Twin Topics
Twin Topics is a monthly column devoted to the operation, maintenance, and ownership of Cessna 300- and 400- series piston twins. It is edited by Mike Busch, CPA’s staff twin expert and the lead instructor in CPA’s twin courses. Mike is a regular contributor to CPA Magazine, as well as editor-in-chief of AVweb, the Internet’s aviation magazine and news service (http://www.avweb.com). He also is the owner of a pristine 1979 Cessna T310R which he maintains almost entirely himself. Vortex Generators: Band-Aids or Magic?
A recent visit to Boundary Layer Research revealed that there’s a lot more to those little wing and tail bumps than meets the eye, and that they’re not just for twins anymore!
by Mike Busch
BEECH BEECH BEECH CESSNA Bonanza 33, 35, 36, 36TC Duke 60, A60, B60 Twin Bonanza 50 340, 340A Baron 55, 58, 58TC, 58P CESSNA Baron 55, 58 402C 120/140 CESSNA 414, 414A Skywagon 180, 185 310G-R,T310R, 320 414AW, 414AW-V 310, 310A–R, T310P–R 335, 340, 340A 421C, 421CW 320D–F 414, 414A 425 (Conquest I) 335, 340, 340A 402B, 402C 401, 401A, 401B 421C 402, 402A, 402B, 402C PIPER 414, 414A Aztec PA23 421, 421A, 421B, 421C Cub J-3, PA11 PIPER Super Cruiser PA12 Navajo PA31-310, 325C/R Super Cub PA18 Colemill Panther I & II Pacer PA20 Chieftan PA31-350, T1020 Tri-Pacer PA22 Seneca II/III/IV PA34 MAULE Super Cruiser PA12, PA14 M-4, M-5, M-7, MX-7 Super Cub PA18 AGPLANES deHAVILLAND Ayres Thrush Beaver Mk I AGPLANES Ayres Thrush, Air Tractor, Cessna 188 AgWagon, Piper PA36 Brave, Dromader, Weatherly
Table 1 — Vortex generator STCs…who offers what? increase. Furthermore, RAM nating ZFW) while the MA STC Bob Desroche, who undoubtedly doesn’t presently have an STC offered no ZFW increase. I con- has more test-pilot time certify- for the 310 or T310. So RAM’s cluded that Boundary Layer Re- ing VGs on light twins than any kits were doubly out of the run- search’s STC for the T310R was man on earth. ning as far as I was concerned. both less expensive and better, I’d also been looking for That left Boundary Layer so I decided to go that route. an excuse to fly up to Everett’s Research and Micro Aerody- Rather than order the kit Paine Field (PAE) because that’s namics, both of whom offer VG and install it myself, I decided to where Boeing builds its wide- kits for the T310R (and for most fly the airplane up to Everett, bodies (747, 767, 777) and I’ve other twin Cessna models as Washington, and have BLR do long wanted to take a tour of that well). Both kits looked good, the installation. Although the facility. So I made an appoint- and both offered comparable VG installation is simple enough ment with BLR for the first week gross weight increases. But (one might even go so far as to in August (while everyone else BLR’s was priced $500 less call it idiot-proof) and can be was off at Oshkosh) and had a ($2,450 vs. $2,950) and offered easily done in one day, going up glorious flight from SMX to slightly better numbers than to Everett would give me the PAE in an easy four hours. MA’s. The clincher was that the chance to learn how these little BLR STC increased the Zero bumps do their aerodynamic Fuel Weight of the T310R by magic, and to do a little flying 385 pounds (effectively elimi- with the master, BLR president Hello, BLR Monday morning at 0800, I taxied the airplane to Hangar C-75 where Boundary Layer Research makes its home. I was greeted by BLR’s office man- ager Jean Wieser and introduced to BLR founder and president Bob Desroche. Bob in turn in- troduced me to Jay Falatko, BLR’s resident FAA-Designated Engineering Representative (DER) and a former Boeing aerodynamicist, and to Dale Lundgren who would be assist- ing Jay with the installation of my VG kit. BLR’s spotless hangar contained Bob’s Beech Duke Figure 2 — Jean and Jay apply wing template using a taut string. which was in the process of be- ing fitted with prototypes of BLR’s new wet wingtips (aux tanks), and a Super Cub be- longing to Bob’s wife Monika that bristled with an eye- catching menagerie of VGs and body strakes. (Monika is an ac- complished pilot and vice- president of BLR.) Bob and Jay pulled Monika’s Super Cub out of the hangar and pushed in my Cessna 310. Within minutes, Jay, Dale and Jean were busily at work on my VG installation.
