An oddd request
 

Hello

I have an idea that i would like your opinion on.

I currently have a jett 50 FIRE

i would like to do a science fair project that is interesting and id like to do it on airflow and propeller Id like to make a jig to measure thrust and bring tip speed up ABOVE the sound barrier to see if you reach a point where you will no longer produce more thrust no matter how much more power you throw at it

So my Q is what size prop would you use for it and what rpm can i use? im thinking that maybe somthing in the 11X5 would work maybe 11X4?

Steven

RE: An oddd request
 

Sounds like a well thought out thesis and project :)
I remember from my science fair days ----- documentation is EVERYTHING... you can never document enough.
Keep good records - even record failures or errors.

More to your point....

If you want to approach sonic tip speeds, you would best not do it with a conventional molded prop. You would run the risk of shattering the blade.

11" diameter = 34.54" circumference * 18,000 rpm = 621720 in/min = 51810 ft/min = 9.8125 miles/min * 60 = 588.75 miles/hr

An 11" prop turning about 18,000 rpm equates to a tip speed around 588 mph

In dry air at sea level with a temperature of 21 °C (70 °F) the speed of sound is 344 m/s (1230 km/h, or 770 mph, or 1130 ft/s)

So the mach number at 18,000 rpm is 588.75/770 = .765

That is approaching M0.8, which is considered where transonic begins - so that is a pretty quick tip speed, and no doubt some airflow over the blade is well exceeding M.8 by a good margin. So you would see some transonic effects and a drop in efficiency.

Why 18,000 rpm here? I do not believe you are going to turn any "usual" 11x4 prop much over 18,000 rpm with a SJ-50, even with a full size nitro pipe properly tunned. Perhaps ya might get up to 18,500.

Mach 1, or a tip speed of 1130 ft/s or 813600 in/min , for the 11" prop requires 813600/34.54 = 23,555 rpm

For your experiment, getting to that M.76 number would be probably useful in showing the performance drop on a curve.
What you would want to do, is use the exact same prop - and use the throttle to set your rpm reference points.

Key here, is you will want to use a carbon fiber prop, just to be safe. There are some available in that size.... you may have to look to Europe, or even look toward some of the CL and FF guys.

One suggestion though.......
You may want to try this experiment with an electric motor.

A few reasons.
1) you are not mechanically RPM limited - Getting to 20,000 rpm with an 11x4 is a matter of AMPS and VOLTS
2) You can measure AMPS and Volts. You know Watts, you can equate/chart the POWER (Watts or HP) needed to turn the prop at that RPM.
3) Electric is smoother - no accelerations - less stress on the blades and prop hub - so you may get away with using a standard blade prop.
4) You can perform the demonstration inside at the science fair without pissing off many people :)

Anyway.... its a good project !! Stick with it.... and stay safe!
Let me know how you make out.....

Bob

RE: An oddd request
 

If you're going to demo that at the science fair, you might want to consider some sort of lexan or similar safety cage around the prop - wouldn't want that to be the time when the prop gave up the ghost!

RE: An oddd request
 

I would definitely be aware of the prop arc and most likly would just have it as a static demo with a video ...

bob27 what E motor do you suggest i really don't want to spend allot of $$

also for the motor what would i need for esc/batt anyways to sue a dimmer switch right from an outlet?

RE: An oddd request
 

Assuming bolly says im good to go for the rpm range what of these props do you recommend and do you know other places to get cf props (other than super cool props they want 110UAD!!!)

RE: An oddd request
 

Im probably the worst person to ask for electric motor stuff :)

But keeping in mind you do not need "flight" hardware, you should be able to locate a DC electric motor that is fairly cheap (ie, big fat and heavy) that will do the job. Im not sure a dimmer switch will handle the current though.

Perhaps ask that one in the electrics forum.... ??

As for props.....

http://pages.prodigy.net/gcleveland_...ls/index6.html
Try Cleveland.. see what they might have.

Also perhaps email Windy.... http://www.windyurtnowski.com/
He may have a few good sources for CL carbon props of the right size.

Also....
You can use good wood props. sometimes those are stronger than all others. Try and find rev-up 11x4 props.

And as long as you are very careful in your testing, and since the prop will not see a flight load, you can probably use the APC prop as well. (You will have to be careful regardless of the prop type and size...)

Bob

RE: An oddd request
 

Il aks in the electric forum but do you really think apc would be able to take the load if so il try it,

BTW do you Have any suggestions for maybe getting a lexan "tube" made? how think does it have to be i got a friend that ocul probably help me out in making it but how think does it need to be to withstand the force if the prop were to decide to...

im looking into bolly props right now but will see what else i cna come up with the link you gave me (thanx ;):D).

Also im thinking that a cf prop will be able to handle any shock waves better than say wood...

RE: An oddd request
 

You do not need a tube....

A box is just as good. In fact, it is probably better, and is easier to work with.

You want as much clearance round the blade tips as possible. Do not create something where the tube will affect the airflow over the blade tips in any way (like a ducted fan arrangement).

Wood is an amazing thing.... its been around a long time, and its hard to beat nature sometimes for continuous fibers :)

RE: An oddd request
 

This is the rig we came up with only thing left to do would be to mount the scale and tech. We wont ahve any kind of prop protection as i wil be the only one anywhere close to this thing and will alwasy be BEHIND the prop and out of danger (hopefully[X(];))

Just a Q for you Bob, Would you be intrested in seeing what numbers (both thrust and rpm) of the differnt props we will be testing (apc 8.8 x 8.5 apc 10 x 6 and bolly 11.25 x 4 CF)

RE: An oddd request
 

Nice setup... should work ok.....

