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Maximum acceleration (Gs) that our miniature turbines can handle

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Maximum acceleration (Gs) that our miniature turbines can handle

Old 04-23-2024, 05:29 PM
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acw
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Default Maximum acceleration (Gs) that our miniature turbines can handle

About 2 weeks ago, I had a flame out with my Ares L. The engine made some unusual noises and then silence. I almost made it back to the runway but slightly damaged the front gear mount after landing in a rough area. After inspection I noticed the electrical connection to the engine popped of. I assumed airflow from intake did that.

I repaired the plane and went back to the field a few days later. The engine failed to start with a bad glow plug message. At home, I removed the engine and found out traces of rubbing on the compressor and a missing blade on the turbine wheel (hot section). The obvious became apparent. I became more comfortable with the plane and was practicing some 3d maneuvers such as hovers, flat spins and square loop. I was having a lot of fun and likely over G'ed that engine causing it to break in flight. The electrical connection popped of due to vibrations after a catastrophic failure.

Obviously when something like that happens, my next step is to understand how to prevent it. One thing I can do is to add some voice warning when the plane exceeds some G limit. But the question becomes: what is the maximum G limit our micro turbines can handle at full power? So for example, if the limit is 15 I could set a warning at 10 etc...

Note: I don't wish to discuss what brand turbine I was flying or which brand X is better than Y. They all have limits. Just trying to figure out what the limits are.

Thanks!

Last edited by acw; 04-23-2024 at 06:48 PM.
Old 04-24-2024, 12:16 AM
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Default g load or gyroscopic effect

Hi there
I do not know how many Gs you put on the airframe during a typical flight, But with an ARES I think you will not get over 10Gs, the wing will stall before that value. What really makes the turbine assembly suffering is the gyroscopic effects. I mainly use Xicoy turbines and they explicitely do not recommend 3D flying (or extreme 3D flying) with their turbines. When you look at your compressor/intake section from the front, where is the rubbing/abrasion? If it is on the side (like 3 or 9 o'clock..) then it can be clearly from too many positive or negative Gs by a massive dive or pull up maneouvre.

Regards
Chris
Old 04-24-2024, 02:57 AM
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The design of our turbines is based on the single / common shaft design , these are susceptible to torsional harmonic issues at certain attitude's - attitude's not altitudes
If it were possible to design in such a small package the turbo fan free shaft style it would make torsional issues a non event
In time we know a manufacture will come up with such design , till then ..... we have what we have

Old 04-24-2024, 03:37 AM
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It is not the G level that is causing the issue. Turbine rub (and subsequent damage) is primarily caused by the gyroscopic forces induced by very high Pitch Rates and/ or Yaw Rates.

Remember how hard it is to rotate a spinning bicycle wheel, or when playing with a gyroscope - our rotating turbine components are exactly the same, but ours are now spinning at 100,000rpm or more.

The gyroscopic forces generated by these pitch/ yaw rates are huge, and are transmitted through the bearings and into the case. With both the compressor and turbine wheels hanging off the end of the shaft, and only supported by the two bearings, they will naturally deflect, to include gyroscopic precession, and may start to rub.

Rubbing at 12/6 o'clock is signs of too much yaw rate, and rubbing at 3/9 o'clock is due to too much pitch rate. Precession effects show up 90degrees away from the applied rotation rate.

If you don't get rubbing and/ or catastrophic failure, you'd at least have to expect not to make the full 25hr bearing life.


There's been more than 1 full-size engine that has had major rub issues by flexing of the rotating parts due to gyroscopic forces. Leading to multiple aircraft losses and loss of life too. And they were designed top handle it too.

Our model engines survive this abuse more by luck than design.



Paul
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Old 04-24-2024, 04:59 AM
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Much if what has been written here, particularly about gyroscopic forces, makes eminent sense.
These engines are not made with 3d type flying in mind.
I donít fly my jets ďhardĒ but I found typical Gs about 4 for a loop or max rate turn. About the same as a fullsize jet trainer.
Even doing square loops with a 120 powered Reaction never exceeded 6 G and that was enough to slightly bend the aluminium wing tubes !
If you want real data, the V Speak vario pro. has a 3 axis a accelerometer, G meter, incorporated.
Old 04-24-2024, 05:13 AM
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Great information on this thread. Thanks all!

