Jeez! I had no idea that the gain was that small.
Facet Opel
- Thread starter Topaz
- Start date
Help Support Homebuilt Aircraft & Kit Plane Forum:
Woodenwings
Well-Known Member
Sorry if i missed this in the earlier comments. There is an aerodynamic problem with plank planforms. Might be good to dig a little deeper. Overcome that problem by passive or active means and you are laughing! I like the design....but would avoid it because it is bound to bite again. Sorry for being a downer!
If you are referring to tumbling then, if the designer is aware of the phenomenon, there are ways to mitigate.
Woodenwings
Well-Known Member
Sadly no. Flutter of the self popagating and amplifying kind. All speed and load related. Better VNE warnings may do the trick. There is a good video about it somewhere on facetube.
Why would planks be any more prone to flutter than any other planform? If nothing else there are fewer surfaces to flutter so fewer critical frequencies in play.
For the small wings related to the Opal and the Pelican control surface flutter is the only real worry - and there well known ways to deal with that. These little wings are already pretty stiff due to the thick airfoils and low aspect ratio.
There are videos of higher aspect ratio flying wings developing span-wise resonance. Very different critters...........
There are videos of higher aspect ratio flying wings developing span-wise resonance. Very different critters...........
Tiger Tim
Well-Known Member
I thought you were getting at the issue I found with my small, indoor RC plank from years ago: up elevator would drive it into the ground.
Elevator portion of main wing chord too large, airfoil possibly had too much reflex, elevator throw possibly too large.
My friends and I flew a few of the old-time unswept R/C flying wings designed by Bill Evans in the 70's (Scimitar, Saracen, et al) and they didn't have those issues. Evans always used narrow chord "trailing edge stock" for the elevons, maybe 3/4 or 1 inch in chord. Easier to build was probably his reasoning. He was a character, and had a few cold solder joints in his head, but his airplanes almost always flew well (for the time) without any of the modern fancy transmitters and radios of today.
nestofdragons
Well-Known Member
Thanks for reminding me not to overdo the elevons. I am thinking about making central trim surface and small enough elevons.
It is not flutter per se, it is the very, very, very close CG/AC/NP proximity to one another that is inherent to the plank configuration, meaning that the pitch channel would be very sensitive, and could lead to divergence of the rigging & loading is not optimal.
This said, I do like the Marske wings; they have some very pleasant flying qualities.
Norman
Well-Known Member
This isn't flutter but is a problem unique to planks that modelers see often. They call it "hyperstall", apparently in the belief that there's separation occurring on the lower surface of the elevons. However droping the nose do to elevon stall would just make the separation worse so I have some doubt that that's actually it. This problem shows up when the CG is too far forward. What happens when static stability is too high and dynamic stability is low? Pitch oscillations are un-damped and the nose bounces up and down!
Thanks, Norman, that’s interesting.
Tiger Tim
Well-Known Member
Probably all of the above, it was built by untrained eye. It was interesting to see the effects of the shortest possible tail moment when I’d feed in a little up elevator to flare and it would just splat down onto the gym floor. Surely not all planks do this, but the careless ones can.
It’s so easy to just slam together a quick foamie to play with and learn about the weirder configurations, isn’t it?
Norman
Well-Known Member
Well actually it's a common characteristic of planks. The elevator of a plank is NOT like an elevator on a stabilizer. When you pull back on the stick it reduces camber and increases the pitching moment. The reduction of camber causes an instantaneous lose of lift but the pitching moment takes a while to overcome inertia so there's a bit of a lag in pitch response. The result is that planks tend to translate downward a few feet while they're rotating to a higher AoA. Airplanes with tails don't do this because elevator commands don't change the wing's camber ie tailless aircraft don't have a tail moment.
Last edited:
Bill-Higdon
Well-Known Member
A few years ago I actually asked Jim Marske about this phenomenon, after reading people's comments here on HBA. Now Jim is definitely not a "neutral party" (he's a strong proponent of his own product), but he said in no uncertainterms that his wings did not ever display that specific issue, and I believed him. His wings are not pure rectangle planks, and the reason for the tapered planform is probably to put the elevator further aft.
- Joined
- May 11, 2006
- Messages
- 2,260
Bill Daniels who flew some of Marske's planes with Jim said that Marske kept the longitudinal stability low to match the low inertia of a tail less aircraft. See Koen's Nest website "Theory of Flying Wings."
Woodenwings
Well-Known Member
Here is one such report to whet the whistles of those willing to investigate:
Investigation: 198902600 - Facet Opal, 0004-10-01, 1 km N Doyalson NSW, 12 May 1989
Investigation: 198902600 - Facet Opal, 0004-10-01, 1 km N Doyalson NSW, 12 May 1989
rotax618
Well-Known Member
I was fortunate enough to witness Scott Winton fly the Facet Opal, and was a friend of his father, Col Winton. The Opal had pretty remarkable performance, but in my opinion was too pitch sensitive for the average pilot.
Planks do fly, some are more docile than others. If I were to design an unswept rectangular flying wing I would design for an AR of not more than 3.5, naturally if soaring performance is what you want a swept or Fauvel planform is best with a higher AR.
Planks do fly, some are more docile than others. If I were to design an unswept rectangular flying wing I would design for an AR of not more than 3.5, naturally if soaring performance is what you want a swept or Fauvel planform is best with a higher AR.
I recall there had been speculation about a change in the spar structure (cutting a hole for the oxygen bottle??) which severely degraded the capability of the spar to withstand G loads. The report referenced above indicated the wing was designed for 8G, but that may have been before a hole was cut in the spar. Is any of this speculation correct, or relevant to the loss of the aircraft?
