TopModel Samsara 3.2 meter glider, build thread on Aloft's Forum.

[Konrad]

Now that's the way to put on the pressure!

TopModel Samsara

The Samsara is an excellent thermal ship that features a sleek and long carbon reinforced glass fuselage that is light yet strong with ample room for gear. A traditional built up, 3 piece balsa wing with D-tube does the heavy lifting. The full flying elev

alofthobbies.com

 

[Nigel]

Hmm, @Konrad I have a question about that tail mount/pivot. I'm building a 3 meter Thermik Dream, and it has the same full-flying tail system. Did you sand the glass down to allow for extra throw? Or was it satisfactory from the factory? I'm only getting between 8-10 degrees up and down movement.

 

[Konrad]

I don't recall doing anything to the rocker other than adding a bit of glue to one side, to seat the stab square to the vertical fin. I think I state the elevator movement I'm using, 12mm up 17mm down. I'll measure when I get home. Now I do like an aft CG, so I cut down on a lot of elevator throw.

 

[Konrad]

In degrees high rate 8° up and 11.5° down. I really don't need that much down. Low rate 5.5° up and 8.5° down

 

[RWallage]

Just to see if the wing fits in my bag, can someone tell me the measurements of the separate wing pieces?

Thanks.

 

[Konrad]

That's a criteria? That might explain why my ships look so ratty!
But here is what I got
1 @ 1350mm x 250mm
2@ 965mm x 250mm

 

[bat68be]

Hi and greets from Belgium!
Maiden flight of my Samsara last week. Not knowing about the incorrect cg, I balanced the glider at 72mm.
Motor: Xpower F3826/10
(183gr)
Lipo: 3s-3000mAh
(234gr)
Added +200gr of lead in the nose to get it at 72mm.
No flutter at all during dives at high speed.
Today, I changed the setup to get rid of that lead in the nose.
Motor: Mega 22/30/3E
(220g)
Lipo: 3s-5000mAh
(367gr)
CG at 80mm.
3 flights today and flutter at every attempt to get some speed.
Moved the CG to 77mm, still some flutter from time to time, but it was worse at 80mm.
After reading some forums like this one, I will try again tomorrow with CG at 90mm.
Change the lipo back from 5000 to 3000mAh.
I will also try to get the play out of both wingconnections using thin tape.

 

[Konrad]

Did the Samsara zoom up out of the dive test with the 72mm CG?

With the larger batteries to get rid of the dead weight did the total weight go up as the CG shifted aft? What is the AUW at the various CG positions?

All models have a speed limit. Don't over speed the Samsara. I think I said I stay within 3 to 4 times the stall speed.

I perform the "dive test" to find the neutral balance point. I don't dive the Samsara. (You can see that the Samsara will dive like on the back side of a loop).

If I need speed I reflex the airfoil. This removes a lot of forward pitching moment from the airfoil. This then unloads the spar allowing for a bit more speed before the structure starts to flutter.

The Samsara is NOT a speed demon! Don't try to fly her like one. See some of my videos to get an idea as to the safe speeds to fly the Samsara.

Always tape the panels, to seal the gap!

Why the motor changes?

Watch the pitch speed!

All the best,
Konrad

P.S.
You need to try to keep the Samsara under 2 Kg. As the Samsara gets heavier the wing flutter speed drops! This is true of all airplanes. The heavier the plane the more the wing needs to work causing more of a pitching moment (twisting force) around the spar.

I think my power set up (Motor/Battery) is about 365 grams

 

[bat68be]

The Samsara recovered slowly from the dive test at 72mm cg. Certainly not zooming up fast, giving me the impression the cg was pretty good.

AUW about 2150gr in all situations (72, 77 and 80mm)

I changed for a heavier motor to get more weight in front. Removed the 200gr of lead just behind the motor. That allowed to put the lipo closer to the motor.
I removed the lead (200gr) and compensated the weight: +37gr motor and + 133gr lipo. So this resulted in a little more aft cg and -30gr in AUW

I'm not looking for a speed bird, but a little bit of speed between termals is welcome.

Don't understand why I was able to fly it very fast first time without flutter... and next time, heavy flutter at lower speed. ?

Anyway, I will give it a try at 90mm cg and see how that goes. Just change the 5000mAh lipo back to 3000mAh and balance again. AUW should than be at 2000gr

 

[bat68be]

Outer wingpanels are fully taped, both sides.
Pitch speed is no issue, I have 12x8 prop.

 

[bat68be]

I agree lower AUW will reduce the risk of flutter.
But I will also take a closer look at the outer panel connections. There is indeed a lot of play there that could trigger flutter.

 

[Konrad]

The Samsara recovered slowly from the dive test at 72mm cg. Certainly not zooming up fast, giving me the impression the cg was pretty good.