But then the key question becomes: how do people with 3d planes mitigating the very real risk of destroying their engines? Is there maneuvers that should clearly be avoided or only be done within some parameters (ex: low speed)? Where is the limit in practice? It appears that the line is blurry and to make things worse, we aren't in the plane to feel it.

V Speak vario pro is interesting and I could setup some warnings for over acceleration on each axis. I'll definitely try that.





Old 04-24-2024, 06:10 AM
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Is there maneuvers that should clearly be avoided or only be done within some parameters (ex: low speed)?
In my opinion, low speed is the worst of all. This is where maximum pitch/ yaw rates can be generated, and with the engine at high power. Just watch how quickly a low speed nose up or down tumble occurs - 1 to 2 seconds to flip fully around. At conventional (higher) speeds there is no way to generate those pitch/ yaw angular RATES, so the induced gyroscopic forces will be way less .

Note that high roll rates don't affect the engine at all.

Personally, like David, I only fly scale type maneuvers. 3D (prop or jet) does not interest me, so I can't comment on long-term engine issues that may be attributed to this style of flying.


Paul

Last edited by JSF-TC; 04-24-2024 at 06:43 AM.
Old 04-24-2024, 06:31 AM
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Originally Posted by acw
Great information on this thread. Thanks all!

But then the key question becomes: how do people with 3d planes mitigating the very real risk of destroying their engines? Is there maneuvers that should clearly be avoided or only be done within some parameters (ex: low speed)? Where is the limit in practice? It appears that the line is blurry and to make things worse, we aren't in the plane to feel it.

V Speak vario pro is interesting and I could setup some warnings for over acceleration on each axis. I'll definitely try that.
There are engines available that are plenty capable of handling the glycolic loads induced by extremely high rate flat spins. Iím currently using a Swiwin 220 on a CARF Diablo and it has been great. The flat spin rate is ridiculous. My estimate is close to 540į/second at full throttle. Iíve seen similar results with the Kingtech 210.

My guess is that youíre using a Xicoy which Iíve seen have issues with rubbing even without high gyroscopic loads. If you have an Ares, you shouldnít be limited by the capabilities of the turbine. There are several turbines that can handle the torture of 3D flight, though you may need to send it in for bearing service at 18hrs instead of 25hrs.
Old 04-24-2024, 06:52 AM
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Part of the solution to getting higher thrust out of our modern and smaller engines is to reduce the clearances between the rotating parts (compressor and turbine wheels) and the adjacent static parts, to reduce the leakage of air.

With reduced clearances, there is less room available to allow the rotating parts to deflect under gyroscopic forces. Adding more structure on the fixed side adds weight, and adding a stronger/ stiffer shaft adds to the rotational inertia, slowing down engine response, so you get into a vicious circle of trying to make a small, powerful, responsive, light-weight and abuse-tolerant engine.

It sounds like Xicoy may have focused on the efficiency, size and light-weight goals, while some other brands may have traded some of that off resulting in it appearing to be more tolerant of the 3D flying style. It's all al balance.



Paul
Old 04-24-2024, 06:53 AM
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acw
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I have many Xicoys but that's not what I had in the Ares...

So you guys flying 3d, what specific engines are you successfully using? Success being defined as 10+ hours of runtime flying 3d without any problem?


Last edited by acw; 04-24-2024 at 07:00 AM.
Old 04-24-2024, 05:04 PM
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Originally Posted by acw
I have many Xicoys but that's not what I had in the Ares...

So you guys flying 3d, what specific engines are you successfully using? Success being defined as 10+ hours of runtime flying 3d without any problem?
Kingtech, jetcat and Swiwin have all been used successfully in 3D jets.

Swiwin 220s and 240s had a recall on the compressor wheel about 12 months ago. There are still many floating around that didnít receive the new compressor wheel.
Old 05-05-2024, 10:10 AM
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Do any of the gyros have an option to program in a max rotational rate so you could still have full throw available for some maneuvers that need it like high alpha flight, but yet not exceed the limits of the turbine in pitch or yaw rates when flipping and spinning.
Old 05-05-2024, 11:54 AM
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Originally Posted by flyinfool1
Do any of the gyros have an option to program in a max rotational rate so you could still have full throw available for some maneuvers that need it like high alpha flight, but yet not exceed the limits of the turbine in pitch or yaw rates when flipping and spinning.
"Max rotational rate"??? You seem to think a gyro is giving the radio inputs for you or can reduce them? That's not how gyros work. Also if you want to 3D your jet, you should know about and pay for the consequences (ie that most turbine manufacturers do not warrant damages due to over G's).
Old 05-05-2024, 01:57 PM
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Actually, Jeff is looking for a feature that could totally be implemented with a modern digital gyro like the Cortex. Basically if rate of rotation exceeds some limit, the device would push back and reduce input or place inputs in the other direction. This would take precedence over the dampening. Hope someone at Bavarian Daemon is reading this.
Old 05-06-2024, 07:07 PM
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I do understand the cost of playing. That does not mean I am not interested in ways to mitigate the cost. A gyro is very capable of adding to or reducing control inputs from the transmitter. This IS what they do to achieve their function.
I would think it would be relatively simple software upgrade for most higher end gyros. Most all gyros already have the needed hardware to do this. Along the same line it would also be very closely related to include a feature to limit G forces for some planes that are not built real strong.

