X8 vs. flat octo redundancy comparison.

[Bartman]

Going back to the original topic of redundancy, how many of you have had hands on experience with one or more motors failing on on Octo? And what were the results? It look like there are not that many!!


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I have a Mikrokopter guided XY8 that had a blinky motor controller. Twice it caused the motor to stop and twice the helicopter took it in stride and kept flying as if nothing was wrong. The first time I saw that the motor had stopped and I landed immediately. The second time I didn't see that one motor wasn't turning until after I had finished the flight and was returning to land.

I also had a motor come apart on my Hoverfly guided XY8 but I was close to the ground and it landed hard. It all happened too quickly to really know if I made the situation worse trying to maintain control after the prop flew off. It might have done better if I just stood there for a second and watched to see what it wanted to do. The prop flew off in an instant with no warning and it was a little bit of a shock, then in an instant it was landing on the pavement, bouncing, then on its head.

Mactadpole also had a coaxial quad lose a motor and it landed without incident, look in his blog for the video.

Bart

 

[DennyR]

Going back to the original topic of redundancy, how many of you have had hands on experience with one or more motors failing on on Octo? And what were the results? It look like there are not that many!!


When I was playing with MK it was quite regular. I have on at least two occasions seen BL-2's burst into flames whilst sitting on the ground looking for Sats. After getting into KK, CC, APM and DJI I stopped seeing that kind of thing because I was now in control of ESC/motor combinations. That changed my whole thinking about the redundancy factor.

I recall Mk selling an aerosol called Contact, If you cleaned your ESC with that stuff it would never dry and would get into the processor and it would never work properly again.

 

[homer911]

Looks like the MK can handle it but what about the WKM???


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[questech]

how is a two motor weight any different from two motors on two adjacent arms? i'm sorry but that's bunky.

One thing for sure is that you are putting more weight and more flight stress on fewer arms. Unless you increase the strength of each arm (now 4 arms have to carry the load and stress of turbulance that 8 use to) it could lead to catastrophic results.

 

[koolkiwikat]

One thing for sure is that you are putting more weight and more flight stress on fewer arms. Unless you increase the strength of each arm (now 4 arms have to carry the load and stress of turbulance that 8 use to) it could lead to catastrophic results.

Im sorry, where is your proof? rather then base your ideas on theory show me the money!, Ive flown coaxial for years under load without having to think of 'thicker arms', never have had an arm or heard of a in flight failure of a boom

 

[DucktileMedia]

In theory that may be true but the arms most of us are using are WAY stronger than necessary for our needs. They only break in a crash from my experience.

 

[questech]

Im sorry, where is your proof? rather then base your ideas on theory show me the money!, Ive flown coaxial for years under load without having to think of 'thicker arms', never have had an arm or heard of a in flight failure of a boom

No Proof required, it is simple logic.

Our gear may be strong enough not to worry about any failures of this type, but my answer is still absolutely true to the question being asked.

As a full scale pilot, we learn early on about the dangers of turbulence and the additional stresses it places on the airframe... just facts.

So when you add more load to a structural component, it doesn't get stronger, it gets one step closer to a failure point. That's just physics.

I didn't say it WOULD lead to catastrophic results... I said it COULD lead to catastrophic results... and it could.

The question was "how is a two motor weight any different from two motors on two adjacent arms?" and my answer is one of the ways it's different.

Again, a fact... no proof required :02.47-tranquillity:

 

[FerdinandK]

Typically arms of a X8 are much shorter than on a flat octo. The shorter the arms the less material needed on them.

The weight of the motors/props on the arms is not counting, since the "lift themselves", when the copter is in the air, only the vibrations and torque caused by the motors are a valid argument.

Not everything a "full scale pilot" tells you is a valid point on multicopter, also airplains are constructed by engineers not by pilots (they only fly them).

best regards
Ferdinand

 

[ary]

with X configuration you can use larger props than flat 8.
Larger props with lower KV motor=more flight time

 

[questech]

Typically arms of a X8 are much shorter than on a flat octo. The shorter the arms the less material needed on them.

