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Aug 10, 2018, 08:02 AM
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Opterra 1.2m spin testing


Well gentlemen (and maybe ladies)

I took the "stock" Opterra out for it's first ride today. Stock PnP that is. I don't fly Spektrum, I'm a Taranis driver, so no "stability" system in this one. I used an old (in time, not cycles)Eflite 20C 3s-3000mah because I had two and they fit all the way fwd and make the model balance right between the marks on the bottom of the centerbody. I taped the wings on to avoid any complications with the plug-in circuits.

The day was low broken clouds with rain nearby, winds southerly (off the ocean) at 6-8 MPH steady with gusts to 10-12.
I was alone at the field so the only recording was thru my headset (actually the best place to see it!).

First flight (very short, <2min) was LOS just to trim her out. no surprises, hand launch was uneventful. landed and hooked up the camera and went off to FPV. Spent most of the next 6 minutes getting used to her. She's a good ship and I found no surprises flying her fast or on approach to land.

OPTERRA FIRST FPV FLIGHT (4 min 15 sec)


Now..... I have built and flown many a flying wings in my 46+ years of flying RC models. I have even scratch built/designed a few. I had a suspicion that the inboard vertical tails were just "decoration" and the second FPV flight showed me that this was true..... but... (and it's a big but), I found out on the third flight that the tails may be responsible for the death spins some have experienced. Read on (and watch the videos) if you're interested.

On the second FPV flight, with the inboard tails removed, after some uneventful hi-speed flight (not terribly fast stock on 3s), I took her upstairs to do some stall/spins. I had NOT yet done any with the vertical tails on!). She behaved EXTREMLY well in the stall and spin. Relaxing the controls after the spin initiates stops the spin almost immediately (power off) and it just takes a little longer power on (low power, about 1/4 stick).

now... let's try it with the tails back on! On the third FPV flight, I really expected there to be no difference. Though they are not "far" behind the CG, they are a bit aft and should contribute "something" to lateral direction stability..... wow.... was I wrong!

On the first stall spin, she whipped around hard and went "flat" before I could do anything about it. Power and full down had no effect. She pancaked in at a slight nose down angle and bent the forebody a bit. Easily repaired. I will NOT try that again until I can devise some way to rescue her from that mode. Might need some sort of active yaw control!
OPTERRA SPIN TESTS (3 min 20 sec)


More as it happens.

Tom
Last edited by Tom Hunt; Oct 28, 2018 at 02:30 PM.
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Aug 10, 2018, 08:20 AM
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After repairing the model, I returned to the flying field a week later and began a new round of spin testing.

I repeated the inboard tails off spins and found it easily recoverable. Relax the controls, wait for the spin to stop and add a little down elevator. easy peasy.

I then added some blue "fan fold" sheet foam extensions to the inboard vertical tails and reinstalled. I taped them on this time due to the higher loads expected. Again, I found the model relatively easy to recover from a spin but it would not stop the spin by simply relaxing the controls. I had to apply opposite command to the ailerons, then push down elevator. Sometimes the opposite command only slowed the spin, so down elevator was added and she came right out. Power on stall/spins (low power) seemed to be a bit slower, but recovery was performed in the same manor.

Then I removed the extensions and returned the model to the stock configuration. This time I was a bit lucky, I was able to enter and recover from spins (I prudently started at a much higher height than last weeks, first flights!)

I did notice however, that the spins were much more nose high (not flat, but certainly saw much more "horizon" visible in the video). Recovery can be a bit scary. It takes full opposite aileron command and then full down elevator to force it out of the spin. Sometimes the model almost went inverted while trying! but in all but the last spin, I was able to recover.

On the last spin in the video below, I let the model get too low and when I tried to recover, it snap rolled in the opposite direction when I pulled "up" to hard. On the last attempt at recovery, it went inverted into the ground. Broke the connector on the camera and split the nose again.

A new camera and a little glue and she will be good to go again.

Where am I going from this? well, you can see that the FX-61 almost refuses to spin. It only has "upright" inboard tails. Some of the tail is below the wing. I might try to emulate this on the Opterra. if this is successful, I may reduce or remove the outboard tails completely.

More as it happens.

OPTERRA SPIN TESTS#2 (4 min 56 sec)
Last edited by Tom Hunt; Aug 10, 2018 at 08:29 AM.
Aug 10, 2018, 02:22 PM
Registered User
Thanks for your work on this. Your experiences mirror mine. I had 2 death spins so far both with only minor damage and was in safe both times but could not stop the spin. This morning I flew again but removed the inboard fins. The wing handled well with no adverse affects that I saw. In fact, looking at the the GoPro session footage that I shot, I think there is less wobble with the fins off than with them on. I realize the wind can play a big part in this but flying in about 8 mph wind with slightly higher gusts the wobble seems less to me . I am flying the safe version without the FPV equipment and fly on 2200 3s and 2500 4s batteries. I prefer the 4s and have the CG just in front of the rear dimples which is my preference. My prop brake was off when I received the plane. You can't set the brake on the ESC with your transmitter on the plane I got it just won't ever go into setup mode. I had to connect the ESC to another receiver to set the ESC brake on and that worked as expected. Don't know if others had that problem but with the prop brake on it glides very nicely.
Aug 10, 2018, 05:10 PM
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Hi fbrunt.

