Tinamou
Revisiting and refreshing an RC Soaring Digest classic from 2008.
Tinamous are strictly neotropical birds— not found outside the tropics, and nowhere but in the Americas. They are large-bodied, almost tailless birds with slender necks and small heads, maybe 30% larger than a bobwhite. They fly but prefer to walk or run, and they’re secretive. — Backyard Nature
Well, so much for the name. My Tinamou is not secretive, as I have plans free for the asking and it launches, flies and lands very well! I’ve always been intrigued by flying wings, especially swept flying wings. Six years ago I happened upon an article about a flying wing built and flown by Herk Stokely. See On The Wing: A Comparison of Two Tailless RC-HLGs linked in the Resources section at the end of this article.
I wanted a bigger flying wing than Herk’s little HLG wing and chose to enlarge his model to 100" wingspan. So I had [the now defunct] CompuFoam cut the wing cores using white bead foam as I intended to use balsa sheeting for the skin. They did a great job cutting the cores, too!
As all this was taking place I got married, bought a house, and was promoted at work. So best laid plans ended up in the rafters of the garage for five years! The end of 2007 I had some free time on my hands and needed a Montana winter project. So I got the foam cores down from the rafters, emailed Herk and started to think about what I needed to do to complete this project.
Originally I wanted to launch her via electric assist and Herk suggested that I reduce the amount of twist or washout from 8 degrees to 4.5 due to the higher speed of the model.
Well, after five years and just like the weather, I changed my mind about how to launch the model and wanted to use a hi-start and to have a thermal duration model, thus I should have used the original amount of twist. This begged the question, “how do you determine the proper amount of twist”?
This is when I was led to the late Dr. Panknin, who devised a complex but fairly simple formula with the use of a calculator to determine the required twist. I have since taken his work and made a companion spreadsheet to Sailplane Calc called Flying Wing Calc (again, see Resources) that will easily calculate the twist.
The Design
Herk wanted a nice flying thermal HLG flying wing and he was successful. He chose the SD7037 airfoil for the center section of the wing as this airfoil was proven to have good characteristics. The SD8020 was chosen for the outer portion of the wing as it’s a fully symmetrical airfoil and its sole purpose is to act as the tail of the wing.
A flying wing has a tail just like your typical tailed model; it’s just that the tail has a very short moment, that is the distance from the wings quarter chord location to the tails quarter chord location. Remember from your basic aerodynamics class that the horizontal tail is used to counteract the negative pitching moment of the main wing by applying a down force, thus pitching the leading edge of the wing up.
The same thing needs to be accomplished with a flying wing. In a swept wing this is done by twisting the outer portion of the wings trailing edge up in relation to the leading edge, thus this applies a down balancing force to the lifting section of the wing.
This is different than with a reflexed airfoil that raises the trailing edge of the airfoil across the entire span, creating a very low negative pitching moment. Very little, if any, twist is required if a reflexed airfoil is used.
Some may ask why not use a reflexed airfoil over the entire span. Well, what I’ve learned during this process is that a reflexed airfoil will suffer in its thermalling or lifting ability. I believe that reflexed airfoils are suited more to slope soaring, electrics, unswept plank type wings or where higher speed is desired. I was looking for a thermal duration model.
Herk then chose a planform, sweepback angle, taper ratio and aspect ratio for his wing. With all of this information he was able place this information into Dr. Panknin’s formulas and determine the required twist angle that was required to be built into the wing. It’s important to note that Dr. Panknin’s formulas require a linear twist. The first 10" of my wing are pure SD7037 with no transition to the SD8020 as I wanted a good lifting section in the center of the wing.
One of the many things I enjoy about aviation is that I am always learning. During this endeavor I learned about airfoil pitching moments, airfoil zero lift angles, The Middle Effect, the difference between reflexed and non-reflexed airfoils, and how to determine wing twist to longitudinally balance a flying wing. I also learned that a flying wing can have flaps, and very effective ones at that!
The Build
The basic building of the wing is quite simple. I took the cores that were professionally cut and chose to use 1/16" balsa wood as the skin with 0.2 ounce carbon fiber in between for torsional strength. The spar is .007 carbon tape top and bottom. I used polyurethane Gorilla glue to attach the balsa to the cores via my home made vacuum bag. I can’t take credit for this method as it was done by Terry Brox and an excellent review is on his website (see Resources).