The weight of the motors/props on the arms is not counting, since the "lift themselves", when the copter is in the air, only the vibrations and torque caused by the motors are a valid argument.

Not everything a "full scale pilot" tells you is a valid point on multicopter, also airplains are constructed by engineers not by pilots (they only fly them).

best regards
Ferdinand

The lift forces on the arms/booms are far greater than the standing weight. The weight on the arms/booms while sitting on the ground, is not the issue, once the motors lift the craft and its payload the stress on the airframe then becomes a real factor. During turbulence, or erratic control input, the stress factors are multiplied and can cause problems.

These models are much stronger in proportion than full scale aircraft, so it isn't as much a factor, but it is still physics and extra weight on these arms/booms will have a physical effect. Whether it's a factor or not is another story.

And not only am I a full sale pilot, but my brother and I build Real airplanes that real people fly in and we maintain general aviation aircraft.

Why the hostility... :dejection:

 

[FerdinandK]

If you have a beam, fixed on the one side, and loaded on the other side, it is important how much load you have, but also (due to lever rule) how long the beam is. On a X8 the arms have typically 1/2 the length than on a flat octo (that is physics double load and half the length does cancel out). Typically the X8 frames will be much more lightweight and still more rigid.
If you want to argument how experienced and knowledgeable you are, show pictures or your copter builds, show videos which document that you can deal with vibrations, show your calculations if you made some. If you build full scale aircrafts, that does not imply that you are a good multicopter builder/constructor/designer , since a lot of different points play a role on multicopter. It is much more difficult to produce/manufacture/assemble a proper and valid wire-tree, than to assemble a rigid frame (in the world of multicopter). On what turbulences and vibrations you are talking about due to a koax setup, I can only tell you from my experience that they are in the same range than on a flat octo.
best regards
Ferdinand

 

[questech]

If you have a beam, fixed on the one side, and loaded on the other side, it is important how much load you have, but also (due to lever rule) how long the beam is. On a X8 the arms have typically 1/2 the length than on a flat octo (that is physics double load and half the length does cancel out). Typically the X8 frames will be much more lightweight and still more rigid.
If you want to argument how experienced and knowledgeable you are, show pictures or your copter builds, show videos which document that you can deal with vibrations, show your calculations if you made some. If you build full scale aircrafts, that does not imply that you are a good multicopter builder/constructor/designer , since a lot of different points play a role on multicopter. It is much more difficult to produce/manufacture/assemble a proper and valid wire-tree, than to assemble a rigid frame (in the world of multicopter). On what turbulences and vibrations you are talking about due to a koax setup, I can only tell you from my experience that they are in the same range than on a flat octo.
best regards
Ferdinand

I'm not on trial, so I don't have to do any of that stuff.

I don't care what anyone thinks about my level of knowledge, that isn't a factor in my life at all.

I was doing some research and read this line "how is a two motor weight any different from two motors on two adjacent arms?"

And I simply made a comment as to one way it was different.

I don't see how aggression is helpful to anyone... but don't worry, I've put my last bit of energy into this conversation.

I wish you well.

 

[Scott Drysdale]

I am inclined to think that the X8 coax is a technically better design all around provided that the camera fov is not affected by the bottom mounted motors. As far as system efficiency, it seems to be that a slight loss of efficiency due to one fixed pitch prop being stacked above another, is made up for by the reduced MR airframe weight. Too bad someone does not make the bottom props with a slight pitch correction to eliminate the remaining stacked prop inefficiency. Howard Hughes team solved that issue back in the 40's with counter-rotating airplane props but using hydraulics to adjust pitch..... but then HH flew it before final engineering QA and crashed it...... pilots should listen more carefully to the aircraft design engineers...... they are most often one step ahead with respect to QA and safety.