I did contact Smooth Flight about this. They sounded positive as they are aware of the turbine issue but had never thought of adding this feature, until now. "The team will be discussing it."
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Old 05-07-2024, 06:03 AM
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How is the gyro going to know that you are about to 3D your plane right now and not just recover from a dangerous situation? Limiting or countering inputs at the wrong moment could be a recipe for desaster. Also it opens a can of worms when it comes to liability for the gyro manufacturer. I don't think that this is ever going to happen.
Old 05-08-2024, 01:04 AM
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Just how many guys are flying 3d in jets ?
My guess is its a tiny percentage of jet fliers and designing an engine to withstand violent flight is likely to degrade the engines for general use by the vast majority of operators. So why bother ?
Xicoy have created a range of truly superb engines, light, compact, low fuel burn, fast starting etc. So why change to suit a tiny minority ?
No, I am not sponsored but have 5 Xicoys, each one a superb pice of miniature engineering, but perhaps not suitable for 3 d !

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Old 05-08-2024, 06:33 AM
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David, maybe I'm missing something but I don't think anyone is suggesting that Xicoy or other manufacturers should optimize all their engines for 3D flying, leading to compromising their offering in other dimensions such as weight etc...
What I would personally like to see, is a systematic publication of limitations for the key components we are using in the hobby, starting with turbines. The maximum acceleration and gyroscopic forces a turbine can handle at some RPM should be computable given the mass and choice of bearings / components.

Today I select engines based on thrust, weight and fuel consumption. I would like to additionally consider maximum acceleration and gyroscopic forces for this selection. This would be far better than assuming that some brands are OK and some are not.
Jetcat started to do this in for example the 250 Pro 2 user manual see: JetCat-P250-PRO-Basic-Technical-Information-V1-2.pdf page 6

This said, I think it would be good for engine manufacturers to optimize a few motors for 3D use. 3D flying is a lot of fun and we need engines to go with the various excellent kits from CARF, Krill, PilotRC and others.
Old 05-08-2024, 11:24 AM
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You may well be right .I was just expressing on opinion on what I have seen to date.
Old 05-14-2024, 06:18 PM
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Old 05-14-2024, 07:27 PM
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acw
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Thanks for sharing. Looks like the gyroscopic forces that people talked about on this thread. The fire post flameout looks like a bearing failure. Really unfortunate crash but great reminder that if the engine quits in a flat spin induced by the vector, there is probably no way out unless the altitude is huge and even then may not happen.

The pilot did an amazing job. Super smooth and controlled flight. Would love to know what turbine was used. Clearly not the one to use for 3D whatever it is.
Old 05-15-2024, 01:51 AM
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Originally Posted by acw
Thanks for sharing. Looks like the gyroscopic forces that people talked about on this thread. The fire post flameout looks like a bearing failure. Really unfortunate crash but great reminder that if the engine quits in a flat spin induced by the vector, there is probably no way out unless the altitude is huge and even then may not happen.

The pilot did an amazing job. Super smooth and controlled flight. Would love to know what turbine was used. Clearly not the one to use for 3D whatever it is.
I'm pretty sure the engine was a JetCat Pro series.
Old 05-15-2024, 06:16 AM
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Originally Posted by acw
The pilot did an amazing job. Super smooth and controlled flight. Would love to know what turbine was used. Clearly not the one to use for 3D whatever it is.
Jetcat 350
Old 05-16-2024, 10:19 AM
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My Eurosport tends to flame out during flatspin, its a pita to get flying after that before ground hits..

I also think that the gyro effects cause the flameout and now I have greatly reduced the rates of TV.

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