Proof of how strong his wing is can be found by clicking on Misc. Stuff. I thought this would be plenty strong for what I needed. The entire center section top and bottom is fiberglassed with three layers of 0.75 ounce cloth starting where the bat tail ends at the trailing edge. Each layer is progressively smaller in span.
The radio gear is mounted in a very simple manner. The battery and receiver are cut out of the top of the foam near the leading edge. The servos are mounted underneath the wing in pockets cut out of the foam; they are centered on their respective surfaces.
I placed hollow pushrod tubes in the wing prior to applying the sheeting so the servo wires were easy to route to the center of the wing. Here you can also see where the ’chute servo and drogue ’chute are placed. More on the ’chute later.
I couldn’t transport the wing at 100" so I made removable wingtips. I built a basswood box that could receive a rectangular carbon fiber rod and simply tape the panels together prior to flight. It’s worked flawlessly.
The carbon rod is six inches long and tapers are cut into the ends of the box to transfer the loads to the wing skins. I chose the length of the outer panel on what I estimated that I needed for elevon size versus the inner section that would be used for flaps.
The photo above, left, shows the completed outer tip. I used 1/8" hard balsa ribs at the root and tip. The tiplets are 1/8" balsa and fiberglassed to the end of this panel. For sizing I use the TLAR method — That Looks About Right. The black line is .007 carbon fiber tape as a spar and the red tube is for the servo wires. This panel is ready to be sheeted.
To the right of that shows an outer wing panel in the vacuum bag. I used a Space Bag (storage type bag) and a MityVac hand held brake bleed kit to remove the air. I easily obtained 7lbs per square inch of pressure overnight. However, the Space Bag won’t hold much more with this setup. I’ve used the MityVac system with a different bag and easily held 15 inches overnight.
Herk suggested a skeg in the aft center of the wing to keep the flaps off the ground during landing and something to hold on to during launch. This has also proven to be very effective and simple.
I used a center 1/8" plywood rib to join the wing’s halves to and the skeg is cut as one piece with this rib which increases the skeg’s strength.
There is something called The Middle Effect. In short this effect is the loss of lift at the center of a swept wing due to the detrimental interaction of vortices at the center of the wing. To counteract this, a so called ‘bat tail’ helps in compensating for this loss of lift. The bat tail is shown in the photos above. You can see the raised triangular area built up over the center section, that’s the bat tail.
You may read more about it the effect at On The Wing: The Middle Effect (see Resources, below).
There are two tow hooks placed approximately five inches each side of the center of the root. The left photo on the next page shows the hardwood blocks I inserted in the foam core. The block is full depth so the loads are transferred to the main wing sheeting.
I made a bridle that attaches to the main hi-start chute, it has two rings that attach to the wings hooks and another ring that slides on the harness (string) and this ring is attached to the hi-start parachute.
The wings tow hooks are placed 1/2" forward of the balance location which is 4% MAC forward of the balance point. This seems too far forward but any further aft and the launch suffered in height and would periodically pop off during launch. The launches have been a non-event, perfectly straight up the hi- start. The initial launch is what concerned me most prior to the first flight but the lack of sleep was unwarranted.
The ’Chute
This hobby is supposed to be fun so I added a drogue ’chute. I saw this on a friend’s Graupner SB-13 flying wing, so I thought the idea would be great to try on my wing. There was room for a ’chute in the area that made up the bat tail. So I simply made a hatch with a release mechanism. The aft part of the hatch is held in place with two plywood tabs that slip under the top sheeting and the front of the hatch has a small balsa block glued underneath with a hole drilled in it that accepts a plastic pushrod.
At the end of the ’chute I tied a loop of string and inserted it from left-to-right through a hole in the center plywood bulkhead and when the pushrod is inserted this holds the ’chute in place. See the photo above right. The pushrod goes through the loop and extends past the hole in the rib into the balsa block in the hatch, thus holding the hatch in place.
Since the ’chute fits snugly in its compartment when the hatch is closed, it applies a little pressure to the hatch and when released by the servo this allows the front edge to pop up allowing airflow to blow the hatch off the wing. The ’chute is attached to the hatch thus it deploys the ’chute.
The servo has two actuations: 1. During the first half of the movement of the servo the pushrod pulls towards the nose of the wing releasing the hatch, but not far enough to expose the hole cut into the rib, as this would release the ’chute. 2. The second half of the actuation exposes the hole in the rib, releasing the loop of string and allowing the ’chute to be emergency jettisoned.
