CENTER OF GRAVITY

center of gravity is one of the most crucial design parameters for all airplanes and it seems to be a subject which mystifies new pilots and builders somewhat although it's not really that hard to understand once you've had a look at what it is and how it interacts with the other design parameters of your airplane particularly the aerodynamic Center center of pressure and neutral point now fixed-wing aircraft will have both lateral center of gravity and a longitudinal center of gravity and the longitudinal center of gravity is the most important parameter in this case partly because the lateral center of gravity is almost always at the midline of the aircraft if it's designed symmetrically with symmetrical identical wings and all of the components are installed at the midline so that the weight is equal between the left and the right sides so we'll put that subject on the backburner for now the longitudinal center of gravity pertains to the balance point between all of the components at the rear of the aircraft and the front of their aircraft taking to account the length of the moment arm that is the lever over which that weight is applied so that any components mounted further from the center of gravity will exert a disproportionately large amount of downgoing force in the back or in the front as the case may be and that is why often the forward compartment of your airplane which contains the battery and heavy electronics is a much shorter lever than the empanadas longer but lighter in any case the center of gravity is the balance point between all of those forces and the lengths of the levers and almost always in a non tapered non swept-wing non canard wing will intersect the airfoil proper now the simplest recommendation for the location of center of gravity and a non swept non tapered wing is typically quoted at being 25 to 33 percent of the total cord rear of the leading edge now though this basic recommendation can vary with a number of other variables including the airfoil shape the DECA lodge or angling of the tail planes relative to the wing plane the angle of attack and certain other design parameters but that recommendation is generally your safest so if you want to skip the rest of the video and just incorporate that knowledge into your airplane design you'll get by 90% at a time with that Commendation but to delve a little bit further into the mechanics of the center of gravity in its relation to the other forces on the airplane let's look into that further surfer swept tapered wings and canards there are calculators for this kind of thing and they'll take into account what's called the mean aerodynamic cord as the center of gravity needs to be located at some proportion forward of the mean aerodynamic cord for a straight non tapered wing fortunately for us the mean aerodynamic cord which is the average of any swept canard or tapered components is actually the actual cord as well so the proportion anywhere on this wing is going to be the same for the center of gravity so in this six inch cord wing the center of gravity located 25% after the leading edge will be found one and a half 1.5 inches rear of the leading edge that is 25% of six inches which is the total cord as to whether to include the control surfaces ailerons and flaps the technical inclusion of those surfaces as they do provide lift drag and other aerodynamic forces is to include them however I often recommend for new builders just to count the airfoil part of the wing that is the camera part of the wing in any case using that measurement to calculate your percent rear of the leading edge you'll always want to end up with a slightly conservative nose-heavy airplane which is certainly always better than a tail-heavy plane center of gravity also varies with angle of attack and particularly with a high wing plane such as this a higher angle of attack will shift the center of gravity forward so you'll see that the balance point at this angle which is quite exaggerated is going to be a further ahead on the wing probably at about 5% of the cord and this does tend to impart some additional stability to a high wing plane for that and a number of other reasons whereas a low wing plane will tend to shift the center of gravity rearward when it's in a higher angle of attack knowing the center of gravity of an aircraft is meaningless unless you take into account its relationship to the aerodynamic center center of pressure and neutral point of a plane now those last three things that I mentioned they're pretty closely related to one another and it's not crucial that you know the distinction between them although I'll go over them except to say that they all must be rear aft of the center of gravity to have stable plane in the both the longitudinal and lateral axis the aerodynamic Center takes into account only drag on an airfoil so if you consider the relative wind coming from this direction the aerodynamic Center is the hypothetical point and some point in the center of the wing which will incur the same force regardless of the angle of attack of that airfoil positive or negative angles of attack theoretically it could be spun around on that point in the relative wind and it would provide an equal amount of drag here and here equal amount of drag here and here so that it could pivot around that point that is the foremost of those three points that I mentioned and is related only to the airfoil that is the wing in this case conversely the center of pressure takes into account drag and lift and those two forces will form a certain force vector which is applied of the upper surface of the wing and differs at different angles of attack so at this position the center of pressure may be here but when a lift is generated at a higher angle of attack and versus the drag vector here the center of pressure may shift rear so it depends on the angle of attack the center of pressure is typically found slightly rear of the aerodynamic center of your wing the neutral point takes into account not just the aerodynamic forces on the wing and its airfoil but also the vertical and horizontal stabilizers and all of the vertical and horizontal surfaces of the airplane itself and this is found typically in a typical planform plane like this further rear even rear of the wing itself because we have further aerodynamic surfaces rear of that exerting this tail moment arm and then providing additional drag here relative to the center of gravity typically found about here so to simplify things I'll discuss all three of those concept in terms of the neutral point because the neutral point takes into account all of the net aerodynamic forces acting on the aircraft and this also can be modified by the Builder by doing such things as adding surface area to horizontal and vertical stabilizer modifying the profile of