Props, supersonic, and safety question

Props, supersonic, and safety question
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All Forums >> RC Airplanes >> Aerodynamics >> Props, supersonic, and safety question

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Props, supersonic, and safety question - 2/8/2008 3:32:44 AM

Grits

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From: , WV, USA
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I hang out mostly in the cars forums but there has been some talk of building test stands for car engines and using airplane props for a load. I don't know much about this subject except that I've heard that if the tip of the prop trys to go supersonic bad things happen. Props fly apart. I have no idea if a .28 size engine has the power to turn a 10 inch prop to 30,000 rpm but my simple math says the tip will go supersonic. I would hate to see or hear of someone hurt from doing this. My post is not to debate power needed or prop pitch, size, or construction. I only want to pass SAFE information. Am I crazy or is there even a remote chance for this to happen and hurt someone?

< Message edited by Grits -- 2/8/2008 3:35:04 AM >

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RE: Props, supersonic, and safety question - 2/8/2008 4:08:31 AM

highhorse

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Joined: 11/16/2006
From: Memphis, TN, USA
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Worry not. The short answer is no. Your engine will come apart first.

These engines just dont have enough power. If the prop is large enough to load the engine down to even the top of it's normal RPM range its too slow to be supersonic. If the prop is too small then the tips will stay subsonic even as the engine screams itself to death.

REGARDLESS, ALWAYS ALWAYS WEAR GOOD EYE PROTECTION WHEN RUNNING ANY PROP. It may come apart anyway, come off the engine, throw a pebble..........

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RE: Props, supersonic, and safety question - 2/8/2008 4:24:18 AM

HighPlains

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From: Over da rainbow, KS, USA
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Tps going supersonic is not the issue. Having enough strength to hold together is. Car engines don't produce enough torque to turn a 10 inch prop anywhere close to 30K unless the pitch was near zero. What I would suggest is either a carbon fiber prop of around 8" diamenter 4 pitch or a maple prop with tight straight grain that starts off with at least 9" diameter and 4 to 6 pitch. Then run to determine rpm, then clip and balance until you reach the desired testing rpm. Avoid having any spectators and remain clear of the line of the prop disk.

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RE: Props, supersonic, and safety question - 2/8/2008 10:54:18 AM

highhorse

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From: Memphis, TN, USA
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I'm assuming he's not gonna know how to "clip and balance" the prop, or he'd already know that he's not gonne get 30k out or his engine

And, IMO, there's no need to burn bucks

on a carbon fiber prop. The props not gonna come apart unless it's damaged. Period.

Just go to the hobby shop and get an APC 9-6 (they are fretty well balanced right outa the box), perhaps a 9-4 also. Try the -6 first. If it puts out the rpm you want you can return the-4.

Wear your eye protection. Have fun.

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RE: Props, supersonic, and safety question - 2/8/2008 12:03:54 PM

da Rock

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From: western, NC, USA
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quote:

ORIGINAL: Grits

I hang out mostly in the cars forums but there has been some talk of building test stands for car engines and using airplane props for a load. I don't know much about this subject except that I've heard that if the tip of the prop trys to go supersonic bad things happen. Props fly apart. I have no idea if a .28 size engine has the power to turn a 10 inch prop to 30,000 rpm but my simple math says the tip will go supersonic. I would hate to see or hear of someone hurt from doing this. My post is not to debate power needed or prop pitch, size, or construction. I only want to pass SAFE information. Am I crazy or is there even a remote chance for this to happen and hurt someone?

First off, prop tips going supersonic ain't no big deal. Because the usual prop for an engine won't be coming close to mach, and because prop manufacturers have been making props for longer than there have been RC cars and know what they're doing. And because if you're not using the usual prop, the engine is probably giving you fits trying to start it. But on the off chance that you do get an engine started with an extremely undersized prop on it, the following chart gives you a clue what the chances are that prop's tips will go supersonic.

Attachments

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(in reply to Grits)

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RE: Props, supersonic, and safety question - 2/8/2008 12:13:47 PM

da Rock

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BTW, IMAC is a good bunch of people. And their website has lots of good information. It's worth the time just to read about those big suckers.

And if you look at their chart, you get a better idea how probable it is that your little .28 engine will wind up a prop and get supersonic tip speeds. If it could swing a 13" prop up to 20,000rpm it could get supersonic tips. Most engines that normally swing 13" props usually don't come close to 20K.

Next time you "hear that...... such and such", you should consider the source. And if you remember who told ya' what you heard about tip speeds, keep this in mind next time they impart wisdom.

(in reply to da Rock)

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RE: Props, supersonic, and safety question - 2/8/2008 12:16:32 PM

B.L.E.