Also shown in this photo is the root rib and skeg that’s cut as the one piece of 1/8” plywood.The ’chute is somewhat effective but nowhere close to effective as the flaps. The ’chute is big in the “Wow!” factor, though! A video demonstration can be found in Resources, below.
Curtis! Why did you name this beautiful model, that flies with the grace and beauty of an Albatross, after a shy semi-flightless jungle bird that probably has the same glide ratio as your transmitter? — Herk Stokely
Tinamou will go straight up the launch with very little correction required. She doesn’t seem to zoom off the launch as well as other thermal duration models. She thermals well, but detecting lift will take a lot of practice as the tail moment is short and it’s difficult to see changes in pitch.
There is a bit of yawing during flight and turns but it’s negligible.
The stall is quite sharp and will catch you by surprise if you crank too much up elevator in a turn. It takes about one-and-a-half turns for the recovery.
I’ve tried wing fences and at first I thought there was a significant difference in handling, but I’ve since removed them and haven’t noticed a significant change with or without them except the stall isn’t as abrupt with the fences.
At times I wish I had a little more elevator control in order to crank her tighter around a turn without feeling I may stall her. Finesse is key in flying her, more so than with my other thermal duration ships as a light hand on the elevator is required. This may be solved with moving the balance point further aft and adding the flaps to work as elevators. Thus more flight testing is in order.
The model was balanced at 8% static margin for the first few flights and later moved back to 3% which proved to be too far aft during the low speed high angle of attack landing portion of the flight. She really got overly pitch sensitive during landing.
I’ve since moved the balance point back to 8% as she launches, soars and lands very well.
The landings are easy. She really slows down well with the large flap area and with more practice spot landings are quite possible. F3J type landings have been obtainable from the first flight, however the USA style of precision landings will take more practice.
Changes I’d Make If I Were to Build Another
Since the twist was reduced for an electric model I chose to increase the wing sweep from 22.5 degrees to 25 degrees at the quarter chord point which effectively increased the amount of twist. This reduced the overall wingspan to 95". I’m currently flying with approximately 1/8" of reflex to maintain a nice thermal speed.
So if I were to build another Tinamou:
- I’d change the amount of twist from the 4.5 degrees to the 8 degrees Herk originally used.
- I’d also change where I have the wings joined, making the elevons longer and the flaps shorter, perhaps a 20% increase in elevon span.
- I’d probably leave the ’chute out as it took quite a while to get it rigged properly and the flaps are way more effective.
- I also would like to try different non- reflexed airfoils, such as the excellent Drela airfoils and perhaps a little bit higher aspect ratio.
But these would be the only changes. I’m so very pleased with how easily she built and how well she flies. Herk Stokely and Dr. Panknin really know what they are doing!
Perhaps in the near future I may make a removable fuselage pod that houses an electric motor in the nose. The pod can be held in place by slipping over the nose then attached to the skeg. It’d be a very simple on/off arrangement.
Credits
There are a lot of people I’d like to thank, beginning with Herk Stokely who patiently answered my daily emails for months. Also Jeff Vrba, Jim Cooney, Shawn Keller, Norm Masters and the folks at RCGroups. Photos and videos courtesy of Jim Cooney, Shawn Keller, Jeff Vrba and myself.
©2008, 2022 Curtis Suter
Resources
- Backyard Nature — “Tinamous are strictly neotropical birds — not found outside the tropics, and nowhere but in the Americas…”
- On The Wing: A Comparison of Two Tailless RC-HLGs — “We recently received a packet of information from Andrew MacDonald, our Australian correspondent, in which was contained…”
- Flying Wing Calc — “Calculate balance point and wing twist using Dr. Panknin’s formulas. Now automatically estimates winglet size and calculate a wing with three tapers! June, 2016, version…”
- Building a Light Set of Wings by Terry Brox — “Welcome to my way of wing construction. I would like it to be understood that I am not saying this is the right way…”
- On The Wing: The Middle Effect — “In the March 1994 issue of RCSD, we discussed Dr. Ing. Ferdinando Gale’s Ubara, a swept wing free flight HLG…”
- Video Demonstration of ’Chute — “This is a demonstration of how the Drogue Chute operates on my flying wing…”.
- RCGroups Forum — Lots of additional information and discussion on the Tinamou.
- Tinamou Video Compilation — “Here’s a compilation of flight videos of my thermal duration flying wing…”
- Tailwind Gliders — my personal website.
- Tinamou — The original PDF of this article as it was published in the RC Soaring Digest in October of 2008.
This article originally appeared in the October, 2008 issue of RC Soaring Digest. All images by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
