28x12 prop at 6000rpm, 5" from a computer...problems?

[Barrett]

Hey guys, I work for a very small company that has been working on a UAV for a VERY specific purpose. I cant really go into details, but I have a question anyway. Instead of designing from scratch around our payload, we opted to use a model that we could easily modify for the job. Basiclly its a "conventional" model (wing, fuse, stab) but our motor is mounted on top of the wing with about a 13 degree up angle. This was done to make room for our payload. This puts the tips of our 28x12 prop about 2 inches above our "payload hatch" We take rpm readings everytime we run the motor, and average about 5600-6000 rpm, depends on the oil and fuel we use. We have a laptop sitting on top of our payload that controls it and stores collected data. The problem is, after about 2 minutes of motor run, we start to see problems with the operation of the payload. The problems start to go away when the trottle is reduced, and come back when it is advanced.

We have tested the payload many times without the motor running and have no problems. We tried physically shaking the payload with no problems. So we think maybe some type of high frequency vibration could be happening. Or, maybe, because the tips of the props are very close to the laptop, and they are moving very, very fast, some type of "percussion shock" to our electronics could be happening.

If we assume that our problems are not coming from loose connections, faulty system components etc... could the prop cause problems to our electronics?

I know this is a very general description, but.....any thoughts?

Thanks in advance,
Barrett

[rakm]

Static electricity generated from the friction of the air moving over the prop blades?

[Barrett]

Could that still be the case even with the hatch closed. It is made of 1/4" plywood, glassed with 4oz and 2oz cloth and epoxy. Can the static electricity penetrate that? Sorry, I know next to nothing
about static electricity, other than walk across carpet in your socks, then zap your bud.

Barrett

[rakm]

Not really sure on that one. Ensure there are no small openings or gaps around the hatch. I'm definately not an expert here, it was just a thought.

[Dick Huang]

Barrett,
The 28" dia prop turning at 6000 rpm has a tip speed Mach number of 0.65. this is very high for a model and may be your problem. It's worth more investigation.

Dick Huang:)

[kist2001]

Build a gaussian cage around the payload. Basically aluminum foil the box and run a grounding strap to a pipe. This way no energy can be transmitted. Run the motor up and see results....my gut feeling is that this is a vibrational problem.

Car manuifacturers spend much time and money figuring out the natural frequencies of things they hang off the chassis. This is to prevent resonance and such.

There is a reason that Mil Spec parts are used....they are a known quantity with vibration. See what the r/c heli guys do to isolate cameras from vibration and copy it. I am sure something in your payload is vibrating at a frequency of your prop. -Ken

[Barrett]

Originally posted by Dick Huang
Barrett,
The 28" dia prop turning at 6000 rpm has a tip speed Mach number of 0.65. this is very high for a model and may be your problem. It's worth more investigation.

Dick Huang:)

Thanks guys. Dick what do you mean? The motor and prop are basically matched for the model size and weight. So Im not sure if
there is a way around that. And yes, that was the first thing I did when this idea was suggested by one of the techs that works for the company that built the payload. I immediatly figured out what the tip speed was. Its very fast I know.

We are back at home now with the plane, making some mods to the airframe. Small stuff... We will go back at the end of this week. Ill keep you guys informed. Keep um coming if you got em though...

Thanks
Barrett

[Dick Huang]

Barrett,
I've read some where that the prop tip speed should not exceed Mach No.=0.5. At sea level it would be 558 ft/sec at 59F. Therefore D*RPM=127893.65.
Dia(in) RPM
28 4568
29 4410
30 4263
31 4126
32 3997
33 3676
34 3762
35 3654
36 3553
I would make some runs with the current 28"dia prop at reduced RPM to see what RPM eliminates the problem, then use that RPM to select a new Dia prop that will keep the performance of the 28x12 prop.
If too high prop speed is not the problem you will find this out when making the runs with the current prop at reduced RPM. This means you need to look for some other cause.
Dick Huang:)

[Marion]

Just a few thoughts not mentioned before.....

Fiber glass epoxy can be a pretty good insulator, and therefore build up and hold static electricity. Painting the part with a high conductive paint, and grounding it to system ground MIGHT help things.

The fact that it takes about 2 minutes of flight before the probem shows itself, could be an indicator of static build up -- as well as a host of other things that could be time dependent.

How about the computer heating during time?? Or it's battery voltage dropping ??

While vibration is always a concern, you should be able to duplicate that on the ground -- almost. The prop speed will increase slightly as the model speed builds up.

Let us all know what you discover the problem to be -- Could save lots of problems for the rest of us...

Marion

[chrisgood]

It sounds like high frequency vibration at a higher frequency than you can generate by hand, or a combination of that plus the EM/RFI generated by the engine. You didn't mention what kind of engine, but if it is any kind of coil ignition it can impart huge amounts of EM/RFI interference to any electronics on board. Start by isolating the electronics physically from the vibration, and then isolate them electronically. A small UAV we put together with a Zenoah G23 puts out a lot of interference on a video downlink at the other end of the plane. They were completely isolated electronically from each other (the video had its own battery pack), but the video still had interference.

Chris Good
Test Manager, AAI Corporation
Shadow 200 TUAV

[Barrett]

Hi Chris. Thanks for the input. Our engine is a 3W 150. It has an onboard ignition module. It was near our payload so we have moved it. We have just begun flight testing with the payload, and it was placed in a spot that kept it away from our autopilot, which has very sensitive electronics also. Now it is away from both. We also have video downlink and a wireless modem to downlink information from our autopilot. So we had to keep those in mind also. We have also installed a device to optically isloate as many signal wires as possible. The weird thing is that our problem dosent seem to present itself for a few minutes, and then it just sort of starts... So we considered some type of "build-up" interference, like static electricity. Which is why I was mostly curous about the prop tips causing some type of shock wave or as already suggested, static electricity.

I am leaving in a few hours to go back to the company that developed our payload. We plan on spending a few days trying to isolate the problem and make changes to eliminate it. Then if all goes well and we have some time left, we will get a few flights in and collect data for processing and validation.

Thanks again to all that helped. I will let you know what happens when I return.

Barrett

[Barrett]

High frequency vibration was the enemy. I came up with an idea to mount the computer on a platform behind the payload. Then the platform would be suspended with bungee cords. The idea worked. the bungee cords did a much better job at seperating the laptop from vibration than our isolation mounts could. We rebalanced the plane, and I mounted a camera inside the plane to watch the movement of the laptop during taxi, high speed taxi, and flight. Surprisingly, it didnt really move around much. The whole contraption looks a little odd, but it works.

Thanks for all the ideas guys!

Barrett

[Marion]

Barrett,

Thanks for posting your solution. We all like to know how to fix problems, so I really appreciate you letting us know what fixed yours.