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Apr 20, 2019, 03:12 AM
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Quote:
Originally Posted by Stuart A
Congrats ettore,I love this wing!
Is some of the ‘buzz’ coming from the shaft extension?
Thank-You Stuart!
Yes. At about 50% throttle stick start a bad resonance and over 80% of throttle stick the resonance disappear.
I can't do nothing to avoid this problem but until now the two ball bearings are 100% ok.

After a flight of about 2:30" the 1.300 mhA. li-po battery is at 80%.......so the motor's amp drawing is very low, something about 8 Amp.
With this very low power requirements even a bad vibration/resonance on the motor shaft is not a problem.

On a big scale DFS-40 model, the motor shaft's vibrations will be a serious problem to solve.

ettore
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Apr 21, 2019, 09:59 PM
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EdSoars's Avatar
A lovely aircraft, well-modeled. Thank you for the story!
Apr 22, 2019, 11:33 AM
Everything's A Composite
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Quote:
Originally Posted by quenda

On a big scale DFS-40 model, the motor shaft's vibrations will be a serious problem to solve.

ettore
In order to reduce motor shaft vibration, consider a support bearing near the mid point. Be sure to offset the location of the bearing so that the shaft on either side is not of equal length. This will help disrupt harmonic vibration.
Apr 22, 2019, 02:33 PM
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IMO 3mm is min thickness for strip planking on anything over 60 cm span, 6mm for larger builds.
Thinner is too weak and there's' not even close to enough material to sand off to fair the shapes decently... even to eliminate all the seam ridges.
Also using water based fillers (anything water based actually) is a mistake.. as water warps balsa skins .. badly.
Even in your pix one sees the visible result, of the sheeting warping between structure.
No biggie for a 'tester' but annoying and avoidable, nonetheless.
Apr 30, 2019, 06:57 AM
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Hello!
Here are four photos of my DFS-40 just painted in light aluminium:

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Description:

Other improvements, I hope...:
- two new digital micro-servos for elevons
- longer (+ 20 mm.) landing gear legs
- added 40 grams of lead in the nose

ASAP I will ask to my friend Giangi Mazzari to do the second flight.

Fingers crossed!

ettore
Apr 30, 2019, 10:20 AM
Registered User
Beautiful work! Extremely impressive. I have a question. You said the battery was as far aft (back to the rear) as possible, yet you had to add lead in the nose to balance. Maybe it was a mistranslation?
If not, you could move the battery forward and remove some lead to achieve the correct balance.

Also, with the shaft harmonics you could have better luck with a wider diameter tube. It would be stiffer and resist wobbling. There would need to be some nifty lathe work done for each end though. There is a video on youtube of a carbon wing with a long propshaft to keep the motor forward. Sounds very cool!
May 03, 2019, 10:46 AM
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Thread OP
Hello Diggerkelly.
Of course is my mistake!!…..you know….I’m really in troubles writing in English.
I mean that the 3 cells 1.300 Mah. li-po battery is in the very forward position….inside the model’s nose. Sorry!

The carbon fiber motor-to-propeller shaft weigh 4 grams. When I installed a stainless steel shaft (20 grams) I had stronger vibrations. So, for now, I leave the CF shaft in place…..until it breaks apart in flight!!!

------------------------------

I've just edited and uploaded on You Tube the video of the second flight.
As you can see the take-off run is shorter than in the maiden flight. So the AoA on the ground is OK.

The pilot says that the model’s balance is almost perfect……the only problem is that on full power the model want to dive down, so for the next flight I will add a “Throttle-to-Elevator” mixing on my transmitter.

Again the bad resonance noise from the CF shaft…..

The landing is something to remember……bravo Giangi!!!

DFS-40 (Delta V) - Secondo volo (3 min 19 sec)


Ettore


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