Dragonfly Tales: Tails, Tips, Tricks, and Treats

From the Towlines column written by Tracy and Lisa, published in Hang Gliding & Paragliding

From Hang Gliding & Paragliding, June 2007
By Tracy Tillman and Lisa Colletti
Photos by Bob Grant

Lisa Colletti and Tracy Tillman
Lisa Colletti and Tracy Tillman

In this month's column, we will discuss some configuration and modification aspects of our Dragonfly tugs, in response to a question that we received from a visiting pilot at our field.

Question of the Month: When I came to fly with your club at Cloud 9 Field, I was very impressed with the climb performance of your tugs and how quiet they are. How did you do it?

Lisa: First, credit must be given to Bobby Bailey and Bill Moyes of LiteFlite in Australia [ref. 1] for the design and manufacture of the Dragonfly. The stock Dragonfly configuration typically uses a 65-horsepower Rotax 582 two-stroke engine, which provides good performance at minimal cost and weight. However, the Rotax two-stroke engines do not provide the higher power output, fuel economy, long-term reliability, and time-between-overhaul (TBO) qualities that are inherent in the Rotax four-stoke engines. Although their four-stoke engines cost more and weigh more, we felt that having more power and reliability would be safer, especially for tandem instruction tows, and that the greater up-front cost would actually be an investment that would be offset in the long term by faster tow turnaround times, greater fuel economy, and fewer rebuilds over time. Less noise generation was something that we did not expect, but found to be a great benefit of the four-stroke engines.

Lisa on Tow in AL12 Sailplane
Photo 1: Lisa on Tow in AL12 Sailplane

Tracy: Use of the four-stroke Rotax engine on the Dragonfly was not our idea. Credit must be given to Russell Brown of Quest Air [ref. 2] for that development. As far as we know, his original 1996 Dragonfly Model A TurboTug was the first Dragonfly to use a Rotax four-stroke engine. We were very impressed when we flew behind it back then, and we were convinced that it was the best way to go for us. Russell has made and supplied most, if not all, of the Rotax four-stoke engine mounts for Dragonflys in the U.S. One of our tugs uses the 80-horsepower Rotax 912 four-stoke engine, and the other uses the Rotax 914 Turbo, both hung on Russell's engine mounts. We usually use the 912 tug for solo hang glider tows, and the 914 TurboTug for tandem hang glider and sailplane tows (see Photo 1). Our 914-powered Dragonfly is a Model A with a reinforced Model B fuselage boom tube, while our 912-powered Dragonfly is a Model B, which came with a reinforced boom tube. LiteFlite is now producing the Model C Dragonfly, which has gained certification overseas [refs. 3 and 4].

Rotax 912 on Dragonfly
Photo 2: Rotax 912 on Dragonfly

Lisa: One thing that we did to both engines to help make them quieter was to add an after-muffler silencer, available from Rotax, part number 978650. Although designed for their two-stroke engines, we have found it to work very well with both of our four-stoke engines, as well. As you can see in Photo 2, we made the exhaust exit point up to the sky, which significantly reduces noise propagation to people on the ground.

Tracy: Not only will that after-muffler silencer configuration work well on tugs powered both by the Rotax four-stroke engines and their two-stroke engines, but a Rotax intake silencer (p/n 825762) can also be added to their two-stoke engines for more noise reduction. For more information about that, you can check out Mike Stratman's article called 'The Stealth Rotax,' which is reprinted in his company's catalog [ref. 5].

Lisa: Another thing that we did to significantly reduce noise and improve thrust was to use propellers custom-designed for our Dragonflys by Lonnie Prince of Prince Aircraft Company [ref. 6]. Whether he uses science, art, or magic, he can really create a great propeller.

78-inch Prince P-Tip Prop on Dragonfly
Photo 3: 78-inch Prince P-Tip Prop on Dragonfly

Tracy: As can be seen in Photo 3, the inboard 60% of our 78-inch-diameter prop is a wide paddle-blade of six-inch chord, while the outboard 40% has a swept leading-edge taper that ends in a curved P-tip. While knowing neither the science nor the art, my guess is (a) that wide inner chord (as often seen on paramotor prop blades) helps to produce more static and low airspeed thrust; (b) the swept leading edge (as seen on high-speed airplane wings) helps to reduce compression at the leading edge and shock-wave formation along the propeller surface, which results in more efficiency and less noise at the higher airspeeds encountered near the tips; and (c) the P-tip helps to eliminate the loud transonic vortex usually produced at the tip and turn it into sub-sonic thrust. In fact, Lonnie claims that his P-tip has the effect of not only reducing noise, but also of adding thrust as if the prop was six inches larger in diameter. If that is the case, we are getting the thrust of an 84-inch propeller from our 78-inch prop ' which is a 22% increase in effective prop disk area ' with much less noise produced because with less diameter, the tip speed is not as close to the speed of sound. Less diameter also helps to reduce stress on the gearbox caused by the prop's moment of inertia.