Installing The Kit My VG kit included about 90 one-inch-long vortex gen- erator tabs machined from a tee- shaped aluminum extrusion and prepainted to match the air- plane’s primary paint color— Figure 3 — Jay installs vertical stabilizer template along a skin lap. white Imron in my case. VGs (That giant VG below Jay’s right arm is my VOR antenna!) located over trim stripes may be complete set of peel-and-stick pulled taut between two refer- painted with touch up paint after templates with little rectangular ence points and the template is installation, if desired, although cutouts where each VG is to go. aligned with the string. the five-color paint scheme on In many cases, such as the verti- Once the templates are in my 310 is so complex that I’ll cal stabilizer and stub wings on position, it’s simply a matter of probably just leave my VGs my 310, the templates are posi- roughening the paint at each VG white. tioned along a nearby skin lap. location with a Scotchbrite pad Positioning the VGs cor- In the case of the outboard wing (or a chisel in the case of the rectly is important, but the kit section of the 310, no conven- 310’s wing-walk area), and then makes that easy by providing a ient skin lap exists so a string is gluing the VG tab in place using How VGs Work With the installation well underway, I asked Bob Desroche and Jay Falatko if they could explain to me the theory behind how vortex generators reduce stall speeds and Vmc. What en- sued was a cram course in Aero- dynamics 101 which I found illuminating and fascinating. VGs are boundary layer control devices, so it isn’t sur- prising that to understand how they work you first need to know something about the boundary layer. I’d certainly heard the term before, but never really understood its significance. Bob and Jay were glad to fill me in, Figure 4 — Nacelle strakes bolt on. and here’s what I learned. When an airplane is in flight, we usually think in terms the provided two-part adhesive detachable capillary tube con- of air passing over the top of the (Loctite 330). necting the instrument to an air wing at the airspeed of the air- Most of the twin kits also temperature probe on the belly craft. But it turns out that the come with a pair of nacelle of the aircraft, and it’s almost viscosity of the air and the fric- strakes that act like large VGs impossible to remove this in- tion of the wing surface cause for the wing-to-nacelle interface. strument from the aircraft with- the air molecules in contact with Another peel-and-stick template out destroying the capillary tube. the wing to adhere to its surface is used to locate mounting holes (I’d love to get my hands on the and therefore have zero velocity. that are drilled in the sides of the yo-yo who came up with that Air molecules slightly farther nacelles. The strakes are then design!) So a technician from away from the wing surface will simply bolted in place. the local instrument shop had to be slowed due to friction with The BLR kit also comes come over to open up the in- the zero-velocity molecules but with a re-marked dial face for strument in the aircraft, install won’t be completely stopped. As the airspeed indicator, and in- the new dial, and recheck the we move still farther away from stalling that turned out to be the instrument calibration. That the wing surface, the air mole- only difficult part of the job. turned out to be a two-hour job. cules will be slowed less and Unfortunately, my airplane came less, until at some distance from equipped with a “true airspeed” the surface a point is reached indicator that has a long non- where the air molecules are not
