P Over D

Lift to drag ratio or L/D is the figure that many crucial aerodynamic criteria are measured by. When soon-to-be pilots study for the FAA written exam, they pore over diagrams of L/D curves and where the most efficient speeds are. They see how glide ratio, climb speeds and best endurance speeds are determined. They see how L/D varies with angle of attack and altitude. They see all this information presented before them and yet are only getting half of what they need to know operationally. The number we’ll be exploring today is the P/D, or payload to drag ratio.

First, what does L/D mean aerodynamically? Quite simple; it means that you have the lowest amount of drag for your given weight. Hence, lift (holding up your weight) over drag. If your L/D went from 10 to 5, it would mean that you are producing more drag for your given weight (whether it is parasite or induced drag depends on the type of speed change). Speaking in terms of the airplane itself, L/D is the determining factor for efficiency. Sailplanes have extremely high lift to drag ratios and are often touted as being the most aerodynamically efficient winged craft on the planet. Sailplane-type aerodynamic efficiency means that making 180 degree turns with no engine at 300 feet above the ground and staying aloft in thermals for hours is completely normal. On the other hand (and there always is another hand), the downside to measuring an airplane’s worth by its L/D ican limit its usefulness. While a sailplane can play in ridge lift for the better part of a day, it’s doing it at the expense of two very important variables: payload and speed.

P/D is the transportation equivalent of the aerodynamic L/D. While everyone talks lovingly of L/D, they almost never ask the question I always ask: “What are you lifting?” It’s similar to me telling a big rig driver that I’m more efficient because I get 100mpg on my moped and he only gets 6mpg. Sure I’ll feel like a eco-hero, but the missing part of the equation is what is my moped can carry and how fast. I weigh around 200lbs, or 0.1 tons, so we can assume that as the moped’s payload. The 18 wheeler on the other hand has a payload of roughly 48,000 lbs or 24 tons. My moped (wide open throttle and downhill with a tailwind) can hit 40mph while the truck can easily cruise over 70mph. What would you rather have deliver 24 tons of groceries to your store from a warehouse site 200 miles away? One truck that takes just under 3 hours, or 240 mopeds that will take 7.5 hours?

The same principle applies to aircraft. Most people buy airplanes for speed. They want to save time over driving or taking the airlines. They want to be on their own schedule. They sometimes just want to be able to say “I cruise at Mach 9.5” to their friends who all cruise at Mach 9.2. Very few people buy powered airplanes for a distinct lack of speed. There is also another camp that values payload over blistering speed. The ability to lift a lot of people or a lot of cargo is critical for airlines and charters in order to make a profit. It counts for general aviation as well since quite frankly, many of the standard “4 place” airplanes are really “2 people plus a couple bags” craft with respect to payload and full tanks. Many planes have been wrecked and too many people killed because pilots failed to fully understand the significance of this nebulous limitation.

Time to compare L/D to P/D. The Schwiezer SGS 2-32 glider has a reported L/D somewhere between 32/1 and 36/1. For it’s gross weight of 1340 lbs, it will be producing between 38 and 42 lbs of drag. Incredibly low numbers thanks in part to it’s low drag fuselage and high aspect ratio wing. This max L/D is achieved at a speed of 45 knots, which is very slow for transportation considerations and totally unusable for any powered airplane’s cruising flight. Assuming 2 large men weighing 250lbs each are on board, the payload is 500 lbs. P/D is between 13.9 and 15.6, which are the real numbers we want. For each pound of drag, the aircraft is carrying an average of 14 lbs of payload.

P/D ratios that high will not be commonplace in powered aircraft simply because they are much heavier in structure and can’t carry as large payload with respect to their maximum weight. A very heavy-lifter, the Boeing 747-400F has an average L/D of 17.5 at 35,000 feet and normal cruise speed, while maximum payload is listed as 248,000 lbs. Assuming an late-in-cruise weight of 700,000 lbs, drag would be 40,000 lbs and P/D therefore is 6.2. Closer to home for many of us, the Cessna 172 has an L/D in the region of 9.0. At 2400 lbs gross weight, the resultant amount of drag is 267 lbs. Assuming 3 people are on board who weigh 200 lbs each, the P/D works out to 2.25. Most light aircraft will end up reasonably close to this number, varying by the number of seats on board.

Why is P/D important to designers? Because while purist pilots may love airplanes, airlines quite frankly couldn’t care less about the aircraft itself as long as it does 3 things:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Turns a profit.

If you wish to build military fighter/attack aircraft, the desires are slightly different:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Carries a lot of weapons.

And if you want to build a business jet…well, you get the idea. The only category of aircraft where payload can be ignored somewhat is the personal pleasure craft, such as certain experimental and ultralight aircraft. They aren’t made to make money, they’re made to enrich the builder/owner’s life. On the other hand, if an airliner has a beautiful shape that inspires poets worldwide, sets speed records and adds alleged prestige to the company, that won’t save it from being cast aside in favor of something more pedestrian but economical (and yes, I was referring to that supersonic work of art, the Concorde).

