So the ask a cop thread has been very useful. I've been at a show in Chicago all week and just realized how stupid most people are. If you have thermo/physics/common sense questions for mechanical engineers. Post them here.

9/14/2014 1:55:58 AM

how'd you handle the labs taught by people who speak no english

that shit is a big fuckin curveball

9/14/2014 2:39:08 AM

you email...

9/14/2014 2:47:41 AM

during or after being handed a crosspeen hammer and told "very hard hit now. not now but is soon the lever for least crush"?

9/14/2014 3:13:29 AM

all replies to questions in this thread are hereby required to be followed by fuckface elipses

9/14/2014 3:15:15 AM

How cool is it that you went to college to learn to drive a train?

Do you wear those goofy hats the engineers wear in the movies when they're driving trains?

9/14/2014 7:44:37 AM

if the treadmill were going fast enough would the airplane take off?

9/14/2014 8:04:07 AM

Quote :

"how'd you handle the labs taught by people who speak no english that shit is a big fuckin curveball"

You give up a little bit and drink a lot.

9/14/2014 8:15:53 AM

Quote :

"How cool is it that you went to college to learn to drive a train? Do you wear those goofy hats the engineers wear in the movies when they're driving trains?"

It's cool as fuck.

And obviously.

9/14/2014 8:19:07 AM

Quote :

"if the treadmill were going fast enough would the airplane take off?"

No you idiot. Lift is created by air passing over the wings. If the plane is stationary there is no air flow.

9/14/2014 8:20:57 AM

NCSU BSME 12/07 here-

Do you find yourself faced with random questions every day that begin with "You're an engineer, how do you [.....] ?"

9/14/2014 8:30:48 AM

and the [.....] always being completely unrelated to your field, like "reprogram a VCR" as an example

9/14/2014 8:48:49 AM

Honestly it's more of people trying to explain shit to me incorrectly. And then me politely suggesting the correct explanation. Being shot down. And then explaining that I'm an engineer.

9/14/2014 8:50:12 AM

You did just preface your question by identifying yourself as an engineer; Your real question is mocking people who do the same for you?

9/14/2014 8:50:40 AM

^no; it's a 'do people do the same thing to you?' question.

It's a constant barrage of questions and scenarios for me at home and work. Computer service, opinions on automotive repair - "my car is making this noise, what's wrong?", purchasing electronics, being expected to have a single-variable equation that answers some random spec on a dynamic, multi-variable process.

9/14/2014 9:30:36 AM

This shit happens to me all the time. Really annoying. Like because I'm an engineer, I obviously know everything

9/14/2014 9:36:32 AM

I'm engineer. I math.

9/14/2014 9:53:55 AM

A lot of people confuse being a mechanical engineer with being a mechanic

9/14/2014 10:04:25 AM

Yeah, mechanics can actually make something work correctly

9/14/2014 10:34:25 AM

Quote :

"I'm engineer. I math."

ISWYDT!

9/14/2014 12:08:10 PM

So at work we are trying to decide whether we want to spend 50-75k to proof of a concept a solution that would help us save 500-700k if it works. We think it will work but before we can even begin the initial modeling phases I need to come up with a plan of action on how to spend as little money as possible to find out if the path we are on is feasible.

The challenge:

An OEM of ours builds a hydraulic motor/gear box combo that they currently production test by connecting it to a heavy flywheel that has a known moment and also have a known drag load that I think is applied by some type of brake setup. The flywheel + drag load simulates the loads that the hydraulic motor sees when it is installed in its application.

We want to have test and repair/overhaul capability of these hydraulic motors in house and we are trying to decide if existing test equipment for other motors we have can be made to do the job or if we need to try and purchase a copy (if they'll even sell it) of the OEMs test stand or to design and build a clone of their stand.

Our current test system has a DC servo motor connected to a gearbox (for torque multiplication) capable of producing 40,000 in-lb loads at ~35 rpm. The servo motor is controlled by a high speed highly sophisticated servo motor controller that could be programmed to changed the load on the fly.

The test process calls to spin the hydraulic motor 8 revolutions. The motor should not exceed 55rpms and the 8 revolutions have to be completed within a given period of time.

I'm trying to figure out where to start with equations to show mathematically and via a model how this will work. Once I can show it on paper we'll get the seed money to create the fixtures to adapt the motor to our existing test system.

[Edited on September 14, 2014 at 12:20 PM. Reason : .]