Figure 5 — Boundary layer changes from laminar to turbulent flow as it moves aft along the wing. take place. (Another easily-seen increasingly slow-moving and example of laminar and turbu- stagnant toward the trailing edge lent flow can be seen by watch- of the wing. It is this ing the smoke rise from a lighted “aerodynamically dead” sub- cigarette in a draft-free room.) layer that allows airflow to sepa- It turns out that laminar rate and the wing to stall. flow is a good-news/bad-news If we could find a way to situation. The good news is that energize this sublayer, flow laminar flow provides greatly separation would be supressed reduced drag compared to tur- and the onset of stall delayed. Figure 6 — Laminar vs turbulent. bulent flow. The bad news is This is precisely what vortex that laminar flow permits the generators do. Each VG creates boundary layer to separate easily a pencil-thin tornado-like cone slowed at all. The layer of air from the wing surface at high of swirling air that stimulates from the surface of the wing to angles of attack. That’s why so- and organizes the turbulent flow the point where there is no called “laminar flow airfoils” of the boundary layer on the aft measurable slowing of the air is (which are designed to move the portion of the wing. The swirl of known as the boundary layer. transition to turbulent flow fur- the vortices pull fast-moving air Near the leading edge of ther aft) tend to provide low down through the boundary the wing, the boundary layer is drag at cruise but nasty stall layer into close proximity to the very thin, and the air molecules characteristics. wing surface, energizing the in it move smoothly and parallel Turbulent flow in the previously-dead air there. The to the wing surface. This is boundary layer produces more result is a wing that can fly at known as laminar flow. But as drag, but is much more resistant significantly higher angles of the airflow progresses aft from to separation (and therefore to attack before the onset of the leading edge, the boundary stalling). However, even in areas boundary layer separation, and layer becomes progressively of turbulent flow, there tends to can therefore achieve a signifi- thicker and more unstable, and be a thin sub-layer of laminar cantly higher maximum lift coef- transitions to turbulent flow in flow in the immediate vicinity of ficient. which intermixing of faster and the wing surface which becomes When mounted on the slower air molecules starts to
Figure 7 — By energizing the boundary layer, VGs allow the airfoil to operate at higher angles-of-attack without airflow separation. wings, VGs reduce stall speed and increase climb capability. Original With VGs Difference When mounted on the vertical tail, they increase rudder effec- Ramp Weight 5535 5720 +185 tiveness and lower Vmc. Gross Takeoff Weight 5500 5684 +184 Weight Increases Zero Fuel Weight 5015 5400 +385 The performance im- provements resulting from the Landing Weight 5400 5400 No Change VG installation on my T310R are shown in Table 2. While the Minimum Control (Vmc) 80 70 -10 numbers mostly speak for them- selves, a few explanations are Stall, Clean (Vs) 79 75 -4 probably in order. The gross weight increase Stall, Dirty (Vso) 72 69 -3 offered by the VG STC is a di- rect result of the reduction in Liftoff Speed (Vlof) 85 75 -10 stall speed. Under the FARs, light twins are required to have an engine-out rate-of-climb (in Approach Speed (Vref) 94 87 -7 feet/minute) equal to .027 times the square of Vso (in knots). If Table 2 — Performance specifications for Cessna T310R, before and you lower Vso by a few knots, after BLR VG kit. (Weights shown in pounds, speeds shown in knots.) the required single-engine ROC goes down. At the same time, ones. For an STC to obtain a be fuel. By increasing the ZFW the VGs actually increase single- landing weight increase would to 5400 lbs (same as landing engine ROC by increasing the involve a landing gear beef-up weight), the VG kit effectively maximum lift coefficient of the and a series of very costly “drop makes ZFW disappear, because wings at high angles-of-attack. tests” to prove that the aircraft if you loaded the aircraft to Thus, the aircraft now has more could handle the additional ZFW you’d have to land on single-engine climb performance weight without structural dam- fumes (or overweight)! than the regs require. The solu- age. BLR actually did this for tion: increase the gross weight! the Piper Chieftain, but it re- Airspeeds Landing weight is a differ- quired strut modifications and ent story. It has structural impli- new torque links, and was quite The most significant air- cations, not just aerodynamic