The more an aircraft weighs empty equates to more structure and materials which directly leads to more cost. The more an aircraft weighs empty, the less it can carry in fuel and payload if it’s maximum takeoff weight is not also adjusted upwards. So not only will it cost more to buy, but it produces less per trip in terms of revenue. From an economic standpoint, an airplane that has lower than average aerodynamic virtues but exceptional carrying capabilities will be more desirable than a craft with a high L/D but limited payload. This explains the ongoing interest with composites to reduce airframe weight without sacrificing strength.

P/D is just another number that can help you analyze performance in a practical manner. For instance, if you’re looking to buy an airplane for personal use, consider who and what you want to take with you. A super efficient powered glider that has only one seat is pointless if your goal is to carry your family on short trips. Likewise, if you find an airplane that has a very high cruise speed but it requires you to lay flat like a Mercury astronaut, that won’t make the kids very happy, especially since there are no windows to see out of. Like everything else in aviation, its a balance with your needs as the fulcrum.

Next time, we’ll talk about all the “best” speeds and why they aren’t speeds.

There Is Never* Too Much Airplane

* Let’s get the disclaimer out the way up front. When I say “never”, I’m not talking about your fringe elements. You know the guy who somehow managed to buy a functioning SR-71 for trips from Tamiami to Beale. Nor am I talking about the person who has 50 hours in a Cessna 152 and buys a P-51D because he’s “always wanted one”. I’m ruling these aircraft choices out because they are pretty rare. This article is meant for the pilot who wants to upgrade from a trainer to a faster/larger aircraft in the same class and category and is intimidated by the change in cruise speed or engine size.

Part 1: Speed

“Speed is relative. If he’s doing 600 and you’re doing 600 what’s the difference? Zero.”     ~ Brigadier General Robin Olds

When sitting around hangars, the talk occasionally brings up some guy who allegedly went from a Cherokee to an Arrow and got so far behind the airplane that he was just contacting departure at the same time he was beginning his initial descent to the destination. And the talk will progress to how complex the new airplane is and how much power it has, which inevitably leads to someone belting out “It’s the same old thing, it was waaaaaay too much airplane for him.” At which point I sigh and hold in what I really want to say.

What I really want to say is “That’s nonsense.” Simple and to the point. Sure there may be some truth in not taking someone who had an intro flight in a Skycatcher and suggesting that they do the rest of their training in a T-38. But the chances of that happening are very, very remote. It doesn’t help pilots to keep them in slow airplanes for hundreds of hours anymore than it helps to teach them how to fly in said T-38. How can I say this with a straight face? Go to the FARs and look at Part 23.49c which regulates stall speed for all single engine aircraft (we’re ignoring twins in this discussion). No aircraft can stall above 61 knots in landing configuration unless an equivalent level of safety is demonstrated with respect to occupant restraint. This stall speed important because it’s a benchmark for any certified single engine aircraft.

Example: Bill Generic wants to upgrade from a Cessna 172 to a Piper Saratoga. Aerodynamically, it’s not a huge leap. In terms of systems of course, it is a completely different aircraft and if Bill doesn’t get transition training, he has a few cotter pins loose. But once Bill finishes training and goes out on his first solo cross countries, he may end up blasting past intersections or call on the wrong frequency because he’s not keeping up mentally with his progress and there’s no instructor/mentor next to him to help coach. The easy way for Bill to fix all that is to simply pull the power back. No he won’t drop out of the sky. No he won’t be hanging on the edge of a stall. Since all singles are created equal in that they cannot exceed 61 knots Vso, it means you can cruise any single between 100 and 120 knots without any superhuman effort at all.

Will the faster airplane like loafing along so slowly? It may not handle exceptionally crisp and depending on how far back Bill has to pull back the power, the engine may start getting warm if he tries this at 2000 feet over Death Valley. But done with agreeable weather conditions, this is a great way for him to get used to progressively faster speeds. It may take 3 or more trips for he becomes comfortable spanning the gap between 110 and 170 knots. Or he may be fine after one leg of continuous increases in speed. But the important part is that he can adapt at his own pace. The throttle doesn’t control power, its controlling time.

The ability to control time means that while doing transition training and even afterwards, a pilot can get used to the checklist items and internalize procedures. Your aircraft upgrade may have a turbocharger or oxygen system or cowl flaps or even extra fuel tanks. More time gives you a cushion for making sure these systems are working the way they should and that you understand why they’re doing what they’re doing. Waypoints passing too quickly and your map (some of us aren’t totally paperless yet) can’t get turned to the proper side fast enough? Pull back the throttle/time controller and slow things down.

