Need Ki84 HAYATE's data!

[Ivan1GFP]

Hi Gman,

As I see it, my idea only adds ONE data point. I am not saying it is the best approach; It is an ADDITIONAL approach. Perhaps the designer would tend to put disposable loads slightly aft of Center of Lift so that dropping them only moves CoG forward?

The problem with just using 25% MAC is that it is probably TOO correct. If this is the CoG estimate and then that estimate is used to calculate performance and handling, then the estimates will give nothing unexpected.

Yes, this method CAN fall apart as you pointed out. Take the case of a late model P-51B/C or P-51D: There is an 85 gallon fuel tank behind the cockpit. That's 510 pounds of fuel well aft of the CoG. The location of the drop tanks probably balances that.

I have been looking at the J2M Raiden and its drop tanks (300 or 400 liter) and to me, it seems as if they are a bit far back in the paintings. I have not found a good side view photograph to confirm.

I understand your See-Saw analogy, but I believe it is really Center of Lift you mean rather than Center of Gravity.

- Ivan.

 

[Gman]

Hi Ivan1GFP

Guess that it is my turn to appologise for taking so long to respond. I read your response some time ago, but the rigors of life left me with little time to get back with you.

Let me start out by also appologising to you if I gave you the impression that I was criticising your idea. It was not my intent to criticise, but to suggest caution in using it, and to cite a reason or two why.

An aircraft's performance is not governed by the CG alone; it is governed by how one assembles each and every one of several aerodynamic elements. So I am not sure how the 25% MAC could be too correct, or what you mean by the estimates giving nothing unexpected. I mean, the whole purpose of designing an aircraft is precisely to minimize the unexpected. Be that as it may, it is a true, unmitigated fact of reality that the CG will be in the vacinity of the 25% MAC. That does not mean that every conventional WWII aircraft- or any other pre/ post war conventional aircraft for that matter- will necessarily have their CG directly on the 25% MAC. But if you find the 25% MAC, you will be acceptably close (depending upon one's definition of acceptable). Besides, we have both touched on the fact- at least tacitly- that an aircraft's CG is not static; it is dynamic. How dynamic it can be will be dependent upon its "Static Margin". Static Margin is aerodynamic engineering parlance for "fudge factor", remembering that we never want the CG to fall behind the NP (Neutral Point). The standard for static margins is usually around 25% MAC, though it can and will vary according to the design and intent of use (the first diagram entry that I made on this forum shows the CG at various locations in relation to the NP and how it affects an aircrafts aerodynamic stability).

While it may be true that some of the information that I have presented here (such as my teeter-totter analogy) may also be usable with other aerodynamic elements, I did my best to present them as they specifically pertain to the CG.

J2M, huh? May I recommend the Maru Mechanic? Several nice pics of it with drop tanks, and cutaways with color coded fuel tanks.

 

[Ivan1GFP]

Hi GMan,

I believe we are still not understanding each other. How can something be TOO correct you ask. Well, here is an example:

Let's say I work as an investigator. There was a burglar at our site. To identify him, I want to know how tall he was. I COULD assume the fellow is average height 5'10 or so. This fellow left some footprints and tracks at the crime scene, so I believe it would be a better idea that we estimate his height from the length of his stride. Perhaps it will be accurate, perhaps not, but it is better than going with an average or "proper" value.

I did round up a Maru Mechanic that has the Kawanishi N1K2-J and the Mitsubishi J2M Raiden. I believe I also might have a FAOTW book on the plane. I bought some stuff at the last Modellers meeting and don't remember if these were in the mix.

- Ivan.