Force of the prop

[murphey]

I'm doing short explanation about prop safety for Young Eagles. Let's not get into the discussion of foot-pounds (rotational force) vs pounds (linear force), nor include the lateral motion the prop travels. Let's assume the airplane is stationary. I want to make sure I've got the right numbers & calculations. And let's not go past 1 or 2 decimal digits.

weight of prop: 34.5 pounds
length: 76 in or 1 blade is 38 in.
revs: 2700 revs/minute (rpm)

Distance the end of the blade travels in 1 revolution:
38 * PI = 38 * 3.14159 = 119.4 in = 9.95 ft.

Distance the end of the blade travels in 1 minute:
119.4 * 2700 = 322,380 in
or
322,380 in/min divided by 12 = 26,865 ft
or
26,865 ft/min divided by 5280 ft = 5.09 statute miles

How far in 1 hour?
5.09 sm/min * 60 min = 305.4 miles

Calculating the torque (rotational force) involves vector algabra so let's simplify. Assume a linear motion, that the 34.5 pound prop is being thrown at you at a speed of 305.4 mph, how hard is it going to hit you? Better yet, do you want to get in the way of ANYTHING going 305.4 mph?

Solution: Stay away from propellers!!!!

 

[Capt. Geoffrey Thorpe]

2*pi*R or pi*d = 72*3.14...

610 mph

 

[MAKG1]

It won't be going that fast in static air, even at full power.

Still, quite dangerous.

If you just want to scare the kids, there is a nasty scene in Raiders of the Lost Ark.

 

[Sac Arrow]

What do you mean by "how hard is it going to hit you?" Do you mean energy transfer? Those numbers are kind of moot because the prop will slice you to pieces before it kills you from blunt force trauma.

Oh by the way calculating the torque is simple. You simply look at the power table for HP vs. RPM and you can directly calculate the torque, no algebra. It's an estimation at static RPM but close enough.

 

[Zeldman]

If you just want to scare the kids, there is a nasty scene in Raiders of the Lost Ark.

 

[Jim Logajan]

I'm doing short explanation about prop safety for Young Eagles. Let's not get into the discussion of foot-pounds (rotational force) vs pounds (linear force), nor include the lateral motion the prop travels. Let's assume the airplane is stationary. I want to make sure I've got the right numbers & calculations. And let's not go past 1 or 2 decimal digits.

weight of prop: 34.5 pounds
length: 76 in or 1 blade is 38 in.
revs: 2700 revs/minute (rpm)

Idling and taxiing rarely require RPMs above 1500. Static RPM at max power is rarely above 2400 for most small planes. If the airplane is stationary, perhaps use 650 RPM?

Distance the end of the blade travels in 1 revolution:
38 * PI = 38 * 3.14159 = 119.4 in = 9.95 ft.

Thorpe already caught this error.
C = 2*R*pi = D*pi

So C = 76*3.1 = 236 in = 19.7 ft

Distance the end of the blade travels in 1 minute:
119.4 * 2700 = 322,380 in
or
322,380 in/min divided by 12 = 26,865 ft
or
26,865 ft/min divided by 5280 ft = 5.09 statute miles

How far in 1 hour?
5.09 sm/min * 60 min = 305.4 miles

Calculating the torque (rotational force) involves vector algabra so let's simplify. Assume a linear motion, that the 34.5 pound prop is being thrown at you at a speed of 305.4 mph, how hard is it going to hit you? Better yet, do you want to get in the way of ANYTHING going 305.4 mph?

Solution: Stay away from propellers!!!!

In the end you didn't figure out the force it was going to hit you, just that the speed is awfully fast. I'd suggest a different approach using the definition that relates energy, force, and distance (E = F*D), dividing both sides by time to yield this equation:

engine_power = force_on_blade * speed_of_blade
(P = F*V)

For simplicity of demonstration explain that to compute prop speed you'll just use a circumference with a radius half the blade radius.

The rest is left as an exercise for the student....

 

[MAKG1]

Idling and taxiing rarely require RPMs above 1500. Static RPM at max power is rarely above 2400 for most small planes. If the airplane is stationary, perhaps use 650 RPM?

Thorpe already caught this error.
C = 2*R*pi = D*pi

So C = 76*3.1 = 236 in = 19.7 ft

In the end you didn't figure out the force it was going to hit you, just that the speed is awfully fast. I'd suggest a different approach using the definition that relates energy, force, and distance (E = F*D), dividing both sides by time to yield this equation:

engine_power = force_on_blade * speed_of_blade
(P = F*V)

For simplicity of demonstration explain that to compute prop speed you'll just use a circumference with a radius half the blade radius.

The rest is left as an exercise for the student....

That will demonstrate that the force the engine is producing at 36 inches isn't that much -- I get about 100 lbf. It's the answer to the wrong problem.