Landing and approaches are another place where the 61 knot rule levels the playing field.  Surely you practiced rectangular patterns at altitude, so making the base and final turns shouldn’t be an issue but all the same, give yourself more time in the form of a slightly wider and longer pattern. On downwind, fly whatever was recommended but 90-100 knots is a safe range (1.47 to 1.63 times Vso) where you have some wiggle room in case your aircraft bleeds speed quickly when flaps or gear come down. If that happens, use the time controller in reverse…push the throttle up so that the bleed rate slows and you have more time to adapt to the aircraft’s idiosyncracies. The standard 1.3 Vso approach speed for the 61 knot aircraft comes out to 79 knots (just say 80). Flying final at 80 knots is an easy and safe target if for some reason everything the instructor taught in transition leaked out of your ears. If you’re being vectored for an approach and the controller wants you to hold 170 knots to the marker and you aren’t comfortable doing that yet, tell them you are unable. They don’t need to know why and if they do ask, mention something about monitoring a fluctuating sonidecimeter indexer. By the time they figure out you’re full of it, you’ll be making the second turnoff.

Part 2: Power

Power is relative too. In my view, too much is made of horsepower being the primary indicator of performance. The FAA categorizes any aircraft with over 200hp as high performance. Because of this, you will often hear the same people who griped about “too much airplane” complaining that a certain airplane had way too much power. While there are cases when this is true, 90% of the time you can never have too much power. High performance is what you make of it, what you wish to measure and what you’re used to.

For a real life example, take a Rockwell Commander 114 and a Pitts S-2C. Both use a 540 cubic inch, 260hp engine and both are technically high performance, but that’s where the similarities end. The Pitts can fit 2 people very snugly and climb at 2900 fpm. The Commander can fit 4 in leather-bound comfort and climb at just under 1100 fpm. The Commander has a 1000nm long-range cruise while the Pitts is limited to 247nm. We could go on and on but the point is that power gives you radically different performance based on what you want to do with it. The Commander is intended to carry people places, the Pitts is made to go up, have some fun and come back within a relatively short time. Which one is high performance? Depends on what you want to do. Flying an advanced aerobatic sequence complete with a pull-push-pull humpty and snap rolls on a 45 downline? The Commander is going to be a letdown. Need to take 3 people nonstop from Teterboro to Nashville in IFR conditions? The Pitts would not be a good choice. What if payload is the determining factor in what you count as high-performance?  Looking at a much larger aircraft, the Cessna Caravan, has 675 shaft hp from its turboprop engine, but because of its rugged design the aircraft max cruises at 186 knots. By comparison the Cirrus SR22T has less than half the power (315hp) but max cruises at 214 knots. But while the Cirrus can only hold 4 people, the Caravan can carry up to 14. If lifting payload is the performance factor your type of flying depends on, the Caravan is the clear winner. And what about your own experience? What you have flown in the past and what you are flying currently has a lot to do with this perception. If you had a PC-12 and are downgrading to a Commander, you may not consider it to be high performance by comparison. If you are coming from an MX-2, then maybe the Pitts isn’t such a wild ride. But if you’re upgrading from a Diamond DA20, you’re most likely going to be wowed by either aircraft. Performance is just as relative as power.

Perhaps the regs should focus on acceleration, climb rate and cruise speed instead of power for a definition of high-performance. This is probably what the FAA intended people to focus on all along but we ended up getting fixated on the semantics of 200 vs 201 hp. Here is an example of why horsepower can be a dead-end. Take an airplane with a pretty anemic power to weight ratio of 16 to 1, meaning there are 16 pounds of aircraft per horsepower. With a 100hp engine, this fictional airplane will weigh 1600 pounds. But what happens when we scale everything up by a factor of 3? The horsepower explodes to 300…definitely high performance by federal standards. But the weight also balloons to 4800 pounds. And in case you didn’t notice, the power to weight didn’t change either which means acceleration and climb is going to be pretty similar (provided the wing loading remained constant). Glancing back at our earlier examples, we’ll compare the difference in power loading for the Commander and Pitts. The Commander has a 12.5lb/hp loading while the Pitts has 6.6lbs/hp. That’s a massive difference in the amount of weight each horsepower has to work with and why the performance figures are so radically different for the same horsepower engine.

Part 3: The Moral

Yes, upgrading in aircraft will take time and money. Get transition training, talk to pilots and owners of the same aircraft first. Learn the little quirks that aren’t written in the operating manual. Study the systems and know little things like how to start the engine. Nothing makes a passenger reevaluate their friendship with you like watching you fumbling around with throttles, mixture knobs and primers on the ramp and saying “It normally doesn’t do this.” Things like starting the engine are and knowing how the fuel system works are pure study items. The things that you can’t learn from a book are your own comfort zones. The only way to learn that is by actually doing it in increments.

Hearing people repeat over and over how hard it is to keep up with fast airplanes and how so much horsepower will ball you up like a Bf109 in a crosswind does not boost confidence. But remember that they’re just repeating what they’ve heard. If flying high performance aircraft was so hard, nobody but astronauts would be doing it and the last time I looked, there’s plenty of non-astronauts in Meridians, Stationairs, Bonanzas and the like. It will just take a little getting used to. Be kind to yourself and don’t expect to go full-tilt right out the gate. Remember, nobody goes 220mph out of turn 4 at Talladega their first time in a race car either.