I just demoed the Falcon 4 195 this morning at the south side, in fact. Compared to Falcon 3 195, Steve Pearson reported that it has a smaller diameter X-bar (52mm with 50mm inner sleeve) for lower drag
Smaller X bar for less drag! I thought they the X-bars didn't created that much drag? So If they really do creat a bunch of drag, why not put FARINGS on them to really reduce the drag, Like North Wing !
dave
jj colorado wrote:
What I found regarding the fairings. I bought some Northwing fairings and cut them down for the Falcon 3.
The glider flew fine with them, After reading the testing report in the magazine I decided to remove them and see if I noticed any difference. I did. It is a little quicker to set up the wing and to get it packed into glider bag when done flying without the fairings. Did not notice any noticeable difference in flying
Campers,
Fairings need to be at least 4:1 (chord-to-thickness ratio) to do much good, which means a two inch crossbar needs eight inch fairings, ideally. Then, they have to be light, cheap, and able to withstand packing (and travel) in the glider bag. It can be done, because I did it on my Seagull crossbars, back in the day. The biggest trouble was, nobody wanted to believe the Angle of Attack needed to get zero incidence to the airflow in flight. They still won't, especially the manufacturers.
If the AoA of the fairing is wrong in flight by more than about ten degrees, there won't be any benefit from fairings, and they may be a real drag instead, pun intended. If anybody expects "self-aligning" fairings to hit zero AoA (or even close), I suggest a simple test. Put the fairing on a crossbar (or any similar tube, such as PVC), hold it out through a car window on an empty road, and drive at twenty mph (30 kph). Add ten to the airspeed, and try again. See how close to zero AoA (horizontal, dead level) these fairings really get. Without even looking, I'd guess that you might get the trailing edge of the fairing to rise to the same height as the bottom of the tube, if you are lucky. That's not even close to zero AoA, for the fairing. Since the AoA is wrong, by a lot, they will provide no benefit. Self-aligning fairings are a pure joke, at HG cruising speeds, because they won't get anywhere near zero AoA. Watch as these "self-aligning" fairings fly up, dive down, and repeat.
This "flutter" in the video would never happen, if the fairings were really self-aligning to zero AoA (zero lift). I used fixed fairings, with the AoA set by streamer observation, and I saw a real benefit. The target spot was behind my landing consistently, for about a month, until I got comfortable with the longer glide, and set up lower approaches.
When you observe the streamer, was it one streamer, or several streamers along the crossbar? I have intended to make a similar observation, but never had the time. The thing is, that " AoA" you are talking about is not actually the real AoA, because the streamer and the crossbar are in the so called "disturbed area" next to the wing, and AOA by definition is the angle between the chord of the wing and the direction of the flight ( which is sometimes described as the non-disturbed air vector of speed, before the wing changes somewhat the airflow) . Anyway, my point is that may be the flow hits the crossbar at different angles along the crossbar at given speed, plus all this different angles will change as you change your speed. So, ideally, those fairings are to be with more complex twisted along shape, and should automatically adjust in flight with the speed changes. Plus , the fairings I see so far are made of some mylar and their cross section is far from ideal. The profile seem to be concave behind the crossbar , and it should be more convex at least.
These are probably the reasons that most manufacturers don't bother to make fairings that will actually work- it is too complex, and the fact is, at low speeds it almost doesn't matter. They may help at high speeds.
When you observe the streamer, was it one streamer, or several streamers along the crossbar?
Anyway, my point is that may be the flow hits the crossbar at different angles along the crossbar at given speed,
plus all this different angles will change as you change your speed.
krassihg,
There were several streamers, located at 1/4, 1/2, and 3/4 length of each crossbar. I took the majority/average for setting the angle of my fairings.
Yes, the crossbars are in disturbed air, and that is the airflow angle that I was using for my setting. I see no alternative to that choice, to get good results.
Lots of pilots told me my fairings were set at the wrong angles for stall speed, and I can only say, we do NOT need to streamline a stall.
My settings were a compromise between slow cruise and high speed, so they were "best" (hopefully) at max glide.
The bottom line for me was, "dang! I overshot the target spot again!"
"Flat plate CD =1.28
Rectangle CD = 0.75
Round tube/wire CD = 0.5
Ellipse CD = down to 0.25 (varies depending on ratio of thickness to length)
Airfoil section CD = 0.10 to 0.04 (varies depending on section)
It appears that a streamlined shape having a 3/1 fineness ratio (NACA 0033) gives the best value of CD , namely 0.045"
Even just an ellipse has about half the drag of a round tube. The fairings I used are close to 8" long but that is only needed if you are trying to get the Huge drag reductions of a good airfoil section.
The fairings did reduce drag, but it didn't seem worth it, especially since, if they got bumped prior to flight, you could have a problem similar to what was shown in the video. Worse yet, that could happen on one side and not the other.
Also remember that single surface gliders have very poor undercambered airfoils. The crossbar goes from about 2 % behind the LE to 20% behind the LE. You need to point the fairing airfoil downward to minimize disturbance and it needs to be twisted for best results.