Lisa: That seems like a pretty educated guess. Maybe you do know something about the science :-).

Tracy: Well, with a little research, we can find that: (a) we want to keep the tip speeds below Mach 0.7 to 0.75 (about 550 to 600 mph) if possible [refs. 7 and 8]; (b) a swept-back leading edge does work to reduce compressibility and shock-wave formation [ref. 9]; and (c) that the P-tip acts as a winglet to control air spillage over the tip to reduce drag, turbulence, and noise, and to increase thrust [ref. 10]. Because winglets reduce induced drag, they have the effect of increasing span and aspect ratio, so that they produce the efficiency of a longer and more efficient wing without it actually being longer [refs. 11 and 12]. Likewise, the benefit of the P-tip is that it produces the thrust of a larger-diameter prop, while maintaining a shorter prop diameter to help keep the tip speed lower. A simplified equation for determining prop speed and Mach number is:[(Engine rpm/redrive ratio) x prop diameter] / 336 = propeller speed in mph. [ref. 13]

For example, the propeller speed on our tugs is [(5800/2.43) x 78] / 336 = 554 mph. The speed of sound at standard 59'F temperature is about 760 mph, so the mach number of our tip speed is 554/760 = Mach 0.71. If we were actually using an 84-inch-diameter blade, our Mach number would be about 0.76, which would result in a greater likelihood of compressibility, shock-wave formation, less efficiency, and more noise generation.

Lisa: The Prince prop is a two-blade prop. What about having more blades?

Tracy: Well, as a kid flying in model airplane contests years ago, I saw that that some speed-plane competitors used counter-weighted single-bladed props, claiming that a single blade is most efficient due to less airflow disturbance encountered by the blade with each revolution. More blades do help to improve efficiency if it allows the diameter to be reduced so that the tips are not spinning so close to the speed of sound. Lonnie's P-tip design helps to resolve that problem, so we can use a large-diameter two-bladed prop to gain more effective prop disk area (not quite that of a helicopter, though ' darn!) with less noise and less loss of efficiency. Single-bladed props are not practical as they become larger and heavier (beyond model airplanes), so a two-bladed prop is probably best.

68-inch Five-bladed Ground-adjustable Prop for Dragonfly
Photo 4: 68-inch Five-bladed Ground-adjustable Prop for Dragonfly

Lisa: What about our 68-inch-diameter five-bladed prop, as shown in Photo 4?

Tracy: We have that prop as a back-up. It is ground-adjustable in pitch, so that we can use it on either of our engines. Note that it has a scimitar-shaped leading edge, which is a good alternative technology for noise reduction. It is almost as quiet as our Prince props, but there are five blades producing noise rather than two. It is a good smooth-running prop, but it just doesn't quite match the thrust produced by our Prince props.

Lisa: How big of a prop can we put on the Dragonfly?

Tracy: Well, the main physical limitations are (a) clearance with the airframe, and (b) the moment of inertia of the prop. We really don't have any more room for a prop with a diameter larger than 78 inches. Moment of inertia is caused by a combination of size, weight, and weight distribution of the prop. The moment-of-inertia limit is listed as 6000 kg/cm2 in the Rotax engine manual. Mike Stratman of California Power Systems provides a great explanation of how to test for propeller moment of inertia in his company's catalog [ref. 14]. The ground-adjustable Ivo Magnum props that we originally used on both of our tugs were fairly heavy and were meant for higher power engines; they exceeded the Rotax moment-of-inertia limit, causing gearbox problems. Our Prince Props are well below the limit.

Lisa: Speaking of gearboxes, readers should know that we put a 914 gear box onto our 912 engine to reduce the tip speeds and noise even more. The stock 80-horsepower 912 comes with a 2.27:1 gearbox, whereas the 100-horsepower 912S and 115-horsepower 914 come with a 2.43:1 gearbox. This change helped to reduce the tips speeds on our 912 by about 7%, so they are the same as that on our 914. Mike Stratman has another article reprinted in his catalog about optimizing gearbox and propeller combinations for two-stoke Rotax engines [ref. 15].

Tracy: Also, you can see in Photo 2 that we mounted two oil coolers back-to-back above the wing, while the radiator is mounted below the wing in front of the engine. We found that the single Rotax radiator (p/n 995697) mounted in that position is fine for both the 912 and 914 engines, while both engines need dual oil coolers due to our slow towing airspeeds, especially for higher tows on hot days. The 912 works great with two smaller Lockwood oil radiators (p/n OILCOOL1), but the 914 needs two large Rotax oil radiators (p/n 886034) to stay cool. We have also found Lockwood's oil thermostat (p/n OILCOOLTH) to be very helpful for getting the oil up to operating temperature before the first tow of the day, and it helps to keep it up to operating temperature during the power-off glide back to the field after each tow. Do you think we should mention anything else?