I have yet to set my brake and may not. I'm not using this model as a soarer, just a sport model. The spinning prop (even in windmill) is a yaw stabilizing force when behind the CG, so I do not want to give that up at this point. having the prop stop and fold may make matters worse in the spin.

Tom
Aug 18, 2018, 08:30 AM
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Next round of spin tests were rather disappointing, but even that is good data!

I removed the "bumps" on the top of the outbd wing panel that are "supposed to be" vortex generators to re-energize the flow on the wing. Although we do these kinds of things in wind tunnel testing, their effect is mostly black magic at these reynolds numbers (generally <100,000). Full scale aircraft with Rn's in the millions, must have fairly SHARP edges on the vortex generators to make them work.

I am not going to provide more "boring" videos of spins this time, because honestly I saw no difference in the way the model spun or recovered from the spin with the inboard tails on or off.

One thing I did notice, however, is that the model now will stall straight ahead if no aileron is input. With the pumps on, even a straight ahead stall broke immediately to the left or right and a spin followed with no aileron input. Does this mean that the flow is now more symmetric over both wings deep into the stall? may be. Some further investigations will include more "proven" low reynolds number "trips" on the LE of the wing.

More as it happens.

Tom
Last edited by Tom Hunt; Aug 18, 2018 at 11:20 AM.
Aug 19, 2018, 11:28 AM
Registered User
Thanks for your work. Did have one more spin but was easy to recover from... Inboard verticals were removed. I am not seeing any adverse handling issues with them off with about 12 flights since I originally removed them. On 4s here.
Aug 22, 2018, 01:26 AM
Registered User

X8 spin testing


Hi Tom,
Never flew the Opterra but tested the X8 some time ago. Opposite controls did not arrest the spin and she flat pancaked into soft grass. Did it a second time but had altitude to try controls into the direction of spin and it arrested the spin. Folding prop with brake was on so no stabilizing force as you mentioned.

I just acquired a “white” X8 and will use windmilling as a anti spin stabilizer.
Aug 22, 2018, 12:07 PM
ab amicis auxilio parvulo
Aerospacer's Avatar

Opterra Stalling


Quote:
Originally Posted by Tom Hunt
Next round of spin tests were rather disappointing, but even that is good data!

I removed the "bumps" on the top of the outbd wing panel that are "supposed to be" vortex generators to re-energize the flow on the wing. Although we do these kinds of things in wind tunnel testing, their effect is mostly black magic at these reynolds numbers (generally <100,000). Full scale aircraft with Rn's in the millions, must have fairly SHARP edges on the vortex generators to make them work.

I am not going to provide more "boring" videos of spins this time, because honestly I saw no difference in the way the model spun or recovered from the spin with the inboard tails on or off.

One thing I did notice, however, is that the model now will stall straight ahead if no aileron is input. With the pumps on, even a straight ahead stall broke immediately to the left or right and a spin followed with no aileron input. Does this mean that the flow is now more symmetric over both wings deep into the stall? may be. Some further investigations will include more "proven" low reynolds number "trips" on the LE of the wing.

More as it happens.

Tom
I have been following your testing and trying to grasp the finer points of this flying wing's aerodynamics. The rows of foam turbulators on top and under the wings are puzzling to me as well. When I saw that you removed some of them, I expected that it could make the stalls more abrupt and spins more vicious. The fact that you didn't see that makes me wonder what they were actually put on there for in the first place. As you commented, they are crude bumps as compared to the carefully constructed turbulators on full scale aircraft.

I haven't induced any spins on my Opterra yet but have purposely stalled it. It mostly stalls straight ahead with an occasional slight wing drop. I do run a non-folding prop and have the ESC set to brake hard to keep from catching a windmilling prop on landing. I quickly found on my Micro SkyHunter that was the best way to keep from going through a lot of props. Also, my Opterra is what I would consider a light wing load (11-12 oz/sq.ft.), coming in at about 36 oz. AUW with my micro FPV gear and 3s-2700 mah battery.

I do have a recurring problem with the Opterra though regarding landing. Our club field has a smooth mat surface runway. I prefer belly landing on that as opposed to setting down on our closely mowed grass areas because there is a fair amount of weed stubble in the grass that grooves and gouges the foam underside of the Opterra. Up until the Opterra I had no problems skidding along to a stop on the mat runway with either my L-39 EDF jet or the Micro SkyHunter. However, with the Opterra it is about a 50-50 proposition. Sometimes it will skid to a stop fine. But other times after about 10-20 feet of skidding it suddenly will raise the nose and one wing and try to cartwheel. I could see that happen if I had bounced the landing but once its down and skidding, it doesn't appear to even have enough velocity to get that kind of lift.

