# Problem with static margin and neutral point

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### ragflyer

##### Well-Known Member
Without fuselage the NP that you get from VLM agrees reasonably with the approximation. In fact if I adjust for tail height (decreases de/da) and the fact that you have an airfoil with NP at 26%, I get 67%. I am assuming in the vlm you included the tail height.

Btw what do you get with fuselage turned on?

Also as Billski points out I am suspicious of tail areas you posted. Maybe there are perspective issues, but it sure looks smaller than the 4 to 1 ratio.

#### Scheny

##### Well-Known Member
You are right that there is a typo for the tail area. It is 1.12m² and the leading 1 got missed.
As this was also asked: the height of the elevator position is taken into account.

Due to wing being at 1800mm station, 1000mm root chord, 814mm MAC (earlier I used the wrong value 849mm*), this results in:
NP with body = 2554mm --> 87% (75%)
NP w/o body = 2625mm --> 95% (82%) (with body panels ignored in analysis)
NP w/o body = 2424mm --> 71% (62%) (with body deleted)

The red values are calculated with a fictive wing which has constant 1000mm chord.
So it seems that the calculations are correct, as 62% without the fuselage looks right and the higher values are due to the fact that the mean chord is so small.

*Most people erroneously call it %MAC, while in reality the correct term would be %MGC. Gudmunsson writes in his book "General aviation aircraft desin applied methods" that MAC and MGC can be considered roughly the same, so I used the MAC value instead of the MGC value (only realiting now in the thread how big the difference is).

#### User27

##### Well-Known Member
Talking about the Neutral Point without the body is meaningless as the Neutral Point is the aero centre of the whole aircraft, wing, fuselage & tailplane. But I wonder if you should consider it at this point in your design process? Should you make the assumption the wing aero centre is around 30% and the static margin should be from 20% to 5% to allow tail sizing? Depending on your modelling capability it may be that those numbers are refined during flight testing depending on the control effectiveness you measure. Often the position of the engine within the fuselage (assuming a jet), the wing in relation to the fuselage or the length/volume of the tail are adjusted the achieve reasonable numbers. Are you fixed on a T-tail? Usually a lower tailplane is lighter and provides better control.

#### Heliano

##### Well-Known Member
Good job, Scheny! As I can see you have a high tail volume and, on top of that, your horizontal tail stands high above the wing - where downwash is somehow reduced, for normal angles of attack (up to, say, 10 degrees). So I see the 84% NP as quite possible. I presume that this NP is stick-fixed, that is, has to do with elevator deflection, not elevator forces.
The only comment I have is: aft NP's as yours MAY mean control difficulties with forward CG. May I suggest that you investigate controllabillity at those forward CG's such as 5-15% if you intend to use that range.

#### ragflyer

##### Well-Known Member
Due to wing being at 1800mm station, 1000mm root chord, 814mm MAC (earlier I used the wrong value 849mm*), this results in:
NP with body = 2554mm --> 87% (75%)
NP w/o body = 2625mm --> 95% (82%) (with body panels ignored in analysis)
NP w/o body = 2424mm --> 71% (62%) (with body deleted)

The red values are calculated with a fictive wing which has constant 1000mm chord.

Am I reading this right? Is the fuselage moving the NP aft? If so you probably have an issue. Fuselages generally move the NP forward.

#### Scheny

##### Well-Known Member
My aerodynamicist was away for a few weeks, but I finally reached him. The biggest input from his side were, that the method that I used for NP calculation is not usable for this usecase, as it only works with thin objects like wings. The fuselage will now be modified to account for the corrected CoG and then I will run CFD to get the exact and correct NP.

Are you fixed on a T-tail? Usually a lower tailplane is lighter and provides better control.
I favor conventional, but the control horn is in conflict with the turbine. T-tail is easier to realize but I will try again.

I presume that this NP is stick-fixed, that is, has to do with elevator deflection, not elevator forces.
The only comment I have is: aft NP's as yours MAY mean control difficulties with forward CG. May I suggest that you investigate controllabillity at those forward CG's such as 5-15% if you intend to use that range.
Thanks! Indeed stick fixed. Most forward CoG was checked with the old configuration, but I will run it again with the modified fuselage.

#### ragflyer

##### Well-Known Member
The fuselage will now be modified to account for the corrected CoG and then I will run CFD to get the exact and correct NP.

You seem to have a lot of faith in CFD giving you the "exact and correct" NP. It is doubtful it will give you that, particularly if you do not have a lot of experience with it. But then again you do not need super precision. Use a semi empirical formula to get you in the ball park, give yourself enough static margin and then flight test to establish your aft CG limit. If needed you can always move things like batteries to fine tune. It is not an airliner you are designing with massive CG range and trim drag sensitivities.

btw you could also fly the model to establish CG range. Generally NP remains constant with scale in the linear range.