Sure, I am always interested in data and information.

Static thrust means only a little, but it is interesting to see sometimes.

Be sure to include all of the variables if possible ..... location, temperature, RH if possible, prop, fuel, and the pipe length. (glow plug to the fat part weld).

Of note, the pipe will tend to keep the rpm fairly consistant, somewhat regardless of the prop. It tends to work like a governor as it attempts to stabilize the system at its natural frequency. Your experiment here may lend some evidence to that aspect. It will be interesting to see.

RE: An oddd request
 

I will do my best to supply all the info i can (when i get the experiment going in by hopefully next week.)
But what is "RH"
What do you mean by the fat part weld? I have never touched the pipe since Dub tuned it for max speed....


Steven

RE: An oddd request
 

RH is relative humidity. I picked up a cheap digital weather thermometer a few months ago ...... gives pretty good temp and rh info on the display.... I think it cost all of $5.

The "Fat" part of the pipe is the end of the divergent section. You can usually see where the weld line is between the front and rear pieces.

Dub did set it up, but that measurement was not likely recorded. It is important to know the pipe length since it is related the rpm and engine power.

RE: An oddd request
 

RH 65%
Temp -3 C
il get the tuned pipe to you

I had the blue tunned pip connector partially break so that was letting out some gasses that may have brought it down a little...

With 11.25X$ i got 16700 rpm not bad thats 75.8% the speed of sound

btw the engine now has almost NO compression is it time for a new sleeve/piston[&:]:([:@])

Steven

RE: An oddd request
 

The only way you would need a piston/sleeve rebuild would be if you really toasted the engine (got it hot, and let it run that way for a while). The construction is pretty solid, lasts for quite a long time. Tends to be pretty tolerant of most brief lean runs.

If the engine gets hot when running, the best thing to do is richen it up a bit, then shut it down. Once it has been over-lean and gets hot, it will not recover to give you any reasonable rpm readings. Best to shut it down, wait 5 minutes or so, open the needle a bit from where it was, and start over.

The engine fit gets better with time... may feel looser .... but that is normal.

If it has zero compression, Id be looking at the head bolts and glow plug for a physical leak. The engine should still have a seal/fit up top. When warm, that should be noticable. The other thing that helps....... toss a 9x7 prop on it for a tank ... let it run a bit.

A cracked header coupler will really mess up the system/function if it allows gasses to excape. But your data looks ok :).

RE: An oddd request
 

Hey!

I was taking a closer look at things today and i noticed the glow plug was loose (that might explain for the lack of compression ;))

so i tighten it back down only to notice that the button piece that it screws into is not turning and not tightening the glow plug.. so i figured the head was loose but it isn't [&:] so im not 100% sure what to do now

Steven

RE: An oddd request
 

well.....

two things to look for....

1) see if the head is loose - you sorta noted this, but is the entire had button turning? If it is turning, turn the engine to bottom of the stroke, remove the exhaust, loosen the head bolts, look into the exhaust to ensure that the sleeve is aligned in the crankcase, and then tighten the head clamp (fins) back down.

2) If the plug just turns in the mounting hole ... the threads stripped out or wore out from a lose plug shaking. In which case you will need a replacement head button.

Loose head bolts and loose plug usually come from the engine getting hot. Be careful.

RE: An oddd request
 

The entire button is rotation before the plugs gets tight

the bolts on the head are tight to i ill remove exhaust and attempt to see if maybe the sleeve alignment is off...

Steven

RE: An oddd request
 

ok.. please check it.

If the button is rotating, the head clamp/head bolts are not tight. The head clamp (fins) hold the liner and head button in place.

If it is rotating, there is a 98% chance you rotated the entire liner assembly.

Remove all of the head bolts ... and remove the head clamp.

You will definately have to inspect and re-align the liner, and then reinstall the head clamp.

After you reassemble, run the engine with a regular (light) prop for a tank. Sorta re-break-in.
And, after shut down, with the engine still a bit warm, check the head bolts.

Bob

RE: An oddd request
 

I will definitely do that once i get things going again in the spring time

btw sorry it took so long but from glow plug to the thickest part of the tune is 12 inches

Steven

RE: An oddd request
 

So i took the engine apart the liner was not rotating at all ,

I took pictures as im not 100% sure what the liner shoudl look like also is it normal that the head clamp i beleive its called is very black while the piston is relitivly metalic/shiny

For some reason i can not upload i will attempt later on. (Sorry)

Steven

RE: An oddd request
 

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RE: An oddd request
 

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RE: An oddd request
 

Just for nomenclature.....

What you have shown there are the piston, sleeve/liner and the head button.

The clamp is the finned section that bolts on to retain the head button and sleeve assembly.

The coloring is generally ok. Nothing real unusual, but darker carbon deposits happen when running the engine a little rich, but it getting hot (over-propped or pipe too short typically).

What concerns me are apparent pits on the top surface of the piston --- it looks like it ate something... ?? Might just be artifacts from the photo.

If everything is in decent shape, and the glow plug threads are not stripped (as you first though) put it back together and run it. I would suggest just tossing a 10x6 or 9x7 on it for a few runs. Make sure to double check the head clamp bolts after the first run.. jsut to ensure they are still snug.

Bob