Examining the skid on the bottom of Opterra, I note it's shaped like a chair rocker. And I do show skid marks on the engine cowl which indicates it is able to get a fairly high angle of attack as it settles in. Also, the CG is a couple inches behind the high point of the rocker shaped skid. That makes me wonder if as it slows skidding on its rocker, it starts pitching up creating enough incidence to lift a wing.

So what I'm going to try is a sub-fin just in front of the motor cowling to prevent any appreciable pitch up as it is skidding to a stop. I wasn't sure what form it should take but given your experience with added inner fin area helping spin recovery, I went with some 1.5mm plywood edged with metal pushrod to make the skid double as a fin. The whole fin is about 3.5" long by about 1" tall. That is about half the area of one of the inner top fins, although it is straight vertical. On the other hand, air flow to it could be blanked by that 1" wide rocker skid ahead of it.

My primary goal with this sub-fin is to stop these near cartwheels on landing. I will see if I can notice any difference in stalls but am not too keen on aggressive enough stalls to go into a spin until you have come up with some more answers here.
Aug 22, 2018, 12:59 PM
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Hello Aerospacer

Though I have not done it to my Opterra yet, I have added skegs to some of my hi-performance sailplanes to keep them on the ground once they contact. The skeg assures that the wing stays below the zero lift AOA of the airfoil.

so far all my (intentional) landings have arrived without flipping over, but I will consider the skeg soon if it does!
Aug 26, 2018, 02:37 PM
Registered User
Opterra landing / skeds modif

hello Aerospacer and Tom Hunt, many thanks for your share

did you notice sensitive improvement since installation of your sub fin /skeg ?? Thanking you in advance
Aug 26, 2018, 05:05 PM
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I think you mean "significant" improvement. Yes. I added a skeg to mine and it now tracks straight after landing and has no reason to "fly" again.
Aug 26, 2018, 05:59 PM
Registered User

Opterra landing - tail sub-fin/skegs


yes Tom, significant is the right word
(being french, i do not master english totally

Many thanks for your experience sharing, we all will benefit of it !!!

i will begin to study how i will build this tail sub-fin/skeg, sounds a good improvement. On my gliders i do not have this pb but as i plan to fly both Opera 2M and 1.2M this modification seems to be excellent
Aug 27, 2018, 12:48 PM
ab amicis auxilio parvulo
Aerospacer's Avatar
Quote:
Originally Posted by Tom Hunt
I think you mean "significant" improvement. Yes. I added a skeg to mine and it now tracks straight after landing and has no reason to "fly" again.
Same here for my sub-fin/skeg addition, it stays on the ground and slides nicely to a stop without bounces or cartwheels. I'm better able to flare a little nose high, drag the skeg on the ground to bleed off a little speed and then let it rotate down flat than I was without the skeg. Maybe it just mental that I don't fear dragging the cowling and prop along during touchdown. Wonder if a small tail wheel would have added anything?

So now I hope that I will be able to reliably land the Opterra FPV on our mat runway like I do my Micro SkyHunter.
Sep 10, 2018, 10:06 AM
Registered User

Opterra 1.2 or 2 M


I really enjoy following your comments. You guys are all very thorough and helpful to all of us. I do not normally post, mostly troll and learn.
I was planning on purchasing a 1.2, but quite frankly I am very concerned with all the issues you fellow highlight. This would be a first wing for me and wonder if I should go to the 2 M version. I really wanted all the bells and whistles that come with the flight controller but not sure I should chance it. Please give me some advice on this as I know you have experience with both birds. My flying is LOS only, would love to try FPV but not yet.
Thanks
Graham
Jan 05, 2019, 11:16 PM
Registered User
Quote:
Originally Posted by LumsdenG
I really enjoy following your comments. You guys are all very thorough and helpful to all of us. I do not normally post, mostly troll and learn.
I was planning on purchasing a 1.2, but quite frankly I am very concerned with all the issues you fellow highlight. This would be a first wing for me and wonder if I should go to the 2 M version. I really wanted all the bells and whistles that come with the flight controller but not sure I should chance it. Please give me some advice on this as I know you have experience with both birds. My flying is LOS only, would love to try FPV but not yet.
Thanks
Graham
I had a 2m Opterra for some time and loved it. It is a floater and was perfect for FPV, Predictable and reliable. I lost it on a long range flight one time and didnt bother replacing it (had a Talon at the time also). Jump forward about a year and I just bought a 1.2m Opterra and I am not impressed with its flight characteristics.

I understand there will be fundamental differences between a 2m and 1.2m flying wing. But the little brother is not to my liking. I experienced the spin as described by several. Was fairly low at the time and was able to repair the minor damage. One plus is that it seems to be pretty rugged, I have rolled it up a couple of times with minor damage. I will keep flying it but it wont be a go to for me.

If you are looking for a fun floater( i actually caught thermals on a couple of occasions) the 2m works fine. Its so big you can get some distance and keep it LOS.

I will definitely be buying another 2m.


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