So the tradeoff is how much L/D improvement do you get, at what speeds, can you set the AOA accurately each flight, and does it make enough difference to be worth it. On an older wing with a 6 or 7 to 1 L/D this probably was worth it. On my F3, so far it has not seemed to make enough difference in glide to be worth it, whereas a smaller diameter bar reduces drag with no setup or storage issues. _________________ jj
H3 - U2 160, Falcon3 195
P2 - BuzzZ4
It might be interesting to see how much drag reduction turbulating the crossbar tube would give.
A couple of strips of Dymo labeling tape would add very little weight, and be unlikely to get knocked out of position.
Someone needs to tow up to 2500 with one cross bar fared and one not if you have a turn you have your answer. I hope to do that with my freedom.
Wayne,
You would need to install an equal weight of plastic pipe inside the crossbar without the fairing, to have the glider balanced left-to-right. Otherwise, the weight of the fairing on one side may cause a turn by weight.
Please, no "self-aligning" fairings. They do not get to the correct Angle of Attack (for the air flowing around the crossbars). They do not even get close to the correct angle.
I have the "new" Northwing farings.They have added an inner velco, about 18 inch's long in two location's to fix the faring at a set angle for the total lenth of the xbar. As to weight, I had not thought of that but the farings weight about 1 1/2 lb's per side and I wonder what I could put out there that wouldn't cause drag.
or just roll up the fairing and stick it in the crossbar on one side. No second guessing about the correct weight or moment. _________________ Airborne Climax 14 (C1)
WW U2
H3
AT, FL,ST, RLF, TUR.
There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. … Clearly, it is this second part, the missing, which presents the difficulties. ~~~Douglas Adams
Gosh, thats a great idea but how do you propose I get it "in" the cross bar?
Wayne,
Use a vacuum cleaner to draw a string through the crossbar, and then use the string to pull the fairing inside the crossbar.
I was gonna say a hacksaw, but Red might have a better idea. _________________ Airborne Climax 14 (C1)
WW U2
H3
AT, FL,ST, RLF, TUR.
There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. … Clearly, it is this second part, the missing, which presents the difficulties. ~~~Douglas Adams
Red, that video is of my Freedom. The flutter was caused when the previous test pilot pio'd on tow, pinned off and then whacked quite hard causing the fairings to shift forward. I removed them , inspected everything and then reinstalled per Northwings instructions. The flutter has not returned. The wing was being set up and test flown at Lookout Mountain for aerotow. They do swivel pretty easily but I have no idea if they achieve zero aoa. _________________ Freedom 170 FR
Youall are just fooling with me cause I'm a yankee, I'm going to through caution to the wind and just fly without any weight on the nonfared side, then we can have a big discussion about the validity of the experiment...if I live.
I see two areas of gain here. First is the reduction of drag of course. Second is gained efficiency of the air foil .
Objects near our wings really screw up the wings ability to do its work > producing lift. Look at a video of a X-bar flying with no faring. You will see the sail turbulated by the turbulence. With a faring it's smooth. The high performance the wing the more good they will do.
If the F4 is high performance the the F3 the would only help.
All the pilots that fly with out them that I have talk to say they love the quiet and they have to set up further back because they glide further into the LZ.
Someone needs to tow up to 2500 with one cross bar fared and one not if you have a turn you have your answer. I hope to do that with my freedom.
Long post follows. Short version: detect the extra drag from the unfaired crossbar by watching a yaw string, not by looking to see if the glider wants to turn.
Hang on, think about that a sec. What you need to do is mount a dowel projecting forward about 1.5 feet from the center of the base bar with a yaw string (telltale) on the end. So you can get a good look at the yaw string in flight with no parallax error. See if the glider flies slightly yawed (slipping), due to drag asymmetry.
The reason that the glider will settle down to some constant slip angle, is that the "upwind" wing or the wing that is pulled further ahead- the wing with the fairing, we expect- experiences a drag increase or yaw torque increase because it is becoming less swept and thus longer in span. Now the net yaw torque is zero and the slip angle is constant. Once the slip angle is constant, we need to be careful about assuming we'll see a turn (steady curve of flight path causing continual change in heading).
A slip due to asymmetric drag is very different from a turn. On a falcon, left yaw (or more precisely, a slipping flow crossing from right to left) may give weak left ("downwind") roll torque at min sink, zero roll torque slightly faster (best L / D?), and increasing right ("upwind") roll torque as you increase speed from there. If you give no roll input, the glider will slowly bank and turn in the direction of the net roll torque, not in direction that nose is shifted (yawed) relative to flight path by the drag asymmetry. Yawing the nose to one side to make the glider fly in a slipping attitude, is not the same as commanding a turn in the direction the nose is pointing.
