# One of the best textbook questions I have ever seen

1. Aug 23, 2008

### Cyrus

I though that was a great question for a textbook that forces people to learn how to communicate effectively.

2. Aug 23, 2008

### lisab

Staff Emeritus
Or how to become great teachers.

3. Aug 23, 2008

### Cyrus

Also, people post questions on here that clearly don't have a technical background. People try to respond by showing a bunch of equations to show off how smart they are. The result the person asking the question has no idea what the hell the person is talking about.

4. Aug 23, 2008

### Astronuc

Staff Emeritus
In that case, I'd use a see-saw analogy, since many people have probably been on one and can relate to the tipping (pitch) if one moves beyond the fulcrum.

5. Aug 23, 2008

### Holocene

Just tell them that if fat people sit in the back of the plane, the pilots will have to use maximum forward pressure just to avoid dropping out of the sky.

6. Aug 23, 2008

### rootX

7. Aug 23, 2008

### LowlyPion

I would simply show them a ruler held in the middle and explain to them that forces acting on the air surfaces of a plane act to lift a plane through a theoretical point referred to as the neutral point and that holding a uniform ruler between your fingers at the 6" mark was like holding it at its neutral point.

Then I would place a small weight like a quarter right in the middle and explain that even with additional weight over the neutral point - the center of mass being still over that neutral point - that the ruler rises and remains level. Then you can show both of the effects of moving the quarter forward of and backward to this neutral point and explain, how the pilot would need to compensate for these effects in flight when making maneuvers as a result of any displacement of the center of mass from the neutral point.

8. Aug 23, 2008

### Cyrus

Guys, the point of this thread was not for you to answer the question......

By posting a solution you're missing the very point of the thread.

9. Aug 23, 2008

### Phrak

What's a neutral point, and does it depend on elevator trim?

10. Aug 23, 2008

### LowlyPion

Maybe explain the purpose then. Apparently you didn't communicate it effectively.

11. Aug 23, 2008

### Holocene

I'm a pilot, and I honstely cannot say that I've ever heard the term "neutral point". There is a "center of lift" which remains fixed, and the "center of gravity", which changes all the time depending on how the aircraft is loaded, or even while it is burning fuel.

With respect to stabilility, the "center of gravity" should remain within allowable distances from a specified point.

Elevator trim plays no factor in these calculations.

Last edited: Aug 23, 2008
12. Aug 23, 2008

### LowlyPion

I think the center of lift and the neutral point are the same thing.

13. Aug 23, 2008

### Phrak

Elevator trim changes the lift of the hoizontal stabilizer, so would change the center of lift, would it not?

14. Aug 23, 2008

### chasely

My interpretation:

When you truly understand the material, you will be able to explain it to others in the simplest means possible. The question above makes the student think critically about the material so that it can be explained to a layperson. Most of the textbook questions out there have the student think very technically about the subject, so they can explain it to their peers in the field. However, by not thinking about it in a manner posed in the above question, they are missing the connection of the material to everyday examples. It is these examples which help to reinforce the learning of the material, and the ability to use it in other applications besides the ones given in technical questions found in textbooks.

Was that at all coherent?

15. Aug 23, 2008

### G01

That is a really good question. I know a lot of science and engineering majors who I have class with that could not for the life of them begin to explain anything they know in simple terms accessible to someone who is not in the field.

I think if more scientists had this skill there would be less misconceptions about science in the world.

16. Aug 23, 2008

### rootX

It takes more time when you use a human language (they are ambiguous and inefficient).

17. Aug 23, 2008

### Holocene

"Center of lift" is with respect to the airfoils (wings), not the stabilizer/stabilator.

18. Aug 23, 2008

### Phrak

Is it already in lay terms? Does "unstable" mean astable or isn't there enough stick to take the airplane to the pitch equilibrium position?

I'm not a pilot, just a former hang glider pilot and former aeromodeler

Edit: Sorry, I didn't see your post, Holocene.

The total lift on an aircraft is due every surface, including the horizontal stabilizer. More down trim reduces the effective angle of attach of the horizontal stabilizer. For a self correcting pitch the horizontal stabilizer needs to have an effective angle of attack less than the wing--otherwise the pilot or autopilot are required as part of the corrective feedback. So it's unclear to me if this problem is about pilot induced pitch oscillation or running out of stick.

Last edited: Aug 23, 2008
19. Aug 23, 2008

### LowlyPion

http://www.phnet.fi/public/mamaa1/einstein.htm

His grandmother wasn't a barmaid was she?

20. Aug 23, 2008

### rootX

Last edited: Aug 23, 2008