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 |
#11
|
|||
|
|||
effect of changed thrust line.
In article
, Alan Baker wrote: In article , Stealth Pilot wrote: On Thu, 13 Nov 2008 22:20:25 -0500, wrote: How does a person determine what the proper height of an engine should be when building an airplane? If a particular engine design mandates the prop is 4 inches, say, lower than where it would be with the engine originally installed, what effect will it have on handling, and what changes in downthrust might be advised? We are building a Pegazair, and my Corvair engine would need to have the cowl higher than ideal to keep the crank centerline at the same hight as say, an O200. Weight wize, the engines are just about identical as equipped Have not determined the center of gravity of the engine yet, to determine the overall length of the mount. For those unfamiliar with the plane it is a highwing STOL 2 placer roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan, ) suck it and see. your elevator should have enough authority to control the resulting couple. you should experience some upthrust but tweaking back the throttle should control it. your question in the first line.... take the centre of mass as the pivot point. you have 4 force couples. lift vs moment arm. elevator down thrust vs moment arm wing drag vs moment arm thrust vs moment arm. they will summate to zero in equilibrium but you probably wont know any of the values o those forces. alan baker will probably give a guestimate. suck it and see. I think you'll need the same side thrust but a little more down thrust as the cessna 150.. Stealth Pilot The math isn't all that hard. Assuming the designed thrust line goes relatively close to the centre of mass, then if you want to keep the torque created by thrust close to the same, you need to change the angle of the engine by arctan(h/l), where h is the amount you're moving the engine up or down and l is the distance between the propellor and the centre of mass. I suck at ASCII art, but: -----T' (new thrust line) ^ | h | | -----T--(old thrust line)--------------------------------C l (CoM) h/l is equal to the tangent of the angle TCT' If the thrust line is not aligned with the CoM to begin with, then situation isn't quite as simple, but for small misalignments the effect is small and for larger misalignments the overall change in torques is smaller in comparison. Draw few diagrams of the situation and you'll see what I mean. I'm not even going to try to draw that situation here. Basically, if the thrust line was already above the CoM and you move it up, then the change is smaller than the arctangent of h/l and if it was below the CoM the change is a little greater than the arctangent. Did a quick little check: As an example, a Cessna 150 is about 25 feet long and from looking at wikipedia's little jpeg, the centre of mass should be about 5 feet behind the propellor disc. So if you raise the thrust line 4 inches, you need to angle the engine up an additional 3.8 degrees; arctan(4/60). -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg |
#12
|
|||
|
|||
effect of changed thrust line.
On Nov 14, 8:25 am, Alan Baker wrote:
Did a quick little check: As an example, a Cessna 150 is about 25 feet long and from looking at wikipedia's little jpeg, the centre of mass should be about 5 feet behind the propellor disc. So if you raise the thrust line 4 inches, you need to angle the engine up an additional 3.8 degrees; arctan(4/60). Don't bother with center of mass. It's not really relevant. Angling the engine up 3.8 degrees would lead to trouble. That's a lot of angle. Most engines are aligned with the longitudinal axis or parallel to it (the waterline) or angled *down* a bit (Ercoupe has lots; Cherokee and its brethren have some, 172 has none at all) and some are angled to the side a bit as well to control P-factor. Thrust works against the center of DRAG, which is much harder to locate than CG. Lowering the thrust line would tend to raise the nose more on powering up, which would require more nose-down trim to control, which would lead to a bigger drop in attitude when the power is removed. But I don't think four inches lower is going to be a big deal. The loss of ground clearance, OTOH, is significant for a STOL airplane. Dan |
#13
|
|||
|
|||
effect of changed thrust line.
|
#14
|
|||
|
|||
effect of changed thrust line.
"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. -- Jim in NC |
#15
|
|||
|
|||
effect of changed thrust line.
On Thu, 13 Nov 2008 23:28:09 -0500, "Morgans"
wrote: wrote For those unfamiliar with the plane it is a highwing STOL 2 placer roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan, Good to see you around, again. :-) Howz the project going? Did you have a web page or blog with your project on it? I lost everything with a total computer melt-down a while back. Project is at www.pegazair.on-the-net.ca/ClareSquared |
#16
|
|||
|
|||
effect of changed thrust line.
|
#17
|
|||
|
|||
effect of changed thrust line.
On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
wrote: wrote: On Nov 14, 8:25 am, Alan Baker wrote: Did a quick little check: As an example, a Cessna 150 is about 25 feet long and from looking at wikipedia's little jpeg, the centre of mass should be about 5 feet behind the propellor disc. So if you raise the thrust line 4 inches, you need to angle the engine up an additional 3.8 degrees; arctan(4/60). Don't bother with center of mass. It's not really relevant. Angling the engine up 3.8 degrees would lead to trouble. That's a lot of angle. Most engines are aligned with the longitudinal axis or parallel to it (the waterline) or angled *down* a bit (Ercoupe has lots; Cherokee and its brethren have some, 172 has none at all) and some are angled to the side a bit as well to control P-factor. Thrust works against the center of DRAG, which is much harder to locate than CG. Lowering the thrust line would tend to raise the nose more on powering up, which would require more nose-down trim to control, which would lead to a bigger drop in attitude when the power is removed. But I don't think four inches lower is going to be a big deal. The loss of ground clearance, OTOH, is significant for a STOL airplane. Dan And angling an engine UP is a real BAD (tm) idea. 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 |
#18
|
|||
|
|||
effect of changed thrust line.
|
#19
|
|||
|
|||
effect of changed thrust line.
"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? 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. -- Jim in NC |
#20
|
|||
|
|||
effect of changed thrust line.
wrote Project is at www.pegazair.on-the-net.ca/ClareSquared Tanks! I'll go have a gander! Or a goose! g -- Jim in NC |
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 |