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Jan 02, 2019, 12:48 PM
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Inside the box of Believer are servo extension cables. Can you tell me what is awg gauge for them? I can not figure it out. And what should Y servo extensions be used for? Thank you?
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Jan 02, 2019, 07:44 PM
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gromitvt10's Avatar
Not sure what the gauge is but the Y-harnesses can be used for ailerons and ESCs. I used a Y for ESCs but plugged ailerons into separate channels on the Pixhawk 2.1 so i could use them as flaperons.
Jan 12, 2019, 09:19 AM
Learn to fly with thumb..
Originally Posted by gromitvt10
My second bird i used some lessons from the first and was able to build much lighter, smaller motors, ESCs, removed some components i didn't need, smaller gauge and shorter wires, etc. Shaved over 3 lbs off, now she flies much better, slower cruise and way easier to launch.
One of these days i'll upload those videos.
Any hint on the spec and flight time?
Jan 13, 2019, 08:15 PM
Registered User
Re efficiency: I don't see anything mentioned about each plane's CG and how far forward or aft of datum they are. If the CG is too far from the sweet spot, you are going to increase drag -- sometimes by a lot, as the nose position will not be at optimal.
Jan 14, 2019, 07:59 PM
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gromitvt10's Avatar
Originally Posted by waladi
Any hint on the spec and flight time?
Motor - KDE 3510XF-475
Prop - APC 12x12E counter-rotating
Battery - (2) Titan 14,000 mah 6s Li-ion
Camera: Sony A6000 with 20mm lens
AUW - 11.2 lbs (my first build was 14.3 lb)
Take-off amps - 56
Cruise amps - 8
Cruise speed - 36 mph
Flight time - The max I've done so far is 2.4 hrs and i had only used approx 40% of the battery. This was on a cool, windless day. I only landed because my time at that location was up. One day, if i have the patience, i'll stretch it out and see how long it can stay up.

Jan 15, 2019, 02:13 AM
Pompecukor's Avatar

Were you thermaling? Your math does not add up.

8a cruise and you have 28Ah on-board. That is a total of 3.5hours to 0Ah. So 40% used in 2.4 hours is off somewhere. Especially on a cold day, you would get less yeild on your battery.

Besides, I think your cruise A is very high in comparison to what I or Nate is getting. Cruise A should be more like 4 or 5 on that setup.
Maybe you current sensor is bad? That is why you have 60% left after 2.4 hour flight?
Last edited by Pompecukor; Jan 15, 2019 at 02:19 AM.
Jan 15, 2019, 09:04 AM
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gromitvt10's Avatar
Originally Posted by Pompecukor

Were you thermaling? Your math does not add up.

8a cruise and you have 28Ah on-board. That is a total of 3.5hours to 0Ah. So 40% used in 2.4 hours is off somewhere. Especially on a cold day, you would get less yeild on your battery.

Besides, I think your cruise A is very high in comparison to what I or Nate is getting. Cruise A should be more like 4 or 5 on that setup.
Maybe you current sensor is bad? That is why you have 60% left after 2.4 hour flight?
Hi, Iím using a Mauch sensor which is usually pretty accurate. I averaged it at about 8A, I did see it in the 4-5A range as well as spikes in the 10-12A range. But it generally bounces between 5-8A. I was not looking at it the entire flight. I actually spend most of it outside watching the airplane go in circles.
But... now that I think of it the battery percentage in MP said 40%, which means that is how much I had left, so I used 60%. My mistake in the wording.
By my figures I could stretch it out to 3-3.5 hours. I could fly slower, stall speed was demonstrated at 25mph, so cruising slower will get those amps down. But I donít anticipate flying any mapping missions for over 2 hours!
Jan 16, 2019, 08:30 PM
Registered User
Is there a section in here where we can ask other q's about the Believer?
Jan 16, 2019, 08:35 PM
Registered User
There's a thread for the Believer.
May 08, 2019, 02:26 PM
Registered User

is there any possible way to find out what is the parasitic drag of an X-8 Skywalker?

I have a fully equipped and reinforced Skywalker X-8 that requires a minimum or 18 amps to keep an stable cruising speed.... The first model weighted almost 16 pounds "Fully covered with Fiberglass / landing gear..." the third generation seats at barely 8 pounds "Where every single possible weight reduction have been done" keeping in mind that if the plane isn't reinforced it will actually have a really nasty fluttering wing behavior that will eventually rip off the tips of the wings....
Now it is something that bother me a lot, to see that even under the most favorable conditions the plane needs a hole 18 amps... sometimes a bit less... but that is a true deal breaker.. I use a 20K mHa lithium battery pack that would give me an incredibly comfortable full hour or so of fly time... but the fact that the wing blended body system have such a large wing cord make me think that the parasitic drag is just to much and as the plane it exponentially compounding the thrust requirements making it an unreasonable platform for a true long range system???? any help will be greatly appreciated.
MaX-8 Skywalker "New Gimbal" II (23 min 19 sec)

Originally Posted by mike_o

Just look closely at the plane's attitude at low speeds: The slower the plane flies, the more nose up it will need to generate the same amount of lift (as it's not getting any lighter by flying slowly), and it's rather intuitively understandable, that this will cause more drag somehow.

