Understanding the Raymer Neutral Point Formula

[natejensen]

I am wondering if anybody understands the Raymer neutral point formula? I have attached the formula for reference.

Even just an example would be good enough.

Thanks,

Nate

 

[viscousflow]

His neutral point formula comes from the datcom method. Are you familiar with that method at all? Also it would help if you could tell me which terms you do not understand rather than explaining them all.

 

[natejensen]

I am not familiar with the datcom method. I have begun to read about it here:
http://en.wikipedia.org/wiki/USAF_Stability_and_Control_DATCOM

I am sorry this is long, and maybe this is more for myself, to clarify what I do and do not know, but here is my understanding of the Raymer neutral point formula.

What I know, or at least what I think I know.

- X_np
The neutral point of the wing in reference to every other point. This is calculated from the Raymer formula.
- C_L_alpha
The coefficient of lift with respect to angle of attack of the wing. This is calculated from PLLT.
- C_m_alpha_fuselage
The coefficient of moment with respect to angle of attack of the fuselage. The only way I can think of to calculate this is to run a CFD program like Fluent.
- eta_h
The efficiency factor of the horizontal stabilizer. This is equal to the dynamic pressure at the horizontal stabilizer over the free stream dynamic pressure. Normally this is just one. I do not know how to calculate this value without the use of CFD.
- S_h
The planform area of the horizontal stabilizer. This is a simple projected area formula, not sure if that is the right term, but you get what I mean.
- S_w
The planform area of the wing. Again, this is a simple projected area formula.
- C_L_alpha_h
The coefficient of lift with respect to angle of attack of the horizontal stabilizer. I assume this is also calculated from PLLT.
- q
The dynamic pressure, using the standard formula q = 0.5*rho*v_inf^2


What I kind of know.

- Xbar_acw
The aerodynamic center of the wing as a fraction of the wing mean chord. I think this is calculated by this formula:
(2/3)*(W_cr+W_ct-W_cr*W_ct/(W_cr+W_ct))
where,
W_cr = Wing chord root
W_ct = Wing chord tip
- (del alpha_h)/(del alpha)
This is the change in angle of attack of the horizontal stabilizer with respect to the change in angle of attack of the wing. I think to do this you have to find the change the aerodynamic center of the entire aircraft, then geometrically calculate how much a change in angle of attack changes the angle of attack of the horizontal stabilizer.
- Xbar_ach
The aerodynamic center of the horizontal stabilizer as a fraction of the horizontal stabilizer mean chord. I think this is calculated by this formula:
(2/3)*(H_cr+H_ct-H_cr*H_ct/(H_cr+H_ct))
where,
H_cr = Horizontal stabilizer chord root
H_ct = Horizontal stabilizer chord tip
- (del alpha_p)/(del alpha)
This is the change in angle of attack of pressure on the wing with respect to the change in angle of attack of the wing. I don't have the formula on me, but I think I could find it.
- Xbar_p
The center of pressure on the wing as a fraction of the wing mean chord. Again I do not have the formula on me, but I think I'll be able to find it somewhere.


What I definitely do not know.

- F_p_alpha
I think this is the force of pressure with respect to alpha on the wing, but I have no idea how to calculate it.

 

[viscousflow]

Hey, sorry I haven't gotten back to you. I've been bogged down the last week with work. This weekend I'll take out my old static stability notes and fill you in.

 

[natejensen]

Awesome thank you

 

[viscousflow]

Hey I checked my old stability notes and the neutral point formula we had was slightly different. Meaning it was meant for a different geometry. However, for all the definitions you posted, you were spot on except, F_{p_\alpha}. I have no idea what that denotes.

In my mind this leaves 1 of two possibilities:

1)I know that Raymer gets his information from many sources and combines certain things. So you'd have to find the source where he used this equation

2)I know that Datcom uses a component buildup method. So I am thinking this aircraft has a certain configuration and utilizes a component which was denoted with subscript 'p'.

I'll have to do some snooping around to see what's going on.

 

[meganlove19]

F_Pα in this formula signifies the propeller or inlet force. I know this is an old thread but for anyone looking for some clarification check out the image I attached it goes over the basics from the text Aircraft Design a Conceptual Approach by Raymer