Two mirrors on Dragonfly
Photo 5: Two mirrors on Dragonfly

Lisa: Yes, the mirrors! For both of our tugs, we put two of the biggest automotive rear-view mirrors that we could find onto both sides of the cockpit, as you can see in Photo 5. Not only is it helpful to have a large viewing area, but it is especially beneficial and safer to easily see what is going on behind the plane, on both sides. This is helpful when tensioning the line, it allows me to see launch assistants on both sides of the glider, I can see wing tip launch assistants on either side for rigid wing launches, I can more easily and quickly see and react if a pilot (the dope on the rope) is getting out of position or locking out on tow, and I can see where you are at the left and right extreme positions on tow when teaching tow-position maneuvers to students during tandem instruction (two dopes on the rope - double trouble!).

Dragonfly Tail View
Photo 6: Dragonfly Tail View

Tracy: Well, thanks, DEAR. I guess that we can finish this month's article with that thought, and a tale of the tail of our Dragonflys. Because of our high climb angles, our planes have a very nose-high attitude on tow. That can cause a lot of stick pressure on tow, so we lowered the leading edge of the horizontal stabilizer by about 1 ' inches on both planes (see Photo 6) for good trim on tow and for the glide back down after tow. When a hang glider pilot is in proper tow position, there is very little stick pressure on tow, and this modification also enables the tug to glide back down to earth power-off at about 40 mph with no stick pressure and positive pitch stability, so that it maintains a 40-mph glide on its own.

Lisa: It is also common on many Dragonflys for the Velcro on the stabilizer gap covers on each side of the vertical fin to not hold after a while. An easy fix for this is to use a large cotter pin on the front, just like a bobby pin, to help hold the gap cover in place at the front (see Photo 6).

Tracy: Well, I guess that telling the tail tale marks the tail end of this month's column. As a reminder, our Towline articles and links to other Sport Pilot-related information are posted on USHPA's 'Sport Pilot' information Web page [ref. 16]. We are happy to receive questions directly from individuals regarding Sport Pilot and other towing-related issues, and encourage you to email us at [email protected] [ref. 17] with your questions. We are always looking for a good new 'Question of the Month'!

References

  1. LiteFlite Pty.Ltd. (manufacturer of the Dragonfly) Web page: http://www.liteflite.com.au
  2. Quest Air Web page: http://www.questairforce.com
  3. 'Interview with Bill Moyes' December 5, 2002 post in {Oz Report}: http://ozreport.com/6.252
  4. 'Bailey Moyes Dragonfly C Model' October 2, 2003 post in {Oz Report}: http://ozreport.com/7.258
  5. 'Part #46: The Stealth Rotax' by Mike Stratman, {The Proper Care and Feeding of the Rotax Motor} series in the California Power Systems Catalog. CPS Web site: http://www.800-airwolf.com
  6. Prince Aircraft Company Web page: www.princeaircraft.com
  7. 'Transonic Airfoils for Propellers' in {Propeller Dynamics} by Stuart Sherlock: http://www.supercoolprops.com/articles/transonic.php
  8. 'Design of high-Mach propeller tips. The Profile Efficiency Problem in Propeller Dynamics' by Stuart Sherlock: http://www.supercoolprops.com/articles/propstips.php
  9. 'Simple Sweep Theory' in {Applied Aerodynamics: A Digital Textbook} by Desktop Aerodynamics, January, 2007: http://www.desktopaero.com/appliedaero/potential3d/sweeptheory.html
  10. 'About Us' Prince Aircraft Company Web page: http://www.princeaircraft.com/html/carbon_fiber_brochure.HTM
  11. 'The Design of Winglets for High-Performance Sailplanes' by Mark Maumer, 2001: http://www.mandhsoaring.com/articles/Winglet_Design.pdf
  12. Wills Wing Winglets Q&A: http://willswing.com/Articles/Article.asp?reqArticleName=winglets
  13. 'Prop Quiz: Speed of Sound Calculation' June 30, 2002 discussion on [email protected]
  14. 'Part #31: Measuring Prop Inertia' by Mike Stratman, {The Proper Care and Feeding of the Rotax Motor Part} series in the California Power Systems Catalog. CPS Web site: http://www.800-airwolf.com
  15. 'Part #6: A Buyer's Guide for Selecting a Propeller' by Mike Stratman, {The Proper Care and Feeding of the Rotax Motor Part} series in the California Power Systems Catalog. CPS Web site: http://www.800-airwolf.com
  16. 'Sport Pilot' USHPA Web page: http://ushpa.org/page/sport-pilot
  17. Tracy and Lisa's 'Cloud 9 Sport Aviation' email address: [email protected]
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