Apr 28, 2019, 08:18 AM
Registered User
Discussion

# Topmodel Discus 2 -

Hi, I have a Topmodel Discus 2a and have made maybe 5 or so flights from the slope with the centre of gravity at around 90 mm as per the instructions (90 to 100 mm). Im trying to get the model trimmed out and struggling a little so looking for some help. I have checked the centre of gravity and checked that the 90 mm is ok using a spreadsheet found on line. The instructions say 90 to 100 mm and using the spreadsheet cof g calculator this corresponds to 5% to 10% static margin. So my last flights have been with cofg around 90 mm, to get level flight i need the elevator trimmed so it is down by several degrees which just does not look right. I would be very concerned about flying tail heavy - i am in two minds if what i am finding is tail heavy (does not appear to be the case with the checks above or how it behaves in flight) or its related to decalage. Before i try something foolish and shim the elevator to reduce decalage, wondering what the opinions are? When i go to dive tests it pulls out - although i think this is more reflection i have not trimmed the model out as yet. Any help/advise would be appreciated.

I have measured the decalage in a couple of ways at its 4 degrees.
Last edited by Andynrose; Apr 28, 2019 at 11:51 AM.
 Apr 29, 2019, 03:34 PM Registered User The first thing to understand is that decalage is function of CG, not the other way around. That means determine the CG you want the plane to fly at and then fix the decalage if necessary. You never move the CG to get rid of unwanted elevator trim. Your observation that the elevator is trimmed down is a reflection of excessive decalage. The horizontal stab is producing excessive down force which the down trimmed elevator is compensating for. A plane like yours typically will have a decalage close to 1 degree when the CG is optimized. Realize that the recommended CG is usually conservative, meaning nose heavy. The optimum CG can only be determined through flight testing. In this case, since your decalage is significantly off, I would start by shimming the leading edge of the stab a bit and reduce the down trim in the elevator. You have already done a dive test which indicates that it is nose heavy so take a couple of ounces out of the nose or tape a 1 oz weight on the boom in front of the fin, the later having the advantage that you can just take it off if your are not happy with it. Go fly it. Trim it for slow level flight. I keep feeding in up trim until it starts stalling then back off until a stable cruise speed is achieved. Repeat the dive test. After you have done a lot of dive tests you will be able to tell how nose heavy it is by how fast it pops up. The other test is inverted flight. Do a loop and come out at the top with some speed. See how much down stick it takes to hold it inverted. If should be 1/4 or less. It just depends on how close to neutral stability you want to fly the plane as to how much down it will take. Repeat the above as many times as it takes. The goal is to push it to neutral stability. As it gets closer to neutral decrease the amount of weight you shift. Once you get it neutral, add a little nose weight to get it back to the positive side. If you find that the plane is tucking at high speed add more nose weight. Now you can fix the decalage. Shim the stab and re-trim the elevator until you are satisfied. I usually do this in small increments as I change the CG so that by the time I'm finished with the balance the decalage is pretty close too. Good luck.
 Apr 29, 2019, 07:17 PM Registered User Some more thoughts on things to observe. As the CG goes back the elevator will become more effective and you should expect to need less throw. Don't confuse a pitch sensitive plane due to too much throw with a tail heavy plane. The former will be fine if you take your hand off the stick whereas the later will go into divergent flight. Essentially a variant of a dive test: after trimming for slow flight which is the trim for the most altitude on the slope, bring the plane down and fly it fast in the stronger lift zone. The amount of down stick you have to hold will decrease as the CG is moved back and the plane is closer to neutral. As the CG goes back less up elevator will be required in turns. As the CG goes back it will be easier to flare for the landing and the landing speed will decrease. With a nose heavy plane there is a tendency to bang the nose down as the airspeed bleeds off and the elevator can no longer counteract the nose weight. A more neutral plane is like a balanced teeter totter. When the teeter totter is moved to a position it holds that angle. A balanced plane is similar. A little blip of elevator on the landing flare and it holds that attitude. I don't know what kind of tip stall the Discus has. If it is a problem, the solution is not to move the CG forward. Tip stall is a wing problem and the solution is washout. On my planes with an aggressive tip stall I program aileron reflex in the landing flight phase. Up high and when the plane is moving fast it is not an issue, but close to the ground you want a stable plane. All of the planes I've done this on have become almost spin proof.
 Apr 30, 2019, 03:33 PM Registered User Thread OP k2k, many thanks for your responses - that is clear and all makes sense to me. Looking forward to the next opportunity to make some adjustments. appreciate your feedback, and will let you know how it proceeds.
 Apr 30, 2019, 07:21 PM Registered User I agree, 4 degrees is far, far too much. Perhaps start with 1-1.5 degrees to save some time (edit: I see Ken already suggested that), and also search this forum for models similar to yours as many others have posted their settings. Ken gives plenty of great advice above. Good luck! Last edited by rjtw; Apr 30, 2019 at 07:51 PM.
 May 07, 2019, 02:39 PM DS will change your life Here is a thread in German http://www.rc-network.de/forum/showt...iscus+topmodel Cheers Soren
May 08, 2019, 01:36 AM
Registered User
Quote:
 Originally Posted by SpeedsterDEN Here is a thread in German http://www.rc-network.de/forum/showt...iscus+topmodel Cheers Soren
Soren, thanks for the link. Useful information in the thread, that i am now feeling more comfortable with making the adjustments. just waiting for the weather.........
Andy
May 14, 2019, 02:08 AM
Registered User
I also own the topmodel discus 2a deluxe.
Its had around 100 aerotows, since new.
The instruction specs for the c of g
are suicidial.
My centre of gravity is at 82mm.
I have found this is spot on.

My Tail incidence is at 1.5 degrees.
Flying weight-5.4 kg with pilot,dual batts for main power,separate battery for the retract,plus x3 switches.
When I first got it,I shimmed the front of the top of the fin, to decrease the tail incidence,
it was around 2.5 degrees from memory.
Great flier,
thermals well,likes to be flown on the pace.
Its a forgiving model,just keep the speed up in the turns,and on final approach,it doesn't like to be flown slow.
I have never sloped mine,only aerotowed it.
I have a fema retract fitted,with a 82 mm wheel.
I installed the fema retract as mine was the deluxe version,withouit the retract,but everything else.
I also have the topmodel discus 2c deluxe -4.5 metre span,which shares the same fuselage as the 2a,
the tail incidence was again decreased by shimming at the front of the top of the fin.
Both are great fliers,
both like to be flown on the pace.
Note-ignore the date stamp on my digital jpegs as I never got around to reset it