expensive. It’s therefore under- speed change resulting from the standable why none of the twin VG installation is the virtual Cessna VG STCs offer a landing elimination of Vmc. Techni- weight increase. cally, Vmc still exists, but at 70 So if you take off at the knots loss of control occurs at a new higher maximum takeoff lower airspeed than the airplane weight, better plan on flying far will fly unless it’s extraordinar- enough to burn of a few hundred ily light. pounds of fuel…or land gently Bob Desroche told me a and don’t tell anyone! funny story from his early VG Zero fuel weight only days with Paul Robertson when comes into play when you want they were getting the original to carry a maximum payload for Cessna 340 STC. Since Vmc is a short distance. For example, predicated on failure of the criti- on a stock T310R with a 3900 cal (left) engine, Robertson pound empty weight, it says that originally applied VGs only to of the 1600 lbs of useful load, no the left side of the 340’s vertical Figure 8 — My re-marked air- more than 1115 lbs may be pas- stabilizer. Les Berven of the speed dial. Vmc is now below Vs. sengers and cargo; the rest must Seattle FSDO did those original certification flights, and after numerous left engine cuts, he was bubbling over about the reduction of Vmc. Then Les tried something totally unex- pected: he cut the right engine, and discovered (to everyone’s astonishment) that Vmc oc- curred at a higher airspeed…the right engine had become critical! Needless to say, Robertson quickly added VGs to the right side of the vertical tail and re- flew the tests! While the reduction in Vmc gets all the glory, the 10- knot reduction in liftoff speed and 7-knot reduction in ap- proach speed makes a big differ- ence in everyday flying. Twin Cessnas are not especially good short-field airplanes, so these improvements are especially welcome. If you’ve been paying close attention, you might have Figure 10 — BLR founder, president and chief test pilot Bob Desroche noticed an apparent discrepancy in a typical pose: on the phone with a customer. in the airspeed figures in Table 2. How can approach speed same question, and the answer is Bob explained that while (Vref) be reduced by 7 knots straightforward: the published the VGs do produce some drag, when the dirty stall speed (Vso) Vso is certified at maximum they also reduce drag by reduc- has been reduced by only 3 takeoff weight (5684 lbs with ing the thickness of the bound- knots? After all, Vref is by defi- the VGs), while Vref is based on ary layer on the aft portion of the nition 1.3 times Vso. I had the maximum landing weight (5400 wing. The net result is about a lbs) at which Vso is lower. Natu- “push” with no measurable deg- rally, at lighter weights, ap- radation in cruise speed. proach speeds should be even Here’s where proper less than the published 87 knot placement of VGs is critical, Jay Vref. chimed in. If they’re placed too far forward, they’ll hasten the Disadvantages? transition from laminar to tur- bulent flow and therefore in- Okay, I thought, this all crease drag. On the other hand, makes sense. But I still had the if they’re placed too far aft, their feeling that there must be some effectiveness will be compro- downside. After all, my daddy mised. The trick is to mount the always taught me that there’s no VGs right at the boundary such thing as a free lunch. layer’s transition zone from For instance, those 90 laminar to turbulent flow. VGs stick up into the airflow How about icing, I asked? and must produce some drag, Won’t the VGs pick up ice? right? Won’t that slow the air- Not unless they’re tall Figure 9 — VGs are applied to both plane down at cruise, I queried? enough to poke up through the sides of the vertical tail! boundary layer, Bob replied. That’s one reason why the VGs are sized to a height of about 80% of the boundary layer thickness. The VGs have been tested extensively in icing con- ditions during FAA certification, and do not pick up ice except possibly when flying in freezing rain or supercooled drizzle drops—conditions in which no portion of the airframe is com- pletely immune from icing.
