possible (within the envelope) to make the plane go faster, which I grant you is counter intuitive. Wouldn’t an aft CG make the plane nose-high, turning it into its own speed brake? Well, an aft CG requires a smaller angle of attack to maintain altitude. This is because, quoting author Richie Lengel’s Everything Explained book, there is ‘less elevator down force to drag around.’ Of course, a lower angle of attack means less lift is required to get the job done and with less lift, there is less induced drag. Less drag, more speed. So, if that is the case, why aren’t all planes built tail-heavy to go fast? Because the trade-off is reduced stability. Still, as most races take only as long as a lunch, shifting CG aft for just a little while is worth the reduction in control for some. I know one racer who joked about sneaking out onto the ramp at night and stuffing two one-gallon milk jugs of water in his tail cone. He called it his Creamland Dairy STC. Or maybe it was not a joke. That plane is pretty damn fast. Of course, for the experimental crowd, they can play with wing shape, wing size, or install winglets to reduce the stubborn induced drag from wing tip vortices; buying ever more speed by toying with the forces and factors that rule flight.
Option four shave weight
The race teams of the tragically discontinued Red Bull Air Race World Championship knew the importance of weight.The Red Bull rules stipulated a minimum weight of just under 1,540 pounds for plane, fuel and pilot and most of the teams achieved this. Why was this important to them? Well, for an acrobatic sport like Red Bull, linear acceleration is one key to victory, as aggressive acrobatic manoeuvres bleed off speed and lighter aircraft can accelerate more rapidly to regain that lost speed than heavier ones can. Equally important, lighter planes require less lift to stay in the air and that means they generate less of that nasty induced drag that is otherwise so hard to get rid of.
To minimise weight, SARL pilots strip their planes of everything not necessary for the race. Baggage, jackets, spare oil, tow bars and even fuel sumps are heaped in race central before the green flag, whilst fuel is calculated to the barest minimum; all to keep light to fly fast.
Thinking beyond the plane
Over the years,I have spent a fair amount of time and money on speed mods for Race 53. I have removed canopy cover snaps, yanked venturi tubes and replaced my strut-based landing light having my mechanics install lower drag in-wing models. I dumped the ‘jam jar’ rotating beacon in favour of wingtip strobes, switched to low-drag antennas, upgraded my engine and installed the vortelator kit on the propeller.
However, I am now in the midst of a modification that will not only be the most cost effective of all, but promises to buy me the most speed, perhaps more speed than all my other mods combined and it also has to do with reducing induced drag by decreasing weight. However, not the weight of the horse in the race, but rather, the weight of the jockey. Yes, I am on a diet. So far, I have lost 33 pounds, with about 10 more to go to reach my ‘ideal’ weight for my height and frame. It is slow going as speed modifications go, but it has an extra benefit: I knew that air racing was good for my soul and now, I am discovering, it is also good for my health.
William E. Dubois is a NAFI Master Ground Instructor, commercial pilot, two-time US National Champion air racer and a World Speed Record Holder.
He blogs his personal flying adventures at www.PlaneTales.net.
Watch the Reno National Championship Air Races video below
https://www.youtube.com/ watch?v=qNWqyR5ZK_8