Need some advice from the sailplane community. I’ve flown my turbine powered U-2 6 or 7 times and each landing resulted in damage. The airplane flies like a dream until you pull the power and slow it down for the approach. The airplane becomes unstable in pitch as the aircraft slows. It appears the tail is stalling before the wing (modified Clark-Y). Increased flap deflections make the elevator less effective.
Take a look at the videos below. The first is not mine but appears to be from the same Kranz kit. Anyone experience anything like this with a sailplane?

[link=http://rcairgallery.com/pages/video.php?type=0]U-2 Land[/link] (Fourth one down under video

[link=http://video.google.com/videoplay?docid=-1718839142269896657#]U-2[/link]



Aloha,

Mike

It seems to me that the stab area is too small for the job.

Whith power, it functions well, but it does not at low speeds, and being affected by the wing wash.

The flaps only impose higher load on the stab, due to the additional pitch torque they introduce.

Is it feasible to increase the stab and/or elevator area(s)?

U-2s don't fly that fast for starters.
And they don't use (or even need) flaps for taking off.
For landings, these models appear to be overcontrolled, even allowing for the wind on the second video.
For the real bird, it goes like this....
http://www.fencecheck.com/forums/ind...c,21325.0.html

The distance from the wing to tail is very short and the tail area is not that big a percentage of the wing. Both of these factors will gang up to produce a very small tail volume coefficient. And that will produce the need for the CG to be located much further ahead than most models.

It's not that your tail is stalling, unless you've got the CG lcoated right at the leading edge, but that you're balancing the model at or behind the neutral point so it's incredably touchy. I suspect that the thrust of the engine is producing a little nose down pitch that you're compensating for with some up trim so it fakes a condition where under power the CG FEELS like it's further ahead and so the model is stable. But once you power back that's gone and it's just a glider.

There's links in the sticky thread at the top of the thread titles in the Aerodynamics forum to some online CG calculators. You input all the factors I mentioned about the TVC along with span and chord and it crunches a bunch of numbers and tells you were the wing's MAC is located and where the CG should be located for the percentage stability margin you request in one of the boxes. Take a few minutes to run the numbers and see what comes out. Run it twice with stability values of 3 and 10 to get a back and forth range of acceptable CG locations. A stability factor of 10(%) is darn close to what a trainer flies like and 3 would be a fairly neutral handling sporty model

EDIT- Here's a direct link to the calculator
http://www.geistware.com/rcmodeling/cg_super_calc.htm

Also I misremembered what stability values you should input. Seems like 5 is about as low as you want to go.

There's also another issue with the U2. The fuselage is very long for the aerodynamic length of the tail. This fact along with the fact that it's a scale model with a fairly heavy engine in the tail is not going to help with the pitch damping in flight. So you're going to want to use an even more forward CG to aid this factor. And on top of all this you're using flaps on a fairly wide wing with a model that has a very low TVC. I strongly suspect you're getting some pitch down from the flaps at the same time that they are lifting. Again this calls for a little more forward a CG location to aid in dealing with that along with setting up some mix into the elevator so dropping the flaps raises or lowers the elevator as required to compensate for any nose up or down pitching. It should also go without saying that if your fuel tank is ahead of the wing that the model must be balanced with an empty tank. Otherwise the fuel load burning away will again shift the CG back too far.

I just wanted to say that I've seen a few models fly that had too rearward a CG and I've even flown one such model. Your videos are very consistent with how such models fly. So use that online calculator to figure out where your CG needs to be and re-trim the model to suit. You can ignore the wheel weight distribution bits. That's for folks using the calculator on full size aircraft.

Thanks all,
What you're saying makes sense. I'll run the numbers. I have 3.5 lbs of fuel sitting on the CG at takeoff. Last flight I moved the CG a bit forward and deployed the flaps to about 40 degrees on approach (10mm up elevator trim compensation). The approach was rock steady, but it wouldn't round out when I attempted to flair. Ploughed in nose first with the stick in my lap.. Either the flaps are blanking out the stab or it's the pitch change you mentioned. Elevator throws are only 10 mm either way so CG must be pretty far aft. I'm thinking about trying a Gyro on the elevator to dampen the pitch and limit flap travel to 25 degrees for the next flight - it's going to land long though..

Also,
I ran the numbers and have a question (2):

Confirm "Y" is HALF the distance from the fuselage longitudnal centerline to the tip?

Does the sweep on the trailing edge matter?

The input for geistware for wing span is for half span. You measure from the CL of the fuselage to the tip. Look at the picture to the right on the input screen that shows the airplane in planform. The "Y" measurement is shown clearly there. It goes from CL to tip for one side. So your input is for half-span.

From the dimensions input, they have enough information that both LE and TE sweep is considered. You've input the root chord and tip chord. The formulas only need one sweep measurement to know what the other sweep is without your having to provide it. The only shortfall of that software is when your planform is more complex than straight taper. It will deal perfectly with the dimensions you provide so long as you have a simple planform with all straight lines.

If you run out of elevator authourity then it's not that the CG is too far ahead but that you need more elevator travel. Use dual rates or exponential for the high speed portion if needed.

If the real one blanketed out the elevator then yours will too. I don't recall any such worry with the real one though. Also we often say this or that was blanketed and that's why this or that won't work. But the number of true cases of being blanketed are few and far between. More typically there is something else at play such as incorrect CG locations or too much or too little elevator travel. The bottom line is that if the tail volume is very low there's two things that are needed to make the design work. First is to move the CG ahead. Second is to use a symetrical or lifting airfoil that has a very low pitching moment. You chose the Clark Y for your U2. It may well be that the relatively strong negative pitching moment of this airfoil combined with using flaps is why you're losing elevator authourity. If this is the case then moving the CG ahead will help but not by as much as it would if you had used a symetrical section or a flying wing airfoil with a very low pitching moment.

Thanks! The CG will move foreward on the next flight with a corresponding increase in elevator throw. It's tough to dial in an airplane when you have to repair it after every flight.. I wouldn't have selected that airfoil either. I bought the kit from Werner Kranz in Germany. It's all molded glass. It came with a Russian three-view with an X over the approximate CG location in the plain view. Even that was wrong.. What's really odd is the horizontal stabs are completely symetrical.

Mike

Given that you'll likely need to move the CG quite far forward it's very likely that the stabilizers will be operating in a negative lift mode much or all of the time (on more regular models the stabilizer actuall lifts up like the wing but just not as strongly due to the typical 30% to 33% CG location). So you may find it helps to replace the stabilizers and elevators with an upside down lifting airfoil to enhance the effectiveness during landings where you need to lift the nose to flare. But start with the new CG location and more elevator travel.

Looking at the pictures of the full size plane if your elevators are scale in size I would not hesitate to set them up so they deflect on high rate to around 30 or even 35 degrees up angle. If you do that you SHOULD have sufficient elevator authourity for landing duties.

land faster