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Jul 16, 2017, 10:28 PM
Ultrabatics enthusiast
lifthunter's Avatar
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Discussion

Small Hill Light Wind Aerobatic Slope Glider


I fly slope gliders on a small hill with generally very light winds (6-8 mph), and my goal is to fly as acrobatically as possible. As anyone who has tried will admit, this is not an easy goal to achieve. I am sure there are other modelers faced with this situation, but little seems to have been written about it. It is my hope that this thread will become an open forum for modelers struggling with this challenge and searching for advice - or just interested.

To my mind, there are two basic areas where progress is required to fly acrobatically on a small hill with light wind: (1) a purpose-designed aerobatic glider with the precise wing loading (WL) required and (2) flying techniques that properly manage the glider's energy (speed + altitude).

It may be appropriate to create two threads – one for each of these areas of discussion. For now, I will try just this one thread.

To kick things off, I thought it might be of interest if I described how I designed my current purpose-designed aerobatic slope glider.

For my part, I believe a fully aerobatic slope glider should:
- be of EPP foam construction for durability and ease of construction and repair;
- have ailerons, 180 degree throw, full-flying stabilizer, and rudder control;
- have large control surfaces for good control authority at low speed;
- and have a semi-symmetrical airfoil so decent inverted flight is possible.

In my experience, wing loading (WL), airfoil, and low drag are key for a light lift aerobatic slope glider. Each hill shape/wind speed combination seems to have a unique range for viable WL. Any WL outside that range - either above or below - doesn't perform very well.

My 16 oz Le Fish will not fly acrobatically on my hill most of the time. I have a Libelle DLG and a Alula-TREK that fly pretty well but are not truly aerobatic gliders. Their wing loadings are:
Le Fish WL: 5.0 oz/sq ft
Libelle WL: 4.4 oz/sq ft
Alula WL: 3.5 oz/sq ft

Based on the way my Libelle and Alula fly on my hill, I decided that I need a glider with a WL of approx 4.3 oz/sq ft. After an extensive search, I was not able to find a suitable glider anywhere. So, last fall, I decided to design a glider from first principles and build it myself. The project took me approx 8 months.

The gliders that come closest to meeting my requirements are the Le Fish and Justin Gafford's Buzz. I purchased a Buzz variant from Justin. Both gliders use the SB96V airfoil which, as will be discussed later, is the wrong airfoil for my hill - in my opinion. I used the Le Fish and Buzz designs as my baseline.

I started by using the XFOIL program to investigate glider airfoils operating at low Reynolds Number. I investigated the SB96V's performance at Reynolds Numbers in the range 63000 to 94000. I also investigated other airfoils commonly used on aerobatic gliders. According to the XFOIL analysis - which is admittedly theoretical - none of the other airfoils seemed to perform any better than the SB96V. Based on a hint in the RCGroups thread "How to Build a Swiss Fish" post 1873 and post 1877, I started investigating the TP96 airfoil.

Link to post 1873: https://www.rcgroups.com/forums/show...s-Fish/page125
Link to post 1877: https://www.rcgroups.com/forums/show...s-Fish/page126

To my surprise, the TP96-1% is theoretically approx 10% more "efficient" than the SB96V in the performance envelope that my glider needs to operate in. By efficient I mean airfoil Lift/Drag ratio. Also, the TP96 airfoil is much thinner than the SB96V airfoil so, for a given wing area, a TP96 wing is much lighter with less drag than a SB96 wing. This breakthrough allowed me to solve the two biggest problems I was facing - reducing weight and reducing drag. I adopted the TP96-1% airfoil in my design.

My other basic design inputs were:
- Reference Velocity: 15 mph (same as the Libelle DLG)
- Overall Wingspan: 45 inches
- Wing Base Chord (including aileron): 9.5 inches
- Wing Tip Chord (including aileron): 7.69 inches

I discovered during my airfoil analysis with XFOIL that an airfoil's coefficient of drag decreases as the Reynolds Number at which it operates increases. I had begun to suspect that minimizing drag was very important. So, I chose these wide wing chords to maximize the Reynolds Number at which the wing operates. I will not know if this wing plan was a good choice until I have more flying time on the glider.

