If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
effect of changed thrust line.
wrote Don't bother with center of mass. It's not really relevant. Indeed Angling the engine up 3.8 degrees would lead to trouble. That's a lot of angle. Absolutely Thrust works against the center of DRAG, which is much harder to locate than CG. Thanks for a verification of my thinking. -- Jim in NC |
#23
|
|||
|
|||
effect of changed thrust line.
In article ,
"Morgans" wrote: wrote Don't bother with center of mass. It's not really relevant. Indeed Angling the engine up 3.8 degrees would lead to trouble. That's a lot of angle. Absolutely Thrust works against the center of DRAG, which is much harder to locate than CG. Thanks for a verification of my thinking. Sorry. But all forces on a body act around it's centre of *mass* when considering its rotation. You can take the moments about anywhere fixed, but the CoM is what actually matters. As for angling the engine 3.8 degrees up being a bad idea, how can that possibly be? When you pull up to a high AOA, does the engine suddenly explode? No. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
#24
|
|||
|
|||
effect of changed thrust line.
In article , "Morgans"
wrote: "Alan Baker" wrote Read this. Read it again and again until you get it: Don't take moments about anything other than the centre of mass. Sorry if I don't take your word for it, so if you have some online sources, lay 'em on me. Sorry, man, but it's just basic physics. You can take them about any fixed point, but the essence of it is that a rigid body rotates about its center of mass. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
#25
|
|||
|
|||
effect of changed thrust line.
|
#26
|
|||
|
|||
effect of changed thrust line.
|
#27
|
|||
|
|||
effect of changed thrust line.
In article ,
cavelamb himself wrote: wrote: Lowering the thrust line to below the center of aerodynamic drag would cause nose up - OK I get that. Now where is the center of drag on a peg? and it will DEFINETLY change with flying attitude - ie with the flaps on, or the slats extended. I guess what it boils down to is it will not be a HUGE effect. On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is roughly 1/2 degree. One 1/8" washer at the firewall and one at the engine rubber on both sides will make 1/2 degree change if I need to do a bit od "fine" tuning. Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75" offset - guess I'll put in about .875 and see what happens I thought about this a bit last night. And a couple of thoughts seemed worth relaying. First, (and most obviously) a new mount will be needed. So build it as close as you can guess to what you'll need. Adjusting the mount at the firewall end strikes me as a bit "iffy". More that a washer or two makes for a noticeable misalignment between top and bottom bolts. When torqued down, something it GOING to give. Either the mount gets twisted or the firewall support structure does. Or both? Not if you use wedge washers... http://www.instron.us/wa/acc_catalog...ref=http://www ..google.com/search The engine end, if rubber cushioned would be a lot more compliant. Might consider all that when designing the new mount. The Corvair would use a bearer style mount, wouldn't it? Rubber pads front and rear would give quite a bit of adjustment room. I think Stealth got it right. Same side alignment and a touch more down. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
#28
|
|||
|
|||
effect of changed thrust line.
Alan Baker wrote:
In article , "Morgans" wrote: wrote Don't bother with center of mass. It's not really relevant. Indeed Angling the engine up 3.8 degrees would lead to trouble. That's a lot of angle. Absolutely Thrust works against the center of DRAG, which is much harder to locate than CG. Thanks for a verification of my thinking. Sorry. But all forces on a body act around it's centre of *mass* when considering its rotation. You can take the moments about anywhere fixed, but the CoM is what actually matters. I know you are trying to be helpful and you know that objects in free space rotate about their center of mass, but I'm fairly confident that aspects such as the center of pressure and affects on control surface authority need to be taken into account when changing the thrust line. As for angling the engine 3.8 degrees up being a bad idea, how can that possibly be? When you pull up to a high AOA, does the engine suddenly explode? No. A high angle of attack during cruise would presumably place the wings closer to their stall angle. I presume that is what makes it dangerous. |
#29
|
|||
|
|||
effect of changed thrust line.
"Alan Baker" wrote Sorry, man, but it's just basic physics. You can take them about any fixed point, but the essence of it is that a rigid body rotates about its center of mass. Yes, but I believe the problem is that you are not seeing the whole picture. For example, what forces are involved that cause an airplane to not be rotating around the center of mass while at cruise? The answer is that thrust is cancel by drag. Where those two forces are located is behind what amount of thrust differential is needed. -- Jim in NC |
#30
|
|||
|
|||
effect of changed thrust line.
In article ,
"Morgans" wrote: "Alan Baker" wrote First: yes, any *fixed* point will do, which the centre of drag is not. Second, the math is easiest when you pick the point that is actually the one about which the body will rotate. I think I get your point, about the plane rotating about the center of mass, but I do not believe that a change in the amount of thrust and its location in relation to the center of mass is what is relevant, in this issue. For a plane to not require a change in trim with a change in power, and not to rotate, the thrust line must be in line with the center of drag in level flight, no? No. Thrust, drag, lift weight are the way we normally coin the forces on an aircraft, but the reality isn't quite that. That formalism has both drag and thrust operating in the horizontal axis, but we know that the engine's thrust will only actually be horizontal at one particular angle of attack. Hence, only then will the thrust line be aligned with the drag line. That's why I keeps saying take all the moments about a fixed point to really have a clear picture. The downthrust or upthrust is added to partially negate the rotation caused due to the distance the thrust is away from the center of drag. Forget all about drag for a moment: you can boil down all the forces of lift and drag -- the aerodynamic forces -- to a single force through the aerodynamic centre of pressure. Now imagine an aircraft with the engine off and the prop feathered and in a trimmed out glide. Got that pictured in your mind? Good. Now: when you start the engine and add its thrust into the equation, what is the one direction in which you can apply that thrust and not cause the aircraft to pitch. Where must the axis of the thrust vector be? Through the centre of mass. Period. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
thrust line for engine and not mounting engine on this thrust line | tommyann | Home Built | 8 | December 15th 06 03:31 PM |
Has something changed | [email protected] | Soaring | 10 | May 3rd 05 08:34 PM |
High thrust line on canard design? | Shin Gou | Home Built | 4 | March 5th 05 03:06 AM |
Getting students to line up with the center line | BoDEAN | Piloting | 27 | April 21st 04 11:23 AM |
I want to tell you something that has changed my life! | C J Campbell | Owning | 11 | January 29th 04 11:34 PM |