9/14/2014 12:18:42 PM

I'm an aerospace engineer so very similar. Anytime something is in the news related to space or aviation I get tons of messages from friends and family asking about it. I usually just read the Wikipedia page and report back.

9/14/2014 1:05:40 PM

Quote :

"Yeah, mechanics can actually make something work correctly"

If they didn't they wouldn't be called good mechanics.

[Edited on September 14, 2014 at 1:38 PM. Reason : .]

9/14/2014 1:36:59 PM

Quote :

"Quote : "if the treadmill were going fast enough would the airplane take off?" No you idiot. Lift is created by air passing over the wings. If the plane is stationary there is no air flow. "

No, you are an idiot. Plane is propelled forward by engines that are pushing against air (in case of propellers) or against gaseous fuel they release in high velocity (in case of jets). They are not propelled forward by the wheels. Therefore, a plane on a treadmill will not be stationary, it will move forward regardless of what tarmac is doing underneath it, and take off just like it would on a regular landing strip (assuming the brakes are not on). You just showed your inability to grasp basic physics concepts, your engineering advice going forward in this thread is therefore declared worthless.

9/14/2014 11:19:14 PM

Put a model plane on a treadmill. Your assuming impossibilities of a real plane fitting on a tread mill and then incorporating jet propulsion. Duh a jet motor moves. You answered that like an engineer and not an engineer trying to answer the actual question.

Does lift occur with no movement?

No

Do jets make a plane work?

Yes.

9/15/2014 12:34:01 AM

Doesn't matter if it is a model or a real plane. The setup goes "put a plane on a big enough treadmill with the belt moving at the same speed as the takeoff speed of the plane, will the pane take off?" And the answer is yes, that plane will move forward and reach it's regular takeoff speed relative to a stationary object, no matter how fast the treadmill spins, because the force is not applied to a treadmill, it is applied to the air. Friction between the treadmill and the plane is negligible. Put a model airplane on a treadmill, and it will take off. Hell, here is a video for you with an actual model plane on an actual treadmill, it looks like Mythbusters did one with a bigger plane:



Don't even try to backpedal you way out of this one, this question is testing not whether you know that lift is created by airflow, but whether you can take several basic engineering concepts and apply them together. And you last response is moronic. "Does lift occur with no movement?" Movement of what? The plane? The air? The ground under the plane? Do you even understand that movement is relative? Lift CAN occur with no movement of the plane relative to the ground, for example if there is a headwind.
"Do jets make a plane work?" What is this, I don't even


Between you and [user]sumfoo[/user] with his solar-panel-is-a battery theory, I think that NSCU must have just gave out an engineering degree to anyone that showed up for class.


[Edited on September 15, 2014 at 7:45 AM. Reason : .]

9/15/2014 7:16:15 AM

Quote :

"It's a constant barrage of questions and scenarios for me at home and work. Computer service, opinions on automotive repair - "my car is making this noise, what's wrong?", purchasing electronics, being expected to have a single-variable equation that answers some random spec on a dynamic, multi-variable process."

I get that a lot too.

9/15/2014 8:46:29 AM

is lion4russell trolling, or does he really think that a plane won't move on a treadmill?

9/15/2014 8:54:27 AM

I'm a Mechanical Engineer as well and people ask me random stuff all the time. The thing is, it's usually about cars or something classically 'mechanical' but that's not the stuff I enjoyed or excelled at in school. I was a fluids and thermo guy all the way. I did not really enjoy anything on the material science, automotive, systems design (gear ratios, bearings, etc.) side which most people think of for Mechanical.

The other part is that I also am not a research or 'invent stuff' type engineer for my job. I actually enjoy the applied side. I enjoy understanding enough about the 'how' side to choose the right solution for the right problem for our clients. Yes there are lots of calculations and still a lot of design, but it's at a little bit of a higher-level. Our vendors and suppliers do the heavy lifting in terms of developing new, more efficient equipment.

[Edited on September 15, 2014 at 9:25 AM. Reason : ]

9/15/2014 9:22:32 AM

I don't understand how the plane/treadmill thing is still even a question. Mythbusters used a full sized plane on a giant treadmill years ago to prove that the plane does take off. We had a thread back in like 2005 that spanned for pages/years of people going back and forth on this -- every time I said why it would take off people just ignored me. After the Mythbusters episode, all the naysayers were completely quiet.

