Building YELLOW F-18 Twin
12-18-2002, 12:38 AM
#28
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Building YELLOW F-18 Twin
Originally posted by YellowAircraft
Hi,
At the risk of evoking the wrath of some of you guys who're more experienced with this bird than me, I'm gonna have to disagree with some of you. You should not statically balance the stabs on the shaft pivot! Here's why: In the case of the Hornet (single AND twin) stabs, the center of gravity and the aerodynamic center of pressure are not at the same point on the stab. Theoretically, these stabilators are designed for optimal performance at a given speed. At that speed, there should be as little force acting on the pivot as possible. There is a moment created by the wieght of the stabilator behind the pivot, and another moment created by the aerodynamic pressure in flight. These two moments counter each other and cancel out most of the force acting on your servo. The way it was explained to me is that it's a case of stable equilibrium vs. unstable equilibrium. With the stabilator NOT statically balanced at the pivot point, it is a 'stable equilibrium' situation in flight (meaning that if your servo arm were to pop off, the stab wouldn't suddenly turn 90 degrees into the wind).
Now, again, I am no engineer, and the information I have about this stuff, I had explained to me. I didn't learn it at Cal-Tech or MIT or the Air Force Academy, so I'm not saying 'I'm right, and Joe Blow is wrong'. I do know, however, that I've logged many flights on the single-engine Hornet, (whose stabs are the same only smaller), including a few turbine flights. I've also flown the F-16, whose stabs are not statically balanced, on turbine power at a very high rate of speed, without the slightest hint of flutter.
I welcome hearing what those of you who know about this stuff have to say.
Hi,
At the risk of evoking the wrath of some of you guys who're more experienced with this bird than me, I'm gonna have to disagree with some of you. You should not statically balance the stabs on the shaft pivot! Here's why: In the case of the Hornet (single AND twin) stabs, the center of gravity and the aerodynamic center of pressure are not at the same point on the stab. Theoretically, these stabilators are designed for optimal performance at a given speed. At that speed, there should be as little force acting on the pivot as possible. There is a moment created by the wieght of the stabilator behind the pivot, and another moment created by the aerodynamic pressure in flight. These two moments counter each other and cancel out most of the force acting on your servo. The way it was explained to me is that it's a case of stable equilibrium vs. unstable equilibrium. With the stabilator NOT statically balanced at the pivot point, it is a 'stable equilibrium' situation in flight (meaning that if your servo arm were to pop off, the stab wouldn't suddenly turn 90 degrees into the wind).
Now, again, I am no engineer, and the information I have about this stuff, I had explained to me. I didn't learn it at Cal-Tech or MIT or the Air Force Academy, so I'm not saying 'I'm right, and Joe Blow is wrong'. I do know, however, that I've logged many flights on the single-engine Hornet, (whose stabs are the same only smaller), including a few turbine flights. I've also flown the F-16, whose stabs are not statically balanced, on turbine power at a very high rate of speed, without the slightest hint of flutter.
I welcome hearing what those of you who know about this stuff have to say.
Besides for every oz you put in those stabs it takes about 3-4 more in the nose!
12-18-2002, 06:17 AM
#29
Senior Member
When you're ready...
Now if you really want to stop the show...
get ahold of these suckras.
Whaddup Mr. Shaun?
When you headin'is way? Dude and me are out (OH) from 23-30.
get ahold of these suckras.
Whaddup Mr. Shaun?
When you headin'is way? Dude and me are out (OH) from 23-30.
12-18-2002, 04:54 PM
#30
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Building YELLOW F-18 Twin
FWIW, I believe that the big AMT F-18 with many successfully flight has a statically balanced Stab. Tim Redelman called his friend Tommy Wood who flew the twin F-18 at Superman last year and flew it at TopGun 2002 and came 4th in the expert class. He recommends balancing the stabs.
I've successfully flown Yellow F-18 single for many seasons without balancing the stabs but I decided to balance the stabs on my new twin F-18 based on expert advice from people that have successfully flown this airplane .
I've successfully flown Yellow F-18 single for many seasons without balancing the stabs but I decided to balance the stabs on my new twin F-18 based on expert advice from people that have successfully flown this airplane .
12-18-2002, 08:10 PM
#31
Building YELLOW F-18 Twin
So, are you saying that Tommy Woods' Hornet has statically balanced stabs? I'd be shocked to hear that he does.
Like I was saying, if it's not good to do it, you wouldn't necessarily know right away. If you're using a mega-servo, then the servo will keep the gremlins away [for how long?]. Really, the question becomes, did the experts with experience with this airplane turn around and balance the stabs because something was wrong and they were doing it in response to that? Or are they doing it because someone thought it was a good idea? I mean, what symptom were they treating with the balancing of the stabs? How did the problem they were addressing manifest itself?
I was hoping to see some input from other aeronautical engineers who might be able to shed some light. The aeronautical engineer who designed those stabs says "NO WAY, you're inviting disaster", but I know he doesn't necessarily have a corner on the information market. What do you other guys think?
Like I was saying, if it's not good to do it, you wouldn't necessarily know right away. If you're using a mega-servo, then the servo will keep the gremlins away [for how long?]. Really, the question becomes, did the experts with experience with this airplane turn around and balance the stabs because something was wrong and they were doing it in response to that? Or are they doing it because someone thought it was a good idea? I mean, what symptom were they treating with the balancing of the stabs? How did the problem they were addressing manifest itself?
I was hoping to see some input from other aeronautical engineers who might be able to shed some light. The aeronautical engineer who designed those stabs says "NO WAY, you're inviting disaster", but I know he doesn't necessarily have a corner on the information market. What do you other guys think?
12-18-2002, 09:19 PM
#32
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Balance Stabs ? ? ? ?
Hi All,
I have flown my F/A-18 at many world comps and haven't ballanced my stabs. To date it has been flown at The JWM 2001, TopGun 2001, the British Nationals and the Scottish Nationals. I would agree with