It's the accelerations during collision that cause damage. The power absorbed by a kid's body during that few milliseconds will be quite a lot more than 160 HP -- it will take the kinetic energy in the prop and change it essentially to zero very quickly. And you need very little force on a good knife to make it cut.

 

[murphey]

It won't be going that fast in static air, even at full power.

Still, quite dangerous.

If you just want to scare the kids, there is a nasty scene in Raiders of the Lost Ark.

One of my favorites!

Another option is to take them out on the ramp and toss a watermelon at a spinning prop.

 

[mjburian]

You're not going to take Young Eagles through the math, are you? You're just trying to come up with a realistic value to tell them? (The math is great, but most Young Eagles I've flown would fall asleep before we got to the end.) To a kid, does 305.4mph mean anything more than 100mph? (Depending on age, I suppose.)

This is generally the approach I take with them about prop safety:
Usually, this spins REALLY fast and has been known to kill people. Even pilots who "know better" have been caught by it, so it's best to treat it as though it's ALWAYS spinning, even when it looks like it's not. It's really that dangerous.

 

[Silvaire]

I honestly don't think there is any need to explain to a kid what will happen if he gets hit by a propeller, everyone knows exactly what happens. The danger of a prop is that you can't see it and when people get excited or distracted they end up walking right into it. So rather than a complex math problem I think it just needs to made clear that there is a specific zone to stay out of whether the propeller is turning or not.

One thing I've found mildly disturbing is how many newer high wing designs, both experimental and LSA, have put the lower strut anchor behind the door. The strut on conventional high wing aircraft has always provided an obstruction that would prevent most cases of someone accidentally walking into a spinning prop. There was always a safety margin with low wing aircraft as well since you always enter from behind the wing.

 

[Dan Thomas]

One of my favorites!

Another option is to take them out on the ramp and toss a watermelon at a spinning prop.

Don't do that. One of our customers taxied into one of those cheap red plastic traffic cones that had been left in front of his airplane. Idle speed. The tip of the prop got bent. A watermelon could do damage, too. Lots of weight there.

All one needs to do is bang his head on the prop while working on the engine or nosegear. It hurts and sometimes draws blood. Not hard to imagine the head/prop collision at a speed about a thousand times higher.

Dan

 

[Sleepingsquirrel]

At 600 RPM a two bladed prop would strike you 20 times in one second with its tip, any Young Eagle would know this exceeds the ability of a Ninja with a Katina.

please check the math.

 

[Witmo]

One of my favorites!

Another option is to take them out on the ramp and toss a watermelon at a spinning prop.

That would make for one very expensive demonstration.

 

[jaybee]

This is what happens when a bird goes through the rotor blade -

 

[Henning]

You lost the Young Eagles on the first line. Take a baseball bat to a watermelon, that will convey everything they need to know without losing all but one of them before you get to the point. Don't try to teach kids safety lessons with math, it's not effective.

 

[Henning]

I honestly don't think there is any need to explain to a kid what will happen if he gets hit by a propeller, everyone knows exactly what happens. The danger of a prop is that you can't see it and when people get excited or distracted they end up walking right into it. So rather than a complex math problem I think it just needs to made clear that there is a specific zone to stay out of whether the propeller is turning or not.

One thing I've found mildly disturbing is how many newer high wing designs, both experimental and LSA, have put the lower strut anchor behind the door. The strut on conventional high wing aircraft has always provided an obstruction that would prevent most cases of someone accidentally walking into a spinning prop. There was always a safety margin with low wing aircraft as well since you always enter from behind the wing.

I find this disturbing as well. Granted the prop should be stopped before people enter and exit, but history shows that it isn't always so. OTOH, it does cull the herd.

 

[Norman]

You lost the Young Eagles on the first line. Take a baseball bat to a watermelon, that will convey everything they need to know without losing all but one of them before you get to the point. Don't try to teach kids safety lessons with math, it's not effective.

Sledge-o-matic?

 

[timwinters]

Sledge-o-matic?

Beat me to it, Norm! Show them an old Gallagher skit!

 

[Everskyward]

You lost the Young Eagles on the first line. Take a baseball bat to a watermelon, that will convey everything they need to know without losing all but one of them before you get to the point. Don't try to teach kids safety lessons with math, it's not effective.

I think it's supposed to be an academic lesson as well as a safety lesson.

 

[Henning]

I think it's supposed to be an academic lesson as well as a safety lesson.

They don't mix well IME. Maybe if the math gets explained second and the 20% with an interest can learn it, but you want to get the point across to 100% of them before you lose the attention of the other 80%.

 

[Sac Arrow]

I think it's supposed to be an academic lesson as well as a safety lesson.