AUW about 2150gr in all situations (72, 77 and 80mm)

I changed for a heavier motor to get more weight in front. Removed the 200gr of lead just behind the motor. That allowed to put the lipo closer to the motor.
I removed the lead (200gr) and compensated the weight: +37gr motor and + 133gr lipo. So this resulted in a little more aft cg and -30gr in AUW

I'm not looking for a speed bird, but a little bit of speed between termals is welcome.

Don't understand why I was able to fly it very fast first time without flutter... and next time, heavy flutter at lower speed. ?

Anyway, I will give it a try at 90mm cg and see how that goes. Just change the 5000mAh lipo back to 3000mAh and balance again. AUW should than be at 2000gr

Every ship is different. Mine had a very strong zoom up at the factory CG.

The structure can have alsorts of things excite it, leading to flutter. I fear that once a structure starts or has been subjected to flutter it will go into flutter earlier the next time. (The joints have loosened up). Re-shrinking the film often helps move the flutter point back up to a higher speed.

I assume your flutter is with the motor off.

With my motor, prop and battery I can't get my Samsara into flutter in level or climbing flight, even at full power (aprox 600 watts).

OK, Running between thermals is done in reflex and should not drive the Samsara into flutter. My Samsara does not go into flutter running away from thermals/sink. I can still drive my Samsara into flutter if I'm trying to dive home (not in crow) to land within the time window.

I too am looking to use a heavy motor and a lighter battery to lower my AUW. Right now I think my AUW is 1.950Kg.

I like the idea of removing the wing joiner slop with tape. There is about 20 grams of mass saving going to carbon tubes from the glass rods. (I detail this, in this thread see post #138 on pp #7)

I only tape the top to seal the joint.

All the best,
Konrad

P.S.
With the 935KV motor I see a no load pitch speed of 126kph (78mph) this is way too fast! Real pitch speed might be closer to 100 kph (62mph). Can you change to a 13x6 prop?

 

[bat68be]

Ok, did some work early morning.
Installed the 3s-3000mAh lipo and balanced to glider to 90mm cg.
It needed about 15gr of lead attached to the front of the lipo to arrive at 90mm cg.
AUW is now 2050gr.

Also added strip of thin tape on the wing joiners. Only the part that goes into the outer panels.
This reduces 1/2 of the play at that area.

Programmed flightmodes an a 3 position switch:
- speed (flaps and ailerons 2mm up)
- cruise (flaps and ailerons at neutral)
- thermal (flaps and ailerons 2,5 mm down)

Didn't change my spoiler setup.
Ailerons start moving up when spoiler stick is at 50%.
So the first 50% of spoiler does only affect flaps.
In thermal I can use the first part of the spoiler (0-25%) to get some extra flaps down.
This worked very nice during earlier flights.
The only thing I changed is the spoiler-elevator mix.
Gave it some extra elevator down mix for the new 90mm cg.
Yesterday, moving from 72mm to 77 and 80mm, the down elevator mix at full spoiler was not sufficiant.
The glider pitched up to much.

I can still use the 3s-5000mAh lipo for more forward CG positions: 77, 80 and 84mm. (see picture)

Weather is fine today with no wind. I'll fly this afternoon and report later this evening.
Fingers crossed!

 

[bat68be]

About the motor/prop combinations.

I started with cheap setup: Xpower F3826/10 motor, allu spinner and cam carbon 12x8

Than I switched to expensive setup: Mega Acn 22/30/3E, RFM carbon spinner and RFM carbon prop 12x8

Guess what… the cheap setup was much more powerfull !!! Rate of climb was about 30% better !!!

So what's the problem here???

Think I figgered it out:
-on the allu spinner, distance between prop bolts is 50mm
-on the RFM spinner, distance between prop bolts is only 36mm
So on the allu spinner, the prop diameter is larger, giving more static thrust and better climb rate.

I think a 13x6 on the RFM spinner is worth a try. But for now, I'm done spending more money on this glider.
Motor performance is not the most important here.
It's al about soaring!

 

[Wayne]

I would have to run the numbers for both motors to see what the ideal prop size would be, but yes, yoke does play into things also.

 

[Konrad]

...
think a 13x6 on the RFM spinner is worth a try. But for now, I'm done spending more money on this glider.
Motor performance is not the most important here.
It's al about soaring!

I agree it is all about soaring not money.

On the power side of things one needs to match the motor, prop, battery and airframe.
As a prop absorbs power as cubic function of RPM one wouldn't expect the higher torque (Kt) motor lower RPM (Kv) to have more power than the lower cost high Kv motor on the same prop. As you noted you are actually using a smaller effective prop diameter. This is going the wrong direction with the lower Kv motor. (A prop absorbs power at the 4th power of the diameter).

Back in the good old days of E-Zone Jim Brouke wrote a nice easy to follow explanation of how to match the prop to the motor.

Control Tower - February 1999 - RC Groups

Control Tower - February 1999 Electric Plane Talk

www.rcgroups.com

This does not explain how to match the prop to the airframe. If you read this thread you can see how I went through the prop selection process to keep the power/speed below the flutter point.

