need help cg and motor thrust
10-10-2010, 09:07 AM
#1
Thread Starter
Join Date: Dec 2005
Location: fort worth,
TX
Posts: 469
Likes: 0
Received 0 Likes
on
0 Posts
need help cg and motor thrust
A buddy of mine has the Pilot Sbach 30cc with the DLE 30 rear carb and I think he is turning a 18x6 prop. This plane tooks awesome, seems to fly well enough but it lands like crap.
The CG is at the center of the wing tube and he has added 5oz of weight to the top of the motor box and both batteries are in front of the tank.
When I say it lands like crap this is what I mean, as he lines up for his approach and starts to drop the throttle the nose starts to balloon up, I am talking like 35 to 40 degrees up. If he brings it in hot for landing its better but when he touches down the plane is still at a good flying speed.
Yesterday after a couple of flights we noticed his up lines pulled to the left so he decided to add 1 degree of right thrust, now the upline are straight and true. On the next flight he added down thrust of 1 degree to the motor, level flight look good, inverted flight caused the nose to drop and drop well. Even before we changed the down thrust of the motor inverted flight did the same thing just not as severe. Now on landings with this new down thrust it basically settled in for a perfect 3 point landing.
Is it time to take out the 5oz of nose weight?
He was told a week or so ago that while in inverted flight the nose should stay level or climb slightly, so if this is a true statement then he is still nose heavy? right?
The CG is at the center of the wing tube and he has added 5oz of weight to the top of the motor box and both batteries are in front of the tank.
When I say it lands like crap this is what I mean, as he lines up for his approach and starts to drop the throttle the nose starts to balloon up, I am talking like 35 to 40 degrees up. If he brings it in hot for landing its better but when he touches down the plane is still at a good flying speed.
Yesterday after a couple of flights we noticed his up lines pulled to the left so he decided to add 1 degree of right thrust, now the upline are straight and true. On the next flight he added down thrust of 1 degree to the motor, level flight look good, inverted flight caused the nose to drop and drop well. Even before we changed the down thrust of the motor inverted flight did the same thing just not as severe. Now on landings with this new down thrust it basically settled in for a perfect 3 point landing.
Is it time to take out the 5oz of nose weight?
He was told a week or so ago that while in inverted flight the nose should stay level or climb slightly, so if this is a true statement then he is still nose heavy? right?
10-10-2010, 12:09 PM
#2
RE: need help cg and motor thrust
That is an awesome looking plane, Wingspam.[sm=thumbs_up.gif]
It seems to me that the plane has too much down thrust angle, against which the trimming of the elevator is fighting during powered flight.
When the power goes to iddle, the downwards component of the thrust force (and the associated nose down torque around the CG) disappears.
Then, the down force of the up trimmed elevator prevails and produces that pitch up just before landing.
This type of planes don't need much down thrust, if any, since they are neutral on purpose (0-0-0 settings).
The designer has located things in a way that thrust and drag vectors cross the vertical location of the CG, so changes in thrust force or speed create zero pitch reactions (opposite to what a trainer does).
I believe that what your friend was told a week or so ago is not correct.
There are several ways to verify the correct location of the CG; however, the sweet spot is to be found by your friend with little relocations of the CG (~1/8" at a time) and flight testing.
Check this trimming process:
http://nsrca.us/all/flying/177-trimchart.html
ORIGINAL: Wingspam
...........as he lines up for his approach and starts to drop the throttle the nose starts to balloon up, I am talking like 35 to 40 degrees up.
Is it time to take out the 5oz of nose weight?
He was told a week or so ago that while in inverted flight the nose should stay level or climb slightly, so if this is a true statement then he is still nose heavy? right?
...........as he lines up for his approach and starts to drop the throttle the nose starts to balloon up, I am talking like 35 to 40 degrees up.
Is it time to take out the 5oz of nose weight?
He was told a week or so ago that while in inverted flight the nose should stay level or climb slightly, so if this is a true statement then he is still nose heavy? right?
When the power goes to iddle, the downwards component of the thrust force (and the associated nose down torque around the CG) disappears.
Then, the down force of the up trimmed elevator prevails and produces that pitch up just before landing.
This type of planes don't need much down thrust, if any, since they are neutral on purpose (0-0-0 settings).
The designer has located things in a way that thrust and drag vectors cross the vertical location of the CG, so changes in thrust force or speed create zero pitch reactions (opposite to what a trainer does).
I believe that what your friend was told a week or so ago is not correct.
There are several ways to verify the correct location of the CG; however, the sweet spot is to be found by your friend with little relocations of the CG (~1/8" at a time) and flight testing.
Check this trimming process:
http://nsrca.us/all/flying/177-trimchart.html
10-10-2010, 12:42 PM
#3
RE: need help cg and motor thrust
First off, that is a wicked lookin' plane!
Ok, ballooning after motor off is a sign of too much down thrust... normally, but after
reading the rest of your thread though it seems you're miles off with your CG theory.
If you want a super smooth flying aerobatic model, F3A style ( I'll use that is it's a great bench mark)... then contary to popular belief, your CG needs to be set forward.
The simple test for this is from level flight, pull up to a 45deg line, roll inverted and let go of the sticks. The model should drop into a in easy curve... down wards!
Another voodoo miss conception is the ol' 0-0-0 set-up... no such thing.
The standard set up in F3A is: wing +0.5, stab 0, engine: right 3, down 1. All of those will be tweeked according to prop choice, CG location etc.
If you're climbing during inverted the CG is way to far back. It's good for a blast... we've all done it... but it has far more negatives than positives, as your friend has discovered.
I'd start by (first removing any mixes you had in) checking your motor angles, wing, stab incidencies.. Start moving the GC forward so you get a nice curved drop during inverted. then do the right thrust to get a good LONG vertical hands off... then work from there.
Should cure the landing problem.
10-10-2010, 12:58 PM
#4
RE: need help cg and motor thrust
There are trim charts available - if you do a google search or an RCU search. There is a method to checking and adjusting all this, and it's really supposed to be done in a certain order. If you add nose weight, change your thrust, then decide you want a little less nose weight, your thrust and other things will become out of alignment.
Down thrust can be checked by the way the plane responds to a sudden chop in throttle - if you are flying at cruising speed, cut the throttle to idle, and the plane climbs - you have too much down thrust, and vice versa.
The plane pulling to the left during a climb can be a result of the right thrust, however it's effect will differ at various speeds. Another solution which accounts for this is a throttle to rudder mix.
For CG, which should be fixed BEFORE you do anything else to the plane, you can pull to a 45 degree climb and roll inverted; plane is still climbing at 45 degrees. When you let go of the elevator, plane should just slightly start to drop it's nose and level off. For 3D, it should hold a solid line, and if you want it super tail-heavy to 'cheat' your prop-hanging, then the nose should slightly climb.
Down thrust can be checked by the way the plane responds to a sudden chop in throttle - if you are flying at cruising speed, cut the throttle to idle, and the plane climbs - you have too much down thrust, and vice versa.
The plane pulling to the left during a climb can be a result of the right thrust, however it's effect will differ at various speeds. Another solution which accounts for this is a throttle to rudder mix.
For CG, which should be fixed BEFORE you do anything else to the plane, you can pull to a 45 degree climb and roll inverted; plane is still climbing at 45 degrees. When you let go of the elevator, plane should just slightly start to drop it's nose and level off. For 3D, it should hold a solid line, and if you want it super tail-heavy to 'cheat' your prop-hanging, then the nose should slightly climb.
