If you’re wondering why you’re told to point your engine to the right,\u00a0sooner or later somebody will mention\u00a0P factor.<\/p>\n
![\"download\"](\"http:\/\/www.balsaworkbench.com\/wp-content\/uploads\/2018\/08\/download.jpg\")
<\/a>As a gardener, I would like to say that this is a beautiful pod of peas.<\/p><\/div>\n
P factor is short for propeller factor. \u00a0You can get a pretty good description of this phenomenon from Wikipedia, so\u00a0I’ll give you the short version. \u00a0Your plane is traveling forward. \u00a0You pull up on the elevator, and now your plane is traveling forward with an increased angle of attack. \u00a0As a result, the downward\u00a0traveling blade (right) is able to produce more thrust\u00a0than the upward (left), thus causing the plane to yaw to the left.<\/p>\n
Therefore we point the engine to the right to counteract P factor. \u00a0This sounds good at first, but it’s not\u00a0correct. \u00a0If right thrust counteracts the left yaw caused by P factor\u00a0when you yank on the elevator, then the plane would turn right when you level off, because the P factor goes away but the engine is still pointing to the right. \u00a0You’re supposed to counteract P factor by using your rudder.<\/p>\n
The\u00a0second most commonly misattributed\u00a0reason for right thrust is engine torque causing the plane to roll to the left. \u00a0Torque does indeed cause left roll, but it is corrected by aileron trim and you therefore don’t have to think about it. \u00a0It’s not much of a factor because there’s more torque at high throttle, which is also when there is more airspeed, which causes a simultaneous increase in the correcting effect of the wings and tail.<\/p>\n
So, why is right thrust needed? \u00a0It’s very simple. \u00a0The propeller creates a spiral slipstream that surrounds the fuselage. \u00a0Most planes have the vertical stabilizer on top of the fuselage. \u00a0The slipstream hits the fin on the left side, pushing it to the right, which causes the plane to yaw left.<\/p>\n
The engine is angled to the right to counteract this effect. \u00a0Both the problem and the solution are\u00a0proportional to the throttle setting, so they balance each other nicely.<\/p>\n But why do different planes have differing amounts of right thrust? \u00a0The two main factors are the location of the tail fin and the ratio of airspeed to slipstream strength. \u00a0High airspeed causes a straightening of the slipstream and thus less yaw, whereas low airspeed combined with lots of engine effort will have more side forces in the slipstream and create more yaw. \u00a0For examples of the two extremes, imagine a pylon racer vs a big, draggy biplane with a long, low pitch propeller. \u00a0This explains why biplane designs sometimes call for as much as 5 or 6 degrees of right thrust, whereas very fast planes typically have little right thrust or even none.<\/p>\n Down thrust:<\/span><\/strong><\/p>\n It’s tempting to think that RC planes have down thrust simply because when the throttle is advanced the nose comes up and the plane climbs too much, just\u00a0because this is one of the effects of applying power. \u00a0A properly adjusted plane will maintain a level attitude at all throttle settings, or\u00a0at least come close to this ideal, and proper adjustment is achieved by pointing the engine up, down, or level as required by the rest of the airframe. \u00a0Airfoil selection and wing location relative to the engine are the key factors determining the proper vertical orientation of the thrust line.<\/p>\n For example, a typical balsa trainer needs a lot of down thrust. \u00a0This plane has a high lift airfoil, which translates to high drag, and it’s on top of the fuselage, so the center of drag is very high in relation to the center of thrust. \u00a0This causes a rotational tendency with the center of drag as the axis. \u00a0The result is that the airplane pitches upward when the throttle is advanced, like when you push a kid on a swing. \u00a0The high angle of attack slows the plane down, and the plane will stall or at least wallow around, or if the engine is very powerful the plane will go slow and climb like crazy. \u00a0Pitching up and climbing can be mitigated by trimming the elevator down, but then you have a twitchy plane with a bunch of lift in the rear, and the nose drops when you cut the throttle and try to land. \u00a0To maintain pitch stability\u00a0at all throttle settings, the ideal thrust line theoretically should pass through\u00a0the center of drag (although this ideal is not achievable\u00a0in most designs). \u00a0The engine should be pointed down, so the imaginary line along\u00a0the propeller shaft will project aftwards pretty close to the center of drag.<\/p>\n I built a 20 size trainer a long time ago and soon discovered that I hadn’t built it with enough down thrust. \u00a0The plane nosed up at high throttle but didn’t go fast because of the high angle of attack. \u00a0If I trimmed the elevator down for speed it would drop like a rock when the throttle was lowered. \u00a0When I added floats the problem was fixed and the plane flew perfectly\u00a0with\u00a0zero pitch change from idle to full throttle, and top speed increased in spite of the extra drag from the floats. \u00a0The reason is because the floats added drag on the bottom, which brought the center of drag down closer to the thrust line projected from the back of the engine.<\/p>\n Another interesting case is\u00a0the 049-assisted two meter glider\u00a0with an engine pod\u00a0installed on top of the wing. \u00a0Because the engine is a few inches above the center of drag, it would cause\u00a0the plane to pitch down if it were installed straight ahead at zero degrees. \u00a0Engines should be\u00a0mounted on this kind of pod\u00a0with several degrees of up thrust. \u00a0Because the center of thrust\u00a0is almost directly above the center of drag, the thrust line can’t go through the center of drag. \u00a0But it’s close enough for government work, and an engine pod equipped plane will also have only a slight difference in trim between power-off operation and power-on.<\/p>\n Engine pod with up thrust. Thanks to whoever posted this photo online.<\/p><\/div>\n Finding the correct vertical thrust line\u00a0can be somewhat tricky because of variations in airfoil, wing incidence, wing placement, and airplane dimensions. \u00a0Different airfoils behave differently as the angle of attack changes. \u00a0For instance, the center of lift of a flat bottom wing moves as speed and angle of attack change. \u00a0So if you are tempted to think that a low wing plane needs the engine\u00a0straight ahead or even pointing up slightly, this isn’t necessarily the case because a flat bottom wing on the bottom of the plane might still pitch up as the throttle is increased. \u00a0Undercambered airfoils, depending on the exact profile, can be more affected by angle of attack and airspeed than flat bottom airfoils, but they also tend to produce less drag in general. \u00a0Symmetrical airfoils maintain the same center of lift at different angles of attack and therefore do not react much to changes in trim or throttle. \u00a0The main factor affecting a symmetrical airfoil is the thickness, which determines\u00a0the amount of drag produced.<\/p>\n Determining the correct vertical thrust line\u00a0is a tricky proposition, dependent upon a lot of variables, and\u00a0difficult to do on paper. \u00a0It’s best accomplished via\u00a0flight testing, which is covered in another article on this site<\/a>. \u00a0I just thought it would be nice to write this article to foster a better understanding of the cause and effect of center of drag and engine thrust angle, so if you design a plane or modify an existing one you’ll know where to start.<\/p>\n","protected":false},"excerpt":{"rendered":" There are a lot of\u00a0people out there explaining why model airplane engines are angled down and to the right. \u00a0 Unfortunately most of them are not correct. Right thrust: If you’re wondering why you’re told to point your engine to … Continue reading <\/a><\/p>\n<\/a>