#### Scheny

##### Well-Known Member
You seem to have a lot of faith in CFD giving you the "exact and correct" NP. It is doubtful it will give you that,...
...btw you could also fly the model to establish CG range. Generally NP remains constant with scale in the linear range.
Exactly what I am doing
CFD is only to verify that I am good to go for the scale model.

#### Scheny

##### Well-Known Member
So, a short status update here. I calculated everything again, using a traditional method of calculation and thereafter changed the wing form (slightly less chord) and its position. Due to higher aspect ratio but less area, the aircraft will behave better now at cruise speed, while landing kept almost the same.

The neutral point now is at 52%, due to skinny wing and high tail coefficient of 0.71 (what is recommended for jet trainers). The CoG will be placed between 32% and 47%. This provides 12% of margin for an average pilot and ~18% for a "stronger" pilot. For super-skinny pilots (less than 150lbs), it will be possible to add a 7kg (15lbs) of steel together with an optional nose bulkhead. Otherwise, CoG will reach neutral point at less than 55kg pilots (120lbs).

As a sanity check I also calculated the JSX-2 and got 38% as neutral point, which aligns good with their stated allowed CoG margins of 18% to 30%. Something I noticed, was the super bad tail volume with a factor of only 0.42 in case that I calculate with the projected surface, but even with full area, I dont get much above 0.5

#### bhooper360

##### Well-Known Member
As a sanity check I also calculated the JSX-2 and got 38% as neutral point, which aligns good with their stated allowed CoG margins of 18% to 30%. Something I noticed, was the super bad tail volume with a factor of only 0.42 in case that I calculate with the projected surface, but even with full area, I dont get much above 0.5

Well, it would make it feel more like a jet. It's not as much fun if you pull back and lose all your energy in one turn.

# # #

Edit: Let me take the opportunity to get on my soapbox here and expound on my criticism* of Raymer's book. The tables you see scattered throughout, are based on parametrized values from successful aircraft. This approach to airplane design is based on analogical thinking.

What does this mean? Example; you sized your tail based on the table value for "jet trainer." Well, jet trainers are used for contact ( formation) training and for instrument approaches, so they benefit from static pitch stability. Jet trainers are also two-seat tandem seating, which means the cg varies over a wide range. Also, military aircraft tend to exist forever, and traditionally they gain avionics weight as they age. A large stabilizer is helpful for this. On the other hand, you will not see a jet trainer optimized for efficient cruise at your thrust settings.

Since you optimized for faster cruise and good takeoff/landing performance, it's not clear why you used the jet trainer sizing. It can be justified: maybe the cg change is comparable because your airplane is a lot smaller, maybe you also want the option to use heavy nose gear, etc. etc. but you haven't actually stated any of this.

To base your solution on first principles, you have to figure out what the tail is actually supposed to do. Maybe you need to list out key points where the tail is supposed to be most efficient, generate the most lift, or whatever. Eventually, instead of the assumption, "my airplane is analogous to a jet trainer," you end up with a more basic assumption, "the thing I put on the back of my airplane is analogous to a wortmann profile wing" (for example). This is a much easier assumption to justify. But it takes longer.

Your tail sizing looks correct. It's a good idea to use the tabular values, because it prevents you from getting bogged down in triviality.And you are also cognizant of the assumptions you make, and to what extent they align with your goals and values.

* I use "criticism" in the literary sense

Last edited:

#### Lendo

##### Well-Known Member
Scheny, 0.5 volume coefficient for the HT is good, 0.42 is low, some say Minimum is 0.45, but I'd prefer 0.5.
George

#### Scheny

##### Well-Known Member
Scheny, 0.5 volume coefficient for the HT is good, 0.42 is low, some say Minimum is 0.45, but I'd prefer 0.5. George

...it's not clear why you used the jet trainer sizing. It can be justified: maybe the cg change is comparable because your airplane is a lot smaller, maybe you also want the option to use heavy nose gear, etc. etc. but you haven't actually stated any of this.

The 0.7 are used, because the allowed CoG variation is in the range of 4-5 inches. With a coefficient of 0.5 it is likely to run into unstable behaviour.

#### Lendo

##### Well-Known Member
Scheny, 0.7 is high, considering larger Volume coefficient leads to excess weigh and Drag. Orien (Bill Husla Dec'd HBA member) an aircraft designer, who's company is Orien, suggested a Minimum was 0.5, but it depends on the Aircraft size weight etc. The lowest I've seen from Books is 0.43 to 0.48, then0.45 from John Roncz, then Orien's recommendation, of 0.5 Min.
I will punch in 0.7 into my Spread sheet and see how it affects the lever Arm or Size.

#### Lendo

##### Well-Known Member
Scheny, 0.7 adds over 64" to my lever arm, way too much.
George