melsman
12-28-2006, 04:07 PM
First, take a very sharp knife and insert between fat layers...
Just kidding.
Anyway, I thought I'd pass along some of my experiences on trimming a tandem rotor aircraft. It's trickier than one might at first think because you're not only trimming each head for balanced flight, but you also have to make sure they're trimmed to each other.
My discussion will focus on the Hirobo tandem, but I'm sure it can be applied to all tandems.
First, start with level swash plates. When I say "level" I really mean "perpendicular to main shaft". This is a very important step and it needs to be very carefully done. Set the collective pitch so that each head is the same. (Yes - I know that on the Hirobo, one head will more than likely have more pitch once all is done, but start equally and work from there.)
In a hover, the heads want to fly level in respect to each other. The heads don't care what kind of angle the fuselage is hanging at; nor at what incidence the fuse sits on the ground. That's why, with the Hirobo, you must first pull aft to lift the nose off of the ground (to level the heads),and then relax to neutral cyclic while pulling up into a hover.
Once lifted into a hover, chances are the nose will want to come back or go forward and will require quite a bit of forward or aft stick to hold in a stabile hover. This initial sensitivity is corrected using collective pitch adjustments. If the nose comes back, increase the aft head col. pitch by one turn on the blade ptich links and decrease the forward head by an equal amount. Then try it again. When you get close, you can try adjusting just one of the heads.
At some point, the nose forward/aft tendency will be reduced to the point where changing the col. pitch adds too much change and it only requires a slight amount of forward or aft stick to trim. At this point, adjust using forward or aft swashplate trim. It should only take a little to get the aircraft trimmed properly. Remember - the swash plate is CCPM and thus multiple links may have to be changed so as to not effect the trim in other axes.
Somewhere in there, you'll probably notice a tendency to roll to one side or the other. This is trimmed by making the same swash link adjustments to both heads on the same side. Again, because the swash is CCPM, be sure to make symetrical adjustments; if you lower one side of the swash, raise the other or else you'll induce a slight forward tilt to the swash... which could be handy if you need a slight amount of forward cyclic trim.
Trimming for yaw (rudder) is a bit more tricky because you have to be very perceptive where the pivot point is. Try to get the other axes trimmed first. Then, lift the nose off while allowing the back wheels to stay on the deck. Does the nose twist to one side or the other? If so, set it back down and adjust the front swash until it lifts cleanly up into the air without veering to one side. Then, lift the aircraft completely into a hover and watch for any additional yaw to occur. If it does, then the aft head needs the adjustment.
Another way to do this is to lift the aircraft completely, and watch the yaw pivot point. If it pivots around the middle, adjust both heads. If it pivots around one of the heads, adjust the other head.
Yep - the process takes time and decent hovering skills. I can easily burn through a couple of tanks just trying to get the bird trimmed.
And one other thing to consider; when you remove the heads to put the body on, the heads may not seat back on the shafts as before and the trim settings will change, causing some unexpected flying follies on the next hover. Double check those heads! (And those head bolts!! trust me on that one...)
Ashley
Just kidding.
Anyway, I thought I'd pass along some of my experiences on trimming a tandem rotor aircraft. It's trickier than one might at first think because you're not only trimming each head for balanced flight, but you also have to make sure they're trimmed to each other.
My discussion will focus on the Hirobo tandem, but I'm sure it can be applied to all tandems.
First, start with level swash plates. When I say "level" I really mean "perpendicular to main shaft". This is a very important step and it needs to be very carefully done. Set the collective pitch so that each head is the same. (Yes - I know that on the Hirobo, one head will more than likely have more pitch once all is done, but start equally and work from there.)
In a hover, the heads want to fly level in respect to each other. The heads don't care what kind of angle the fuselage is hanging at; nor at what incidence the fuse sits on the ground. That's why, with the Hirobo, you must first pull aft to lift the nose off of the ground (to level the heads),and then relax to neutral cyclic while pulling up into a hover.
Once lifted into a hover, chances are the nose will want to come back or go forward and will require quite a bit of forward or aft stick to hold in a stabile hover. This initial sensitivity is corrected using collective pitch adjustments. If the nose comes back, increase the aft head col. pitch by one turn on the blade ptich links and decrease the forward head by an equal amount. Then try it again. When you get close, you can try adjusting just one of the heads.
At some point, the nose forward/aft tendency will be reduced to the point where changing the col. pitch adds too much change and it only requires a slight amount of forward or aft stick to trim. At this point, adjust using forward or aft swashplate trim. It should only take a little to get the aircraft trimmed properly. Remember - the swash plate is CCPM and thus multiple links may have to be changed so as to not effect the trim in other axes.
Somewhere in there, you'll probably notice a tendency to roll to one side or the other. This is trimmed by making the same swash link adjustments to both heads on the same side. Again, because the swash is CCPM, be sure to make symetrical adjustments; if you lower one side of the swash, raise the other or else you'll induce a slight forward tilt to the swash... which could be handy if you need a slight amount of forward cyclic trim.
Trimming for yaw (rudder) is a bit more tricky because you have to be very perceptive where the pivot point is. Try to get the other axes trimmed first. Then, lift the nose off while allowing the back wheels to stay on the deck. Does the nose twist to one side or the other? If so, set it back down and adjust the front swash until it lifts cleanly up into the air without veering to one side. Then, lift the aircraft completely into a hover and watch for any additional yaw to occur. If it does, then the aft head needs the adjustment.
Another way to do this is to lift the aircraft completely, and watch the yaw pivot point. If it pivots around the middle, adjust both heads. If it pivots around one of the heads, adjust the other head.
Yep - the process takes time and decent hovering skills. I can easily burn through a couple of tanks just trying to get the bird trimmed.
And one other thing to consider; when you remove the heads to put the body on, the heads may not seat back on the shafts as before and the trim settings will change, causing some unexpected flying follies on the next hover. Double check those heads! (And those head bolts!! trust me on that one...)
Ashley