If you have a beam, fixed on the one side, and loaded on the other side, it is important how much load you have, but also (due to lever rule) how long the beam is. On a X8 the arms have typically 1/2 the length than on a flat octo (that is physics double load and half the length does cancel out). Typically the X8 frames will be much more lightweight and still more rigid.
If you want to argument how experienced and knowledgeable you are, show pictures or your copter builds, show videos which document that you can deal with vibrations, show your calculations if you made some. If you build full scale aircrafts, that does not imply that you are a good multicopter builder/constructor/designer , since a lot of different points play a role on multicopter. It is much more difficult to produce/manufacture/assemble a proper and valid wire-tree, than to assemble a rigid frame (in the world of multicopter). On what turbulences and vibrations you are talking about due to a koax setup, I can only tell you from my experience that they are in the same range than on a flat octo.
best regards
Ferdinand

 

[Stacky]

I am inclined to think that the X8 coax is a technically better design all around provided that the camera fov is not affected by the bottom mounted motors. As far as system efficiency, it seems to be that a slight loss of efficiency due to one fixed pitch prop being stacked above another, is made up for by the reduced MR airframe weight. Too bad someone does not make the bottom props with a slight pitch correction to eliminate the remaining stacked prop inefficiency. Howard Hughes team solved that issue back in the 40's with counter-rotating airplane props but using hydraulics to adjust pitch..... but then HH flew it before final engineering QA and crashed it...... pilots should listen more carefully to the aircraft design engineers...... they are most often one step ahead with respect to QA and safety.

The savings in weight with shorter arms are not enough to make up for the loss in efficiency, in fact the weight savings in losing 4 arms are not as great as people think. You still end up with a similar amount of wiring going through 4 arms. I found this when trying to work out if I wanted a X8 or an Octo. Have been through the measuring and weighing process twice on this one. There is some weight saving but its not as much as people imagine. X8 arms are not half the length of Octo arms in my experience, closer to 2/3rds.
Most people run their X8's with a higher pitch prop on the bottom to help with the efficiency issue and again it just isnt enough to cover the efficiency losses compared to an Octo.
People have been fighting this problem for a few years now and I havent seen any ideas that have solved the problem when tried in real life.

 

[jes1111]

I don't see why this flat vs coax thing is such a big issue. What does "It's less efficient" really mean? Shorter flight time for the equivalent configuration. Big deal! - why would maximum flight time be the highest priority for AP? I've gone for coax because flat layouts are just too darn big. Period. I don't give two hoots about the "loss in efficiency".

 

[Stacky]

I don't see why this flat vs coax thing is such a big issue. What does "It's less efficient" really mean? Shorter flight time for the equivalent configuration. Big deal! - why would maximum flight time be the highest priority for AP? I've gone for coax because flat layouts are just too darn big. Period. I don't give two hoots about the "loss in efficiency".

Well if you are one of the guys shooting video for a tvc for example getting a 12 minute flight over a 10 minute flight might just give you one more pass on a shot the director is trying to get you to grab without the delay of a battery change.

 

[FerdinandK]

That is the old story, people building flat octos claim that they are more efficient (compared to X8), and people (like me) building X8 claim, that the loss in efficiency can be covered by weight reduction.
I can build a frame for a 5kg (AUW) X8 with 150gr (14" props) , I am afraid, no flat octo frame with 14" props less than 500-700 gr is possible (with comparable stiffness). If you cannot save weight at the wiring when switching from an flat to an X8, there is something wrong.

Here an X8 with 60 min flighttime:

This copter has 14" props, and the copter has only 1,35kg possibly somebody can share the weight of a flat octo with 14" props for comparison ...

best regards
Ferdinand

P.S. I already thought, that the times of 10-12min copter has long gone ...

 

[Stacky]

The weight saving from the wiring is minimal. You still have 8 motors requiring wiring. The arms are not halved in length and you are not saving half the wiring weight of each motor.

What payload is the 1.35kg copter capable of carrying? You need to compare like with like. Have messed with both formats and I went through the weighing and comparison side of things with the 2 different builds I tried. As it is I ended up with X8's because im mostly just doing stills and am not concerned with longer flight times.

 

[jes1111]

Well if you are one of the guys shooting video for a tvc for example getting a 12 minute flight over a 10 minute flight might just give you one more pass on a shot the director is trying to get you to grab without the delay of a battery change.

Fit a larger battery?

 

[Stacky]

Fit a larger battery?

yeah as tongue in cheek as that presumably was in a real world situation within the context of the bigger picture that probably isnt really the solution for a variety of simple reasons.