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Old 09-29-2013, 08:55 PM   #17731
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I'm trying to explain a serious breach of personal safety a person did with a hands on demo of how stable a quad is. It's the same one I fly so I jumped right on how unaware the guy in the demo was as to how bad he could get hurt if the quad got out of hand. The troll seems to think the "little" quad wouldn't hurt you if it hit you. Can't get through to him so I need to try another approach.
I need to calculate how fast in miles per hour the tip of the prop blade is going. Here's the details.
2 bladed prop. 8 inches total length. 4 inches hub to tip.
Motor can do 4000rpms but in this case let's say 2000rpms.
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Old 09-29-2013, 10:28 PM   #17732
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Quote:
Originally Posted by setishock View Post
I'm trying to explain a serious breach of personal safety a person did with a hands on demo of how stable a quad is. It's the same one I fly so I jumped right on how unaware the guy in the demo was as to how bad he could get hurt if the quad got out of hand. The troll seems to think the "little" quad wouldn't hurt you if it hit you. Can't get through to him so I need to try another approach.
I need to calculate how fast in miles per hour the tip of the prop blade is going. Here's the details.
2 bladed prop. 8 inches total length. 4 inches hub to tip.
Motor can do 4000rpms but in this case let's say 2000rpms.
Not as fast as you would think. I got 69.81 feet per second, which would be 47.6 miles per hour. This is at 2000 RPM, as you requested.

If someone wants to check over this, it'd be great. It's pretty basic, but my math skills are extremely rusty. Here's my reasoning:

2π x 4 inches = 25.13272 inches / revolution
x 2000 revolutions per minute = 50265.44 inches / minute
/ 12 = 4188.786 feet /minute
/ 60 = 69.813 feet / second.

From there I was lazy and plugged it into google's conversion calculator to get miles per hour.
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Old 09-30-2013, 12:35 AM   #17733
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How to Convert RPM to MPH With a Calculator | The Classroom | Synonym on Synonym
Try that one. I get 71.63 mph. Here's the thread at HeliFreaks that started all that mess. Not real bright - HeliFreak
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Old 09-30-2013, 12:48 AM   #17734
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How to Convert RPM to MPH With a Calculator | The Classroom | Synonym on Synonym
Try that one. I get 71.63 mph. Here's the thread at HeliFreaks that started all that mess. Not real bright - HeliFreak
That's pretty much what I did before, just in a different order. I have no idea how you got that answer. I got the exact same result as before when following that guide.

Edit: Read that link. I understand what you did now.

Quote:
You figure an 8 inch prop at 1/2 it's length and use the hub to tip length of 4 inches. The actual distance of the circle the tip travels in 12.56 inches. That's 1.05 feet.
What's the size of this blade? 8 inch total diameter, 4 inch radius, correct? That's what I got from your first post. If so, here's what you did wrong:

To calculate the circumference (total size of a full revolution) you'd do 2πR, or 2π times 4 inches. this would mean the full revolution draws a circle of 25.13272 inches. Given that you ended up with half of that, I'm going to assume you forgot that pi is the ratio of the diameter to circumference, rather that radius (what you used.)

You then multiplied the (incorrect) circumference by pi again for some reason . I assume you thought the formula wanted circumference, but the correct value would have been the diameter of the blade, or 8 inches/0.6667 feet.
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Old 09-30-2013, 01:04 AM   #17735
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8 inch tip to tip. 4 inch hub to tip. One blade at 4 inches travels 12.56 inches or 1.05 feet in one 1 revolution.. You just figure one blade.
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Old 09-30-2013, 01:06 AM   #17736
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8 inch tip to tip. 4 inch hub to tip. One blade at 4 inches travels 12.56 inches or 1.05 feet in one 1 revolution.. You just figure one blade.
I just edited my post. Read it again. If you still don't understand, say something and I can draw a diagram.

Remember, a full revolution would mean each blade making a full sweep around to its original point. The fact that there are two doesn't make any difference
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Old 09-30-2013, 01:38 AM   #17737
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The 25 inches per rev is wrong. It's correct if you assume 8 inches from hub to tip on one blade. It's only 4 inches so the travel is 12.56.
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Old 09-30-2013, 01:53 AM   #17738
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We're literally finding the circumference of a circle here. 2πr would be the correct formula. You seem to be caught up in the two blades thing here. That's completely irrelevant. It could have six blades, and the answer would be exactly the same so long as they maintained the four inch radius.
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Old 09-30-2013, 02:14 AM   #17739
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There's an error in the online formula. It states to put the cir in in feet. Then does an inch to feet conversion. That's what's throwing it off. It gives a higher mph than it really is. I nabbed another formula that gives a more accurate answer as it's consistent feet through out the equation.

---------- Post added at 01:14 AM ---------- Previous post was at 01:09 AM ----------

Thanks for your help though. You think your math is bad, mine must really stink not to catch that error. Now I have to go back and edit my post.
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Old 09-30-2013, 02:14 AM   #17740
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Thanks for your help though. You think your math is bad, mine must really stink not to catch that error. Now I have to go back and edit my post.
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