But I think that many model pilots tend to not fully comprehend the nature of drag, or they don't look closely at the attitude vs speed. Basically, were all left with the same, old dilemma of induced vs parasitic drag. There are only two ways to optimize on a given wing profile; keep weight down (surprisingly often ignored or not taken to the limit) and keep the surface and structures aerodynamically clean.

Reducing weight will lower the induced drag curve, and improving aerodynamic airflow will lower the parasitic drag. I think that reducing weight is not taken seriously enough.
May 08, 2019, 07:27 PM
Registered User
I am not at all expert at all on X-8, and wings in general, but...your plane seems to be pretty streamlined from the few seconds in the beginning of the video. If your plane weighs 3.6 kg + 1 kg 20 Ah battery + 0.5 kg FPV, the total should be around 5 kg. 18 A /4 S for a 5 kg is extremely bad value, it means it is actually flies like a brick...

1. Are you sure your current sensor is OK and calibrated?

2. Prop pitch / motor kV combination? If you have very low or very high pitch compared to optimum pitch, the current can be way off; if pitch is too high, at slow speeds the prop will be bashing air (stalled) and that generates a lot of drag on prop and high current. If this were the case, you would see improved current or at least same current at higher air speeds; Also, the climb performance would be terrible. if pitch is too low, the top speed will be very reduced, but your climb performance should be good.

Trying to calculate parasitic drag is a big project, and probably pointless. What I always do though, I fly all new airplanes at different power settings (something like 8-10-12-15-20-30 A) and take note of the airspeed, and then make a simple graph which indicates the efficiency at a given speed. It is not the same as the drag/speed curve, but it gives you pretty much the same idea.

One important point is that any, even small structure on the airplane (like a a small gymbal) can have a dramatic increase on the overall parasitic drag. A 5 x 5 cm. square plate (equivalent to a runcam 2 HD on two pan and tilt servoes) can easily generate 10 % of the total drag of a 2 m. span airplane. People who put rectangular Gopro actually put a very efficent airbrake which reduced the efficiency by up to 20 %. However, from the looks, your airplane does not have anything like that.

I would look into Motor / Prop issues, and you probably should give more details on that and on the weight of your wing
May 10, 2019, 06:01 PM
Registered User
After reading this thread, I was surprised that square props (12 x 12, etc.) seem to be more efficient than those with lesser pitch. I have MTD and have been using 9 x 6 prop with 1000 kV motor. Today I tried running 9 x 7.5 prop, and the results were very disappointing. The efficiency was about 7-10 % less for all speeds (50-110 km/h) for the 9 x 7.5 prop. The setup with 9 x 7.5 was 4600 grams versus 5000 grams for 9 x 6 prop (I did the series the other day), the altitude was almost the same (800 m for 9 x 7.5 versus 1200 m for 9 x 6 test), considering that the airplane was lighter, the overall efficiency loss is probably in the range 10 - 15 %. And no improvement was seen in the higher speed range, the efficiency drop was very even.

I also bought the 9 x 9 props, but in light of the previous test decided not to try it out (I am worried about the overcurrent and the stalled props which could make launching more complicated).
Last edited by chileflora; May 10, 2019 at 06:13 PM.
May 10, 2019, 06:07 PM
Registered User
The "positive" thing about 9 x 7.5 was that at full power I could hear the prop sound at a distance of up to 2.4 km!!!

In light of the above, I am inclined to try out 10 x 5 (could not find 9 x 5), not sure if someone had the experience... Strange that my results seem not to agree with the detailed tests run by the topic starter...

Here is my data:

9 x 6, 5000 grams 1200 m:
km/h motor power W
56 129
69 186
81 229
90 292
112 430

9 x 7.5, 4600 grams 800 m:
km/h / motor power W/ interpolated efficiency loss
50 124 =-7%
61 180 =-12%
75 260 =-7%
95 337 =-10%
110 500 =-14%
May 10, 2019, 06:22 PM
Registered User
The other strange thing was that the climb performance was indeed the same or slightly better than that of 9 x 6:
I got about 2800 mah at average 14.5 V per 1000 vertical meters and 4600 weight
I usually get 2800 mah at average 16 V per 1000 m for 5000 gr weight.

Correcting for the weight difference and voltage difference, the 9 x 7.5 seems to be marginally more efficient than 9 x 6 for climbs; that is very counter intuitive...

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