These design inputs, taken together, dictate an AUW of 11.5 oz. Based on my experience with the Le Fish and Buzz gliders, I believed I could achieve this AUW, so I started drawing up the glider.

The actual glider came in at an AUW of 9.8 oz which gives a WL of 3.7 oz/sq ft. I exceeded my design goals. I may find that I need to add a small amount of ballast! I can live with that. Photos of the glider are attached. I have flown the glider a few times, and its performance seems to be “in the ballpark”. I will need more flying time on it to know whether my design decisions were sound.

I have named this glider "Kestrel-TP".
Last edited by lifthunter; Jul 24, 2017 at 04:56 PM. Reason: Add link
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Jul 16, 2017, 10:55 PM
VTPR & Slope Aerobatics
surfimp's Avatar
I love what you're doing here and hope your glider works out! Those Austin-area slopes have always been a tough nut to crack, and trying to fly them aerobatically - even more so. I wish you the best of luck.

I assume you're using flaperon mixing combined with always-on snapflaps. If not, give it a shot - a small amount of snapflap can really make the airfoil come alive, and I know, from my personal correspondence with Thierry Platon (the "TP" in your airfoil) that he designed most of his airfoils with this mixing in mind.
Jul 17, 2017, 12:27 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Thanks for the encouragement surfimp. Your flying videos have been my main inspiration to learn to fly aerobatic slope gliders. "It's a gas" as we say in Texas.

I do use flaperon mixing combined with always-on snapflaps. I learned that from building and flying your Le Fish. It is good to learn that I may have gotten lucky with regard to snapflap performance when I chose to use a Thierry Platon TP airfoil.

Jul 19, 2017, 07:26 AM
Bay Area Model Flyers aka BAMF
Diceman89's Avatar
Very cool! I am sure all of us would like to see some video of the planes performance.
Looks like you did all your homework and should be a good plane. Congrats!

Jeremy
Jul 19, 2017, 01:18 PM
Registered User
I think what plane works best heavily depends on flying style. I like some energy when I fly and dlg are perfect. I am now going to build a 0 dihedral zone 2 wing for my longshot in the hopes of even better energy and inverted performance. Personally I prefer this platform over the fishy type. I have even experimented with fins for increased side area and they work well.
Jul 19, 2017, 02:05 PM
auto-tune remix
slopemeno's Avatar
Any plans to open-source the plans for the SHLWASG?
Jul 19, 2017, 05:55 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Quote:
Originally Posted by mnavarro
I think what plane works best heavily depends on flying style. I like some energy when I fly and dlg are perfect. I am now going to build a 0 dihedral zone 2 wing for my longshot in the hopes of even better energy and inverted performance. Personally I prefer this platform over the fishy type. I have even experimented with fins for increased side area and they work well.
I agree. This quest for a light lift aerobatic slope glider can be thought of as a quest for energy. I thought a lot about making it DLG style for that reason. In the end, I decided to go fish style because I had accepted the Le Fish and Buzz as my baseline. If it turns out that knife edge is not possible to any reasonable degree on my hill, my next iteration will probably be DLG style.

I'd love to get more information about your "fins for increased side area".
Jul 19, 2017, 06:04 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Quote:
Originally Posted by Diceman89
Very cool! I am sure all of us would like to see some video of the planes performance.
Looks like you did all your homework and should be a good plane. Congrats!