Why So Pricey?
While I had Bob’s ear, I Figure 11 — It doesn't look very different, but it flies like a new wing! figured I might as well go for broke and ask him the $2,500 popular models like the Cessna tude at 85 knots…no faster.” question: why do VG kits cost so 310, but you’d be wrong. Sepa- Bob warned me that this would much when the materials cost is rate STCs (and flight tests) are feel at first like an unnatural act. clearly not very great? Of required for the “tuna tank” He was right…it took all course, I already knew the an- models, the narrow-chord ai- the faith and backpressure I swer—it costs a lot to get the leron models, wide-chord ai- could muster, and the airplane FAA to certify these things—but leron models, the long-nose R- (with three people aboard) broke Bob gave me some details that model, and the turbocharged ground early and climbed at an helped put things into true per- models. So the market for each awesome deck angle with the spective. of those STCs is still pretty VSI nailed at 2,000 FPM. Thirty He said that it can easily small. To make matters worse, seconds later, we were at pattern cost between $250,000 and the popular models like Barons altitude and hadn’t even crossed $500,000 to get a VG kit certi- and Twin Cessnas have two or the departure end of the runway fied. Why so much? In essence, three companies competing for yet. the FAA requires that almost all the limited market. Bob directed me to a of the airplane’s original flight It’s a tough business. practice area over Puget Sound testing be repeated. For instance, Work the numbers. I think I’ll and had me fly a series of steep for twins that were certificated stick to writing. turns, slow flight exercises, and for known-icing (i.e., most of stalls. I found the airplane rock them), the icing tests have to be solid at indicated airspeeds so reflown (which means finding Lets Go Flying! low that they’d have freaked me sufficiently bad natural icing With the VGs installed, out before. We flew a series of condition, flying behind a spray the airspeed dial changed, and low-speed maneuvers with the plane, or gluing styrofoam the logbooks and 337 forms stall warning horn blaring con- “shapes” to the unbooted areas signed, it was time to go flying. tinuously, yet roll and pitch of the aircraft to simulate ice). Bob likes to go up with new VG control remained crisp and re- For singles, the spin tests have to customers for 45 minutes or so sponsive. be reflown (which means fitting to give them a checkout in their Then we did a series of the aircraft with a spin chute and new-and-improved airplane be- stalls, with and without power, water ballast). fore turning them loose. It didn’t clean and dirty, level and turn- To make matters worse, take long for me to see why. ing. It was really interesting: as the market for most of these We taxied out to PAE’s the airplane eventually ap- costly-to-get STCs is depress- 9000-foot main runway, did our proached a stall (with indicated ingly small. BLR’s first VG STC runup, and Bob briefed me for airspeeds down in the 60s), it was for the Beech Duke, of the takeoff. “I want you to rotate would start buffeting like a which only about 500 are flying. at 75 knots—the new Vmc plus bucking bronco, yet with no loss You might think the situation five—and climb to pattern alti- of altitude. Bob explained that would be a lot better with more this was the airflow separating Bob thinks that similar re- over the stub wings (between the sults could be achieved on the fuselage and nacelles), but that Cessna 182, and expects that the nacelle strakes created a BLR will start working on the large vortex that acts like a stall STC for the Skylane in a few fence and prevents the stall from months. At this point, Bob is on propagating outboard of the na- the lookout for a few 182 own- celles. All I know is that even ers who’d be willing to make with the stall warning horn dis- their airplanes available for VG coneected and the airspeed indi- certification work. (He’ll need cator covered up, you’d still both straight-tail and swept-tail have to be comatose to get the airplanes, with and without airplane into an unintentional leading-edge cuffs.) If you’re a stall. 182 owner and think you might We were running out of be interested, give Bob a call at Figure 12 — BLR’s Spring Thing time, so we decided to skip the 1-800-257-4847 or drop him an installed on the nose compartment Vmc demonstration and head e-mail at