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You would probably have to go to a 8 or even a 7 inch diameter prop to get 30K from a .28 . Doubling the rpm quadruples the torque needed to drive the prop and increases the power needed to drive the prop by a factor of eight.

A Cox Texaco .049 could turn a 8x4 about 8000 rpm. 16000 rpm would need four times the torque so it would need a 4x bigger engine, .20 in other words, 32000 rpm would need the engine to be four times bigger again, .80 in other words.

(in reply to da Rock)

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RE: Props, supersonic, and safety question - 2/8/2008 12:53:28 PM

Grits

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Thanks peeps. I just went with 1130 feet per sec and know nothing about power needed, aerodynamics of a prop, or anything supersonic. My uncle was a flight engineer and told me about the prop tip and ss problem. I just wanted to make sure someone didn't get hurt. While rc is scaled way down, physics and dynamics still apply, I guess. Anyhow, I probably over worried the issue and I thank you for your comments.

(in reply to B.L.E.)

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RE: Props, supersonic, and safety question - 2/8/2008 4:12:40 PM

HighPlains

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Plastic sport props have a rather poor record at high rpm. The reason I suggest using either carbon fiber or maple in the sizes indicated is only to have a hub size that will have enough strength. The diameter you end up with will be much smaller, but if you start with a smaller prop of the same diameter it will not be strong enough to hold together. I have run racing engines for over 25 years, and break-in and high rpm test props have to be strong to survive.

A friend of mine nearly lost an eye from a prop explosion (and he had on a eye protection goggles), while using a plastic sport prop that was cut down.

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RE: Props, supersonic, and safety question - 2/8/2008 5:21:22 PM

dick Hanson

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From: slc, UT, USA
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plastic sport props - typically are made from nylon which becomes brittle as moisture content decreases -
early on nylon props had cautionary instructions - to boil them occasionally
When I was a kid - I used "SCAMPER" props on my Wasp - . these 5+ inch props turned 15000plus at the time

Looking at old ones - it was easy to see small strain lines in the materials especially at the hub.
One feature of the plastic prop was that it survived small bups with the ground and smacks with chicken sticks etc..
too many users see these things as safe toys just like the little plastic helis now available .
No prop should be viewed as safe
No - NONE - NADA.

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RE: Props, supersonic, and safety question - 2/8/2008 5:30:42 PM

HighPlains

Posts: 2640
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From: Over da rainbow, KS, USA
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I found an old email from my buddy that experienced the prop failure. He is much better with his description:

quote:

Despite my best efforts at "hiding out" for a few days, enough folks have been asking about my new Phantom of the Opera look that I figured I should say something. Skip this if you have more important things to do. Last Saturday I was "breaking in" a racing model aircraft engine by running it on a test bench using a small, high-airflow propeller to keep it cool. This is a familiar procedure among those who, like me, are addicted to speed but don't have the $2 million for an Indy car or a war surplus Gumman Bearcat. The engine is about the size of a russet potato, and generates about 3.5 horsepower at 27,000 rpm. The tip speed of the propeller is somewhere around 400 mph. I was wearing earplugs and a pair of polycarbonate wraparound sunglasses from Performance Bike Shop. I always wear some sort of eye protection because you can never be sure the propeller won't fling a grain of sand or something. There's also the remote possibility the propeller itself could fail, although in 30 years of messing with these things I'd only seen that happen twice. And of course the odds are that if a blade did come off, it would find some other spot to land -- with 360 degrees of arc to choose from and all the aerodynamic forces pulling it forward, not back behind the plane of rotation to where I was standing. Do I even need to finish this story? Direct hit. Fate is the hunter. Polycarbonate works. Wear your seat belt, hug your children, etc. But now, I need some help finding a more socially acceptable cover story -- something a little more macho, a little less Jimmy Neutron-like. I have come up with the following candidates so far. Your additional suggestions are welcome. 1. Tae Kwon Do class, first day with the nunchaku. 2. Answered casting call for Siegfried & Roy. Tryout did not go as well as hoped. 3. That Mike Tyson can get SO mouthy sometimes. 4. Second place, 2003 Miller Genuine Draft Longneck Sparring Championships. 5. Pierced tongues out, pierced eyeballs in. 6. Recruited as third Blues Brother. Got the shades, still shopping for a guitar and 310 slightly used police cars. 7. Eh'll be bock. Thank you for your support!

< Message edited by HighPlains -- 2/8/2008 5:31:30 PM >

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In politics if you want anything said, ask a man. If you want anything done, ask a woman - Margaret Thatcher

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