Flight Test:- X8 a game changer?

Having designed, built, and flown flying wings since 2000, I believe that the Spectra flying wing has to be one of the best flying airframes I’ve ever wiggled sticks on. Not that it didn’t come easy – wings are known for some pretty nasty habits like center of gravity sensitivities, stall-snap-spin issues, just to name of few. It took some years to figure out the best way to tame those nasty habits and turn this airframe into a docile, stable flyer. So with that background, great interest was taken in the announcement first of the small X5, then the much larger X8 flying wing.

Being a good “alpha-geek” I have a web snoop on my webpage so I can see who is visiting. I watched in curiosity as a Chinese entity parked themselves on my website for several weeks in April of 2011 – specifically the one where the Spectra wing is animated and can be spun around 360 degrees. After the announcement of the X5 flying wing, then the X8 wing in December 2011, I had my suspicions that some of the Spectra’s bloodlines might be included. I waited until the initial clamour died down until I ordered, and finally received, my X8 from BevRC. I am pretty shocked at how close they came. Other than the winglets and some other notches here and there the resemblance is uncanny. 

Being a good business man, it occurred to me that if these lads had produced a good product that flew well and had all that room, it would behoove me to see if I could cut some of my manufacturing times by using this offering as a base to start with. So curiosity got the better of me and I ordered one. At around $200 USD I believed that I could make a few modifications and incorporate the X8 into the line up of tools that I use in search & rescue (SAR) and other photographic efforts.

OK, I’ll admit up front – I originally tried to order a DOZEN of these as I just KNEW it was going to be a good airframe. My goal was to see if the “copy” can be modified to something past what the normal hobby builder might do and make it worthy of the field tried and tested Spectra line. Contrary to what some might think, I really don’t care who makes the air frame. I have no problem ordering a $200 air frame because it would save me labour time and money in the long run to do so. It must, however, perform to the standards that we have come to expect, otherwise it’s not worth anything other than a goof around airplane.

Over all, I really like the lines of the X8 (I guess I would, huh?) and how they have implemented the foam molding. I was surprised at the weight difference of the Elapor cores vs. some similar that we have cut using 2 lb EPS. Significantly heavier to start and I knew that I was going to add more weight to “harden” it to the rigors of field use. I found it interesting that there are no build instructions included with this kit, but since I would’ve tossed them anyway, this is not a big deal either.

The X5 – Legacy of the X8?

I guess I should have paid attention to the comments concerning the X5, but I was confident that lessons learned there had been incorporated into the bigger X8. Some of the usual lines should have sparked memories that are now embedded in my DNA. Things like “under powered”, “hard to launch”, “won’t slow down” should have set alarm bells off, but I soldiered on with my build.

Modifications – Making a silk purse……..

Wings – Joining the two

As documented in other threads, this aircraft is prone to wing “flap” or excessive wing bending (as I predicted) and they need to be stiffened. I opened up the wing slots to accept a 7/16” tube. We use a T6 heavy wall aluminum tube in the Spectra which makes the wing stiff enough to allow us to fly in 50mph winds. Another nice thing about this mod is that will allow some fixed anchor points in the fuse itself while providing some additional center line stiffness. The wings will gain some additional stiffness when the tubes are epoxied in place as well. I also wanted to leverage the embedded carbon spars already in the wing by adding a carbon stringer on top of the wing. Not exactly a “D” box construction but should stiffen the wing up with adding very little weight. The wings will still require some additional stiffening on the outer third of the span, but I have another trick up my sleeve for that.

Wings – Outer panels

We used small diameter carbon rod on top with fiberglass tape on the bottom. The wings werel then covered in EconoKote since this provides significant support to the foam structure while adding a small weight penalty. This should pretty much take care of the dreaded “flutter” or even “flap” that occurs at higher speeds and wing loading. We kept the color scheme to our tried and true black bottom and white top.

I hate to put anything of any weight out on the wing tips that are aft of the CofG but I really do think that the “drooped” winglets help improve low speed handling of just about any wing. Addition of a small fin that protrudes approx. 30% below the bottom of the wing helps break up vortices generated by the swept, tapered plan form of the wing. It has been noted that this aircraft can have some vicious stall tendencies and I believe that a modified winglet configuration would prevent this. The upswept winglets look really cool and all, but in practice, the drooped winglets would help both in flight, and in landing as a skid. I did NOT make this modification as I wanted to keep the airframe as “stock” as I possibly could to determine its’ base flight characteristics 

Wings – Controls and electronics.

The wing servos were also set up to come out of the top of the wing rather than the bottom. I’ve always been taught it is better to “pull” for up controls surfaces rather than “push”. This will also get the servo arm/control horn out of harms way on landing in tall grass, weeds, etc. The elevons need stiffening too, just not sure I want to replace them with balsa just yet. I’d like to stick with what came in the kit. I opted to use Hitec HS-82 metal geared servos too. They shouldn’t send their servos out in the clear plastic cases – I saw the plastic guts in there and just said “uh….. no…..” Just to keep things easy I’ll let the pushrod come out of the Econokote and will not even mess with faring it even though I do have some PETG ones vacuformed.

Fuselage modification

The one thing that I just could NOT abide was the square boxy appendage that hangs from the bottom of the 

fuselage. To me, it just adds up to more drag for little benefit. Surgery was called for on this growth.

This modification caused me some extra work to make the new “scar” a bit more cosmetic, but it was not difficult to produce a vacu-formed “shield” to glue over the now smooth bottom of the X8

Other than that, there was little to do to the interior of the fuselage. I could see many possibilities for batteries, equipment, and sensors. Lots of wide open spaces.