Jeremy
Thanks for the encouragement Jeremy. I'm afraid the video will have to wait - no camera, no editing software. But I will be providing some detailed flight reports when I get more flying time on the glider.
Jul 19, 2017, 06:29 PM
Bay Area Model Flyers aka BAMF
Diceman89's Avatar
Quote:
Originally Posted by mnavarro
I think what plane works best heavily depends on flying style. I like some energy when I fly and dlg are perfect. I am now going to build a 0 dihedral zone 2 wing for my longshot in the hopes of even better energy and inverted performance. Personally I prefer this platform over the fishy type. I have even experimented with fins for increased side area and they work well.
You got to take in consideration the flying location as well. I know Miguel here gets to fly off bluffs mainly which product nice vertical lift versus a hill. So your plane for your area will need to meet different requirements.
You guys sound like you know what you each need for you own hill to fly off of.
Keep on going!
Jul 19, 2017, 06:36 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Quote:
Originally Posted by slopemeno
Any plans to open-source the plans for the SHLWASG?
The glider needs a name, but I haven't been able to think of one. I'm not good with names. But I hope SHLWASG doesn't stick.

Anyone have a good name?

Regarding your question, I will be happy to make the plans available in a build log. I must stress that, at this time, this is a one-off, experimental design with very little flying time. That being said, let me know anytime if I should publish the plans.

On a related note, I would really like to get this glider into the hands of experienced slope pilot(s) in California where its performance can be better evaluated. I have an extra wing already finished, and I can easily cut another fuselage. I would be willing to give these, along with material for the tail feathers, to a CA pilot who is willing to finish the model and publish some flight test results. This proposal is strictly in the interest of speeding up the learning curve for those of us who are interested. I have no plans to ever produce and sell this or any other model. Any taker?
Last edited by lifthunter; Jan 03, 2019 at 04:49 PM. Reason: Reword
Jul 19, 2017, 08:10 PM
Registered User
Dlg are designed to work on flat land, they will work anywhere.

https://www.rcgroups.com/forums/show...9&postcount=63
Jul 19, 2017, 08:17 PM
Bay Area Model Flyers aka BAMF
Diceman89's Avatar
You know I heard really good things about the Ahi. You might give that a whirl.
Jul 20, 2017, 01:46 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Quote:
Originally Posted by mnavarro
Dlg are designed to work on flat land, they will work anywhere.

https://www.rcgroups.com/forums/show...9&postcount=63
Cool idea for increasing the side area on a DLG style slope glider! (See photos in link above.)
Last edited by lifthunter; Jul 20, 2017 at 02:00 PM.
Jul 20, 2017, 01:58 PM
Ultrabatics enthusiast
lifthunter's Avatar
Thread OP
Quote:
Originally Posted by Diceman89
You know I heard really good things about the Ahi. You might give that a whirl.
Sorry to say, in my opinion, an Ahi is too heavy for my small hill light wind conditions. Based on the way my Libelle and Alula fly on my hill, I decided that I need a glider with a wing loading of approx 4.3 oz/sq ft. The Ahi's wing loading is 5.4 oz/sq ft minimum. My 16 oz Le Fish has a wing loading of 5.0 oz/sq ft, and it often struggles to fly acrobatically on my hill. I would love to be able to fly an Ahi though.
Jul 20, 2017, 04:48 PM
Registered User
Quote:
Originally Posted by lifthunter
Sorry to say, in my opinion, an Ahi is too heavy for my small hill light wind conditions. Based on the way my Libelle and Alula fly on my hill, I decided that I need a glider with a wing loading of approx 4.3 oz/sq ft. The Ahi's wing loading is 5.4 oz/sq ft minimum. My 16 oz Le Fish has a wing loading of 5.0 oz/sq ft, and it often struggles to fly acrobatically on my hill. I would love to be able to fly an Ahi though.
The ahi may actually do better than the le-fish because the airfoil is more semi-symmetrical, seems to have a reasonable chord length, and is designed to create more lift. While my dlg weighs about the same with a 1.5 wing span, the ahi felt just as light in the air. Sometimes it's not strict wing loading, the airfoil is involved too. You might even be able to do a very lite version of it using smaller servos, batteries, pull string, etc.


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