On a hang glider or any other aircraft with minimal side area, the slightest bank will cause a turn in the direction of bank even if the nose is yawed strongly the other way, relative to the flow. In theory if you keep the wings ABSOLUTELY level there will be a very slow turn in the direction that the nose is yawed, due to the weak sideforce from the sideways flow hitting the wing, but our view perspective relative to the wing which is high overhead, no aircraft structure directly in front, etc, makes it very hard to be sure we are keeping the wings 100% level in a hang glider. MUCH better to detect the yaw (slip) by watching a yaw string, rather than trying to note a turning tendency while hoping to keep the wings 100% level-despite any strong left or right roll torque that the sideways flow may be creating.
Another way to detect slip without having keep the wings 100% level is to watch a bubble level mounted on the base bar (it deflects toward the "downwind" or trailing wing, regardless of bank angle) but the deflection is VERY small, and maybe the base bar doesn't stay 100% fixed relative to the span as we make roll inputs, which could introduce error, so much better to use a yaw string.
It all has to due with anhederal. These comments are based on tests with a small drogue chute flying from one wingtip. I've spent a lot of time experimenting with this sort of thing. It might be that in the part of the envelope you are most interested (max l/d?) even a strong yaw (slip) due to asymmetric drag makes no roll torque and thus no significant turning tendency. Even if you detect a turning tendency, unless you are flying quite fast where anhedral clearly dominates, you may not know for sure whether you are in the part of the flight envelope where the glider wants to turn downwind (toward the draggy side) to due to positive effective dihedral, or upwind (toward the clean side) due to negative effective dihedral. Maybe at some loadings even a falcon turns (rolls) upwind even at min sink, who knows. And I sure don't know for a freedom-- or for a falcon 4-- as I have never done yaw tests on either.
You can get around all this confusion by using a yaw string, rather than watching to see if the glider wants to turn. The yaw string will shift toward the draggy side.
You do want to do the tests holding the glider basically on a constant heading- turning will create additional deflections of the yaw string which will complicate things, and rolling even more so. But small deviations in heading (low rates of turn) won't affect things too much- the yaw string should, on average, still be deflected toward the draggy side.
I can post some pics and videos of the bracket I used to mount a yaw string probe (made from 1/4" fiberglass driveway reflecter pole) on the base bar centerline. The bracket involves a short 1/4 " ID tube epoxied to a few inches of aluminum angle (for round base bar) or flat bar (for streamlined base bar), and is velcroed or taped to the base bar, and is small enough not to be a problem during aerotow. The fiberglass rod is easily removable for launch landing and aerotow, easily mountable in flight, stored in small tube on left lower rear wire during when not in use. An inch or so of ziptie around the end of the rod makes a flexible standoff to hold the yarn a bit away from the surface of the rod- it works better this way, otherwise the yarn seems to stick against the rod due to some sort of aerodynamic suction.
I'd be happy to try to explain this better or answer any questions. If you do this test, I would be most interested in the results, go ahead and note the turning (rolling, banking) tendency as well the yaw or slip tendency (yaw string deflection), at various airspeeds or bar positions. Assuming that the half-faired cross bar DOES cause significant yaw (slip), this will give us one more set of data points as to how a given glider tends to roll in the presence of a sideways flow. At high airspeeds the glider will tend to roll "upwind", and progressively less so as the glider is slowed, but I'm always interested to know at what airspeed or bar position the roll torque switches to neutral and then to "downwind". In some gliders we can slow even below min sink without seeing this reversal but I think in a falcon the neutral point where the "effective dihedral" is zero (hence no roll tendency) might often be around best l/d, or somewhere between best l/d and min sink, with a "downwind" roll torque (positive effective dihedral) at lower airspeeds.
Short version : detect the drag from the unfaired crossbar by watching a yaw string, not by watching for the glider to turn.
Thanks
Steve
Last edited by aeroexperiments on Fri Aug 10, 2012 3:42 pm; edited 5 times in total
PS- besides "to detect asymmetrical drag, fly with a yaw string", there is one other PRACTICAL lesson to be learned from the ideas in my previous post.
If you are intentionally flying with a drag asymmetry and you have a roll control problem, DON'T stuff the bar in the hope that this will give you more roll control power. At high airspeeds (low angles of attack), all flex-wing hang gliders will show a powerful tendency to roll and turn AWAY from the draggy wing, due to anhedral.
I know this from experience as well as theory. Some of my tests with the small drogue chute deployed from one wingtip, were, shall we say, a bit "interesting"!
Youall are just fooling with me cause I'm a yankee, I'm going to through caution to the wind and just fly without any weight on the nonfared side, then we can have a big discussion about the validity of the experiment...if I live.
I don't think the weight will endanger you but it will interfere with your ability to detect a small turning tendency. But like I'm saying, the tendency to turn (roll, bank) is not the relevant test anyway. You need to look for the tendency to fly sideways. Slip. With a yaw string that you can easily view in flight. Steve
OK, I think. Having sold insurance for 40 year's and NOT being an aeronautical engineer I'll have read this over a few time's to understand what your talking about but I think I should be able to figue it out. I'll get a Gopro and record what happen's, that in it's self should be interesting.
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