As I'm not using high power set ups. I find the low cost Aeronaut carbon props more than adequate. I'd save the RFM props for application using much more power.

I'd spend the few extra dollars to make up some carbon wing joiner. Loosing 20 grams from the tips does make it easier to read the lift off to the side of the aircraft.

 

[Konrad]

Ok, took some measurements of my elevator rigging. This is with my CG at close to 95mm.

In neutral trim the rocker bed is at 20mm.

In full crow (flaps down at 85°-90° and ailerons up about 25°) the rocker bed is at 15.5mm. 3/4 of this down comp. comes in during the first 1/4 of the stick (crow command).

 

[bat68be]

Few more flights this afternoon.
Started with the 3s 3000mAh and CG at 90mm.
I'm pretty happy with the result! This is the perfect spot to float this ship.
Risk of flutter is further away. I only got it in to flutter once at cg 90mm, but I had to push it down real hard to get there.
It flies great in this configuration with aft cg and lower AUW.
The only negative here:
It does not auto-recover from a dive. When you speed it up and let go of the elevator stick, it keeps on diving untill you hit the ground.
But hey, it's a soaring machine, not a speedbird.

Also did some flights with the 5000mAh lipo at 84mm CG. Not bad at all, but with only 130gr extra weight it's a different feeling.
The heavier lipo will be used in more windy conditions and for those extra long flight times.

My flight mode setup was not a succes. Tuned the thermaling mode to aileron 2mm down and flap 5mm down. That works just fine.
But the speed mode... not found the right setting yet.
Tried different settings:
- aileron 2mm up, flap 2mm up
- aileron 1mm up, flap 1mm up
- aileron 0, flap 2mm up
In all 3 configurations, the glider picks up speed and keeps on accelerating...

Long story short: it flies the way I want it to, and the way it was designed for.
The more aft cg in the range of 90mm is the sweet spot, but not the best spot for less experienced pilots. They should fly their cg in the range of 70mm to get a more stablr plane.
Combination of both is not possible I guess... the nose section is to short.

Another issue on my Samsara that I haven't mentioned before is the wing servos: 4 x HS82MG.
All 4 have quite some play. (Ailerons and flaps can be moved 1mm up/down)
I should have invested a little bit more money there.
Just wondering what's the position of the flaps in flight (when neutral on the ground). Think I'll install a small camera on the fuse to check this in flight.
Not sure, but I think the flaps and ailerons are a little bit up in flight due to the play in the servos...

 

[Konrad]

Alright!

It is sounding like you are getting the trim close to what I like. I hate auto correcting (nose heavy) trim. What you call a negative is what I call a positive trait. I like this trim for both my speed and slow thermal ships (neutral trim). With a glider this kind of trim results in the lowest possible drag trim. Meaning that the glider airframe will generate more lift for any given speed. In a speed ship I'm not having to hold forward stick as the plane goes faster and faster.

The flat bottom AG airfoil does NOT respond to reflex as well as some of the aft loaded (slight under cambered) airfoils you might be use to using. Yes, in reflex you will need to re-trim (two or three clicks of up) the elevator to keep the airframe from diving (accelerating).

As to the last comment, can you put the thermal mode on a pot? This way you can preload the camber to take up the slop in the linkage. But yes the if there is any slop the flaps the slip stream will push the flap up. So to be in profile in flight you might have to droop the flaps a bit on the ground (static trim). Honestly the airframe is a bit loose that I don't think you really need too high a quality gear train. That having been said, yes the flaps need the best gears you can afford. After all you are likely to be driving the flaps close to 100°. Over this range of motion any slop becomes obvious. With your radio can you over drive the servo to maybe 120° (60° +) rotation? Most radio default to driving the servo for only 90° (45°+).

I love the CG at 92 to 95mm. But you are correct this is not a beginner's sailplane.

Even with my 2250 mAh 4 cell I can fly all day and only take out half the energy from the battery.

All the best,
Konrad

P.S.
I think everyone should start with the CG at 85mm. This is a safe nose heavy setting. 70mm is just gross! Way too nose heavy, making it almost impossible to fly slowly. It also makes controlled flight very difficult as the trim setting are always changing as the speed changes. Again 70mm is a gross setting!

Have you been able to do any thermal work? I think you will really like the 1.8m long fuselage!

 

[Konrad]

Wow, Topmodels has changed the manual. Original CG is shown as 67mm to 75mm (See back page 10). The front page shows the CG as being 75mm to 85mm. This is good move in the right direction.

Now what is bad is that Topmodels has moved the elevator servo to under the wing. This just makes the model heavier and adds slop to the control system. What I'm sorry to see gone is that the push rod set up for the properly placed elevator servo in the vertical fin is gone. Again you can still see the proper placement of both servos in the tail in the last pages (page 10).

Adding the weight of the push rod and still keeping the wires going aft for the rudder servo does NOT reduce the need for nose weight.