9/15/2014 9:44:20 AM

i don't see how there would even be a debate, let alone a debate among college educated people (and engineers!)

9/15/2014 9:51:15 AM

^Even leading up to the Mythbusters thing there were youtube videos of small RC planes on a treadmill moving forward under its own power without trouble. All of the naysayers also conveniently ignored them.

9/15/2014 9:55:00 AM

did anyone in the debate remind people that planes can takeoff from water, snow and ice? and did that still not trigger anyone into realizing their mistake/misunderstanding of the issue?

9/15/2014 9:57:08 AM

I love that the Airplane treadmill question came up in the "Ask an Engineer" thread... and the "engineer" said

"Do jets make a plane work?

Yes."

9/15/2014 10:07:09 AM

^^I mentioned ice at one point but nobody even responded so at that point I think I just gave up trying to convince anyone since they had already made up their minds.

9/15/2014 10:58:00 AM

re: plane on treadmill

I think most of the confusion comes from the stipulations in the question. Simply asking if a plane will take off on a treadmill is not enough information. The short answer is, if the question's stipulations imply that air move over/under the wings, then yes; if not, then no.

9/15/2014 11:14:19 AM

lullz.

BUT THE WHEEL SPIN GUISE!

9/15/2014 11:17:35 AM

Its still hard for me to wrap my head around the fact that if a truck/trailer with a ramp is going down the highway, and I drive up on it after matching its speed, that I stay on the trailer.

I've been in tons of physics classes with that question, seen the mythbusters episode, but I still reject the reality.

Maybe I dont so much think that I'll accelerate up the trailer ramp and "jump" it, but that I will at least need to apply some brakes to slow down. All the videos I've seen of this stunt the cars just go from 60mph on road to 0mph (its speed) on the trailer in a split second.

9/15/2014 11:26:22 AM

Quote :

"No you idiot. Lift is created by air passing over the wings. If the plane is stationary there is no air flow."

So you're saying if the plane is stationary and air is moving fast enough relative to said plane, it wont take off?

9/15/2014 11:28:35 AM

^^Yeah that one I agree with you on. All of my instincts in that one lead me to believe that the car would just speed over the ramp and hit the front end of the trailer but all the videos of demos I've seen obviously show otherwise. It is weird.

9/15/2014 11:32:42 AM

if you are going the same speed as the truck, you won't be going up the ramp. going up the ramp requires going slightly faster than the truck, so you will still need to brake

9/15/2014 11:36:00 AM

Where's Silverberg when we need him!

9/15/2014 11:36:29 AM

with the car on a ramp, it's not like your brakes would need to stop the car's momentum... just the rotation of the wheels. a quick downshift into first would do it probably?

or hell, just make sure you have enough relative velocity to make it up the ramp and go up in neutral.

[Edited on September 15, 2014 at 11:37 AM. Reason : ]

9/15/2014 11:37:07 AM

Wow I cant believe people still dont get the treadmill plane thing, especially someone who is claiming other people are so stupid. I am hoping you were making this a place holder for other mechanical engineers to answer questions and you are not actually one otherwise

9/15/2014 11:38:50 AM

^^there will be no forward momentum of the car relative to the truck. The only momentum that will need to be overcome by any braking action is the rotational momentum of the wheels. That momentum can be stopped by brakes in less than quarter of a revolution of the wheel. Take your bike, pick up the front end, get the wheel spinning, and hit the brake. You will see that will will stop almost momentarily. Unless you are driving a steamroller, the rotational momentum of the wheel will be almost negligible.

Yes, obviously you'd have to disengage the power train once the driving axle is on the trailer, and use either momentum of going slightly faster than the trailer, or on of the two axles to make it up the ramp. This is not possible on a full-time AWD car, but possible on both fwD and rWD cars, just a slightly different execution on how to push it up the trailer

[Edited on September 15, 2014 at 11:47 AM. Reason : .]

9/15/2014 11:44:16 AM

I understand how the car/ramp thing works it just seems so counter-intuitive to me.

9/15/2014 11:44:31 AM

but what if the car and ramp are on a treadmill and there is a plane in front of them?

9/15/2014 11:51:47 AM

Quote :

"I'm engineer. I math."

ISWYDT!

9/15/2014 1:32:02 PM

^^LOL.

Or, what if it's a car that can turn into a plane

9/15/2014 2:11:39 PM

Nah, turns into a submarine and dives below the ground.

9/15/2014 4:36:23 PM