your aircraft fore and aft to keep that aerodynamic center in check relative to the center of gravity so let's reduce our concept of the aircraft down to this arrow that I'm balancing on my fingers so the centre of gravity is the point at which all of the gravity exerting effect on this arrow are balanced at my finger in the midpoint so this part way is the same as this accounting somewhat for the additional lever arm at this end of the arrow for example but that is the center of gravity now not hard to imagine where the neutral point of this arrow is as this shaft on your right presents very little aerodynamic forces in the form of drag and it is fairly symmetrical because this is a constant diameter with the drag produced by the shaft in the rear the difference is the rear end of the arrow has fletching or fins and that does exert an aerodynamic force therefore that shifts the neutral point rear of this point let's imagine it's about here given the size of the fletching so the forces at hand are the momentum of the airplane acting at the center of gravity which in forward flight towards you the forces exerted here and then the neutral point aerodynamic forces are exerted here so applying that moment arm lever that obviously has the effect of stabilizing the arrow to come right at you the greater that distance between the centre of gravity and the neutral point the more stable your plane will be in a longitudinal and lateral stability so it acting that concept to a wing just the wing itself you can imagine the centre of gravity being 25% rear of the leading edge and the neutral point let's just say it's 50% rear on the cord well that's only an inch and a half difference it's a very short moment arm therefore it takes a great deal of force to keep such a wing stable now though there are flying slab wings they can be very delicate to balance because the neutral neutral point is so close to the centre of gravity it must be balanced perfectly and have nicely tuned control surfaces but what are some things we can do to bring the neutral point further rear of the centre of gravity for increased stability one thing that can be done is to sweep the wings so this place is more aerodynamic surface rear of the centre of gravity and it does shift the center of gravity back somewhat but most wings and tailless planes locate most of the electronics batteries of worth further upfront so this sweep has the effect of sending the neutral point further err of the center of gravity it also has the added effect of placing the control surfaces for the rear of the center of gravity and the center of gravity of this plane happens to be right here and so that you can see that the effect of tail moment arm that's the lever over which these elephant's act is about four inches rear of the center of gravity at the root and about 7 inches rear at the tip if we were to extend the center of gravity all the way out the mean aerodynamic cord of a swept but non tapered wing like this as you'll see illustrated on one of the calculators I'll link to below is approximately here for this wing just accounting for the left wing here you'll see that this triangle for the forward wing and this triangle for the rear wing or about symmetrical to one another therefore their aerodynamic forces against those two parts is balanced at this location so the approximate aerodynamic Center because in this case it has no tail it's equivalent to the center of pressure as well is located about four or five inches rear of the center of gravity of this wing the more traditional approach to shifting the neutral point rear and further rear of the center of gravity is to add vertical and horizontal stabilizers or a V tail or other means to provide a greater aerodynamic surface exerting its force over a longer lever far rear of the center of gravity so each of these has in the case of the rudder lateral a moment arm and in the case of the horizontal stabilizer an elevator vertical moment arm acting about the point which is the center of gravity so this plane when the momentum acting of the center of gravity is pulled forward either under power or glide there's always a resisting force of aerodynamic oncoming air pushing against those surfaces in the rear to keep the plane pointed forward and wings level flight so the main factors at play there are the size of the vertical and horizontal stabilizers and their length from the center of gravity now hypothetically you could have an infinitely long tail and an infinitely small control surfaces but of course these things weigh and the further back this is the heavier this tail moment arm so any components placed back here including the very lightest of control surfaces will exert a disproportionate amount of downward force and that alone can shift the center of gravity rear so there's usually a happy medium as to the length now as for providing larger vertical and horizontal stabilisers this can be accomplished easily during building and we often end up with a plan form that looks something like this other important considerations are the lateral surface area of your plane don't neglect the fuselage when computing your neutral point of the plane or at least anticipating the neutral point take this plane which has a very narrow tail section here although it's equipped with vertical stabilizer but a fairly broad fuselage forward of the center of gravity any lateral surface area forward of the center of gravity here effectively deducts its equivalent amount rear of that so in this plane if we have two inches of one inch tall foam board presenting its lateral surface to the aerodynamic forces that is offset by the equivalent amount even a little bit longer because this is tapered so anything from here forward is balanced out from what's in front of the center of gravity leaving only this component here and back in the vertical stabilizer to broad that lateral stability so the lesson there is just to say that if you design a plan with a very long nose with very broad sides expect to have some yaw instability unless you compensate by adding a sufficiently sized vertical stabilizer so hopefully we've got that down to a simple operation for you while you're building and at the field knowing with confidence that if your plane has sufficiently sized vertical and horizontal stabilizers a reasonably balanced profile and your aft and four fuselage that you can simply know your center of gravity is 25 to 33 percent rear of that leading edge and you can just plop your battery in do this test at the field and you'll be ready to go and safely fly a good stable airplane

Channel: Experimental Airlines

Views: 195,305

Keywords: center gravity, airplane, scratchbuild, model, rc, radio control, scratchbuilding, foamboard

Id: URglaHEt5Pk

Length: 12min 46sec (766 seconds)

Published: Tue Apr 19 2016