The problem is, the only academics they will understand will be gross and inaccurate generalizations. The math that goes in to describing the damage done to flesh by collisions is complex and largely empirical.

 

[Everskyward]

The problem is, the only academics they will understand will be gross and inaccurate generalizations. The math that goes in to describing the damage done to flesh by collisions is complex and largely empirical.

I don't think it needs to be that technical. Saying the tip of the blade is going X mph is probably enough.

 

[Henning]

I don't think it needs to be that technical. Saying the tip of the blade is going X mph is probably enough.

Not really. Most kids would associate you saying "90mph" with getting hit by a baseball because that is the frame of reference they have, they haven't gotten into Mass x Velocity = Kinetic Energy. Tell them 'It's going nearly supersonic' and you have entered fantasy land because they don't really have sense of the value of the reference to them. When you need to relay important information to someone, you need to do it in a frame of reference they can internalize and understand or you risk not getting the point clear.

Baseball bat, melon: "This is what happens when a prop hits you...splat... Stay clear of props." That is a frame of reference kids can understand the full meaning of with no doubt the point is conveyed and understood.

 

[AggieMike88]

 

[AggieMike88]

One of my favorites!

Another option is to take them out on the ramp and toss a watermelon at a spinning prop.

The early Mythbusters episode where they did the spinning prop versus cessna/piper empennage.

 

[Everskyward]

Not really. Most kids would associate you saying "90mph" with getting hit by a baseball because that is the frame of reference they have, they haven't gotten into Mass x Velocity = Kinetic Energy. Tell them 'It's going nearly supersonic' and you have entered fantasy land because they don't really have sense of the value of the reference to them. When you need to relay important information to someone, you need to do it in a frame of reference they can internalize and understand or you risk not getting the point clear.

Even a kid can see the difference between a baseball and a prop.

 

[Henning]

Even a kid can see the difference between a baseball and a prop.

Handle em how you want, I've had literally thousands of kids on big old sailboats trying to keep them safe and having about 10 minutes to convey a lot of information. I learned quickly what didn't work.

 

[JeffDG]

To calculate it mathematically, you'd need to do F=mv across the length of the prop, so near the hub would be high-mass, low velocity, and at the tip would be low mass, high velocity.

Because it varies across the entire length, you're looking at an integral.

 

[Sac Arrow]

Even a kid can see the difference between a baseball and a prop.

In terms of pure math (e.g. weight, speed, kinetics), a baseball bat is roughly equivalent to a Katana class samurai sword. Both have markedly different effects on the body.

 

[Everskyward]

Handle em how you want, I've had literally thousands of kids on big old sailboats trying to keep them safe and having about 10 minutes to convey a lot of information. I learned quickly what didn't work.

It's not me who is doing this. I just figured Murphey was also trying to throw a little math/science in there too. But maybe I am wrong.

 

[Henning]

It's not me who is doing this. I just figured Murphey was also trying to throw a little math/science in there too. But maybe I am wrong.

I understand, and I don't disagree with tossing in some math and physics, but there are other opportunities for that as well. Critical points are best left in the clear and simple. Want to talk about where the noise comes from? Now is a good point to address the math and physics and then you can make the energy connection back to prop safety for the ones who are still participating (20%). One thing to remember about children is that their brains don't always make the connections an adult brain does.

 

[MAKG1]

To calculate it mathematically, you'd need to do F=mv across the length of the prop, so near the hub would be high-mass, low velocity, and at the tip would be low mass, high velocity.

Because it varies across the entire length, you're looking at an integral.

Might want to proofread that.

You can estimate the energy in the spinning prop reasonably without going to calculus, but only if your kinematics is correct to start with.

As Sac pointed out, the energy isn't really the correct variable here. The energy in a food processor and a hairdryer are very nearly equivalent. Sticking your fingers in them isn't equivalent. Virtually nothing in your house that plugs into 110V single phase can produce much more than about 1500 W (or it would be tripping breakers), but some of it can make a huge mess.

 

[MAKG1]

It's not me who is doing this. I just figured Murphey was also trying to throw a little math/science in there too. But maybe I am wrong.

There is a whole lot of science to throw into aviation. Just, this is a bad target. Safety needs to be clear, concise, and really obvious.

If there are any jets around, one of the Top Gear videos of the effect of 20,000 lb thrust on a 2,000 lb car might be useful.

 

[Henning]

There is a whole lot of science to throw into aviation. Just, this is a bad target. Safety needs to be clear, concise, and really obvious.

If there are any jets around, one of the Top Gear videos of the effect of 20,000 lb thrust on a 2,000 lb car might be useful.

We used to park in McDonnell Douglass's parking lot in a 1977 Coupe de Ville on the other side of the fence from where they chained down the F-15s and ran them up. The car used to slide backwards on locked brakes.