The rest of the electronics were installed in their usual places. Motor, speed control and batteries went to their respective places with the included mounts. I was pleased to see that the battery bay area was large enough to hold two 5Kmah batteries which should extend the range of the X8. As stated, the entire aircraft was covered in Ekonokote and it ended up looking like a very sleek bird. The fit is a little rough but what can you expect on foam parts? It wasn’t so bad that it wasn’t fixable and the finished product doesn’t look bad at all. The covering stiffens the wing up considerably and gives it a nicer finish than just the bare foam, and doesn’t weigh much more if you used paint.

   

I had to use two 5K maH batteries to get the CG close to the fingerholes as has been the suggested location. The two packs fit snugly in front of the spar tube. No camera gear will be flown on the maiden so it will be as light as it can be, which is just a shade under 6 lbs (2.73kg). Should be easily hand launchable.

The motor and ESC are not the greatest combination in the world, the ESC is very slow to start up. It has no instructions so I’ll have to dig around to see how to turn the prop brake on. That works about as good as a “folderable” prop and a lot less mechanical doodads back there to boot!

Maiden Flight

The estimated CofG was incorrect! The first launch ended up being a good test of the ABS skid/shield. The resulting launch ended up being an immediate stall, snap/pin into the ground . The shield did its’ job, but the nose still got crinkled a bit. It would have been much worse without it though. This turned out to be foreshadowing of what would follow with subsequent flights. Opened the hatch and moved the 10Kmah pack forward of the bulkhead in what I would consider the camera bay. Not having anything to secure it, I took off my T-shirt and stuffed that in their too. Hmmm…. I guess I should weigh my shirt now. Regardless, the next launch went as expected. The motor/ESC combination starts rather slowly and the X8 was drifting earthward by the time motor got up to speed and started producing thrust for flying speed. The motor was turning a 13 X 8 APC prop pulling about 29A out of a 11.1V – 3S2P 10Kmah pack at full throttle.

Got some altitude and tried to induce some tip stalling and snap spins but she wouldn’t do it. More from an extreme forward CofG than anything else. The controls were sluggish and it would cruise like a majestic kite with about 1/3 throttle. Total time of the flight was 12 minutes and it burned 1093 mah. I’m sure the few minutes of thermal flight helped but even if you cut that by a third to say 8 minutes, that would still be over an hour of flying time.

This is a big bird and has a lot of wing. Longer swept wings are just prone to flap and flutter and this one is no different. Even with all the stiffening measures I took, there was a visible oscillation in the entire wing in shallow dives, even with the T6 aluminum spars! If you held the dive long enough, you could induce control flutter as well. Servos are locked in place and we upgraded the pushrods to larger 2-56 with clevis and control horn locks. You could chop power and gently pull out of the dive and recover. 

I suspected that the cheesy little magnets wouldn’t hold the hatch in place, and they didn’t. At full throttle the low pressure over the body sucked it right out. Recovered it no problem, but that is something to rectify.

The ESC get’s no ventilation way back there, so need to put a vent in for that.

The motor mount pulled loose on landing – I know, use the folderable prop…. I think if the brake was applied it would be less likely for that to happen, but that remains to be seen.


Continued flight testing

Took the big wing out to do some more testing before putting an autopilot in it. It was a bright sunny day, with variable winds, and some considerable thermal activity. Launches with no wind were problematic in that it was difficult (not impossible) to get the speed required for takeoff. There were some close calls, but no crashes. Weight was the same as the earlier flights and the CG was the same. We used a 10Kmah 3S pack and had a Panasonic Lumix in the nose, with about 3 oz. of lead to get the CG right.
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In the air, the bird didn’t seem to like the thermal transitions too much. As it would pass in and out of the updrafts, the wings would flex and even flap some of the time in level flight. When slowed to an optimum cruise, the controls would become quite ineffective even on high rates. The alternative was to leave the throttle on to keep the speed up, which ended up being a vicious cycle of speed up, flap/flutter, slow down, mush, rinse and repeat. It got quite unnerving having to manage it so closely. I believe a bigger spar would help this situation, but so would glass and carbon. Take your pick. Several pilots tried the X8 and all agreed there are some inherent attributes with the air frame that aren’t very desirable.

This is a big aircraft and it can get heavy and there in lies the problem. Because of its’ weight, all the rules of kinetics are in place. When you get it up to speed, it tends to retain that energy even the throttle is completely closed. When you do finally get it slow down, it tends to KEEP slowing down until you get to the point there is a stall imminent. You can add full power, but that mass tends to stay put! The added torque of the motor tends to jerk the slow moving wing to the left ending up in all sorts of mixed inputs from the pilot. The only real way to keep this from happening is keep the throttle set to a point that retains a minimum airspeed. The trade off there is, once the nose gets pointed down, it will accelerate again – a vicious cycle.

General impressions…..

This is a very sharp looking aircraft when completed. In straight and level flight she is smooth and would seemingly make a great FPV aircraft or photography platform. But plan your flights carefully as wind could put you in a critical portion of the flight envelope. This is a great hobby aircraft and that is its target market. With a 3S pack launches can be a problem. An upgrade to the motor and batteries could be of benefit to this aircraft. I don’t know if it would stand up to the rigors of field use in a SAR situation but I guess I’m pretty spoiled by the tough as nails Spectra. I would consider the X8 an intermediate skill level wing just because of its size and need for attention to details concerning control slop and proper CofG attainment. Don’t let that huge pod with all that room catch you though! You can put a lot of gear in there, but if it doesn’t balance and those wings are stiffened up, it will be in the alfalfa in a heartbeat!!