Original Posted By: "bender"
Subject: Re: Pietenpol-List: CG QuestionOK Wayne,I was right.But, I was not smart enough to argue my point of view. So I got me a Big Gun. DrDave Rogers is on the Beechcraft Bonanza list and has a good track record ofteaching us Luddites about aerodynamics. He knows a little about this since heteaches aerodynamics at a small school in Annapolis, Maryland, that school hasas something to do with ships. Some of their graduates fly off of very short(though paved) moving airstrips. Dave is able to take some highfalutin science talk and dumb it down enough so thateven a Marine can understand or better an Army Grunt, (BTW a blade antenna,mounted properly is much more aerodynamic than a rod antenna. He has me convincedthat about the only thing aerodynamically worse than a round pole is maybea square block) Below is the original message I sent to him:Dr. Dave, in a discussion with a fellow pilot. I made the following statement:"In layman's terms, the reason a plane gets more "Twitchy" with an aft CG is theshortened distance between the CG (moved aft) and the elevator. Move the CGforward and the plane becomes more stable. When the CG is too far forward, theplane becomes so stable that the required response may be too slow or not possible.IE too far forward a CG and you can't get a good flare when required."The other pilot said: "I thought the reason for the increased sensitivity was dueto the Center of Gravity becoming closer to the wing's Center of Pressure."Which is correct? Or is it like Bernoullis sucking or Coandas blowing. Luckily the end effect isthe same. I think I got my idea from the Gleim Pilot handbook recently studied for an FAAtest. Blue Skies,Steve DHIS ANSWER:G'day Steve,You are.On 9/16/2011 3:06 PM, Dortch, Steven D MAJ NG NG FORSCOM wrote:>Dr. Dave, in a discussion with a fellow pilot. I made the following statement:>>"In layman's terms, the reason a plane gets more "Twitchy" with an aft CG is theshortened>distance between the CG (moved aft) and the elevator. Move the CG forward andthe plane>becomes more stable. When the CG is too far forward, the plane becomes so stablethat the>required response may be too slow or not possible. IE too far forward a CG andyou can't get>a good flare when required."This is a reasonable layman's description.Technically, there is a point as the CG moves aft at which the stability becomesneutral. That point is called the neutral point (stick fixed, stick free, with/withoutpower as the case my be). If the CG is aft of the neutral point, then the aircraftis unstable.When the CG is too far forward a number of things may occur examples are: (a) thereis notenough elevator power to flare the aircraft for a full stall landing; (b) thestick force is too large; (c) the stick force gradient is too large.There is a small discussion of the forward CG question on the Technical Flyingwebsite.>>The other pilot said: "I thought the reason for the increased sensitivity wasdue to the>Center of Gravity becoming closer to the wing's Center of Pressure."This is wrong. Technically this is associated with the balance of the aircraftandnot the stability.Simply put, an aircraft must be both balanced and stable (modern tactical fightersare an exception to the stability requirement). In a conventional configuration,i.e., with a horizontal tail, it is balance that requires the horizontal tail withtypically a down force on the tail. It is the stability requirement that limitsthe aft movement of the CG. Control requirements determine the forward CG limit.Dave RogersE33A for saleBEFORE I COULD GLOAT HE ANSWERED ANOTHER BONANZA OWNERS COMMENT, BELOW IS HIS EDITEDCOMMENT:A reasonable simplified explanation. Improved:1. Stability (tendency for the aircraft to return to the trimmed speed when displaceda small amount from the initial trimmed speed.)2. Sensitivity to controls (the tendency for the pitch to change a lot, vs a little,with a given movement of the yoke).Other Pilot comment: 3. Moving the CG forward tends to make the aircraft morestable. If the CG is moved too far forward control issues result, e.g. stick force becomes too large.DAVE'S Answer: Moving the CG toward the elevator makes the aircraft less stableand makes it more sensitive to elevator inputs.for the same elevator force a shortened lever are would result in LESS momentto pitch the aircraft.Dave RogersE33A for saleSo I was right, but I sure did not understand why until he told me why. Blue Skies, Steve DHis WEBSITE (listed Below) has Beech Bonanza centered articles under TechnicalFlying, but there is a lot of generic aviation information. I do not do math inpublic and it makes my ears bleed if I don't understand it, I prefer to justbelieve.Dr Rogers signature block below: David F. Rogers, PhD, ATPProfessor of Aerospace Engineering (Emeritus)Annapolis, MDRogers Aerospace Engineering & ConsultingAnnapolis, MDOver 50 years of experiencewww.nar-associates.com410 271 1968 (c)________________________________________________________________________________Subject: Pietenpol-List: Re: Flitzer