Return-Path: Sender: (Marvin Kaye) To: lml Date: Mon, 21 Jun 2004 12:16:01 -0400 Message-ID: From: "Marvin Kaye" Subject: Re: [LML] CG and Gross Weight X-Original-To: Sky2high@aol.com X-Mailer: CommuniGate Pro WebUser Interface v.4.2b6 X-Original-Date: Mon, 21 Jun 2004 12:13:39 -0400 X-Original-Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for Sky2high@aol.com: Cary, Cafe Problems: Generally, many people have chosen to represent CG calculations from the TE of the prop spinner. This can lead to false results because of different propeller setups (wood/Hartzell/MT), long/short engine mount, angle of the extended nose gear, placement of rear wing spar (fuel), etc. For example, You have placed the mains at P73.75 and the nose at P29.5 (P= from prop spinner TE). My measurements are P76.6 and P32.6, aproximately a 3 inch difference. It would be best to convert your measurements to FS (aft face of fire wall) since the firewall/nose/wheel/wing relationships are equivalent across Lancair 320/360 regardless of engine/prop/engine-mount combination. In my airplane, FS0 = Px - 36.25. Side notes: Remember that retraction of the nose wheel moves the CG back somewhere between 1/4 to 1/2 inch. Gross Weight calculations are related to what the wing can carry relative to stall loadings and max G's. Also, it is important to consider what forces the landing gear was designed to carry. You may be operating out of a 7000' low altitude airport, but some day you will want to land somewhere else and, possibly, also take off from a higher altitude on a shorter runway. My airplane is a slow-built 320 equipped with: Hartzell CS prop and 12 lb harmonic damper on the flywheel, short eng mount, small tail, 9 gal header tank, 43 gal total and 1210 pounds empty. 1. Exceeding Gross Weight - I have flown (very carefully) at approximately 1960 pounds with the CG about 2/3rds back. The autopilot could not hold a strictly level altitude (loss of pitch stability) until 60 pounds of fuel were burned off. I would not consider future flights at anything over 1900 pounds and would seriously consider whether a flight over 1800 pounds is worth the added risk. There is no way to load these up without moving the CG back. I understand that the 3 inch longer engine mount moves the CG forward about 1.5 inches. 2. Do your own calculations for items moving forward. new CG = ((old GWT moment) - ((change in arm)*(item wt)))/(old GWT) Using your example and a 22 pound battery moving forward 40 inches: new CG = (129960-(40*22))/1938 = 66.6 (old= 67.1) would move the CG forward .5 inch. 3. My header tank carries 9 gallons at FS7.5. Converting to your example, 7.5 + 36.25 + 3 = P46.75 and the moment would be 9 x 6 x 46.75 or 2524. 4. Of course the bigger tail moves the CG back and the long engine mount was made available to help correct for that condition. The long engine mount is useful even if you don't have the bigger tail. Let's say the h-stab center of weight is about P240 and the engine center of weight is about P20 so that each pound added to the tail requires 12 pounds added to the engine to balance it without a change to the CG. 5. Wingtip extensions - Hmmmm, more weight/lift on an already highly loaded wing spar. I don't know nuttin about no stink'n extensions... Scott Krueger AKA Grayhawk Sky2high@aol.com II-P N92EX IO320 Aurora, IL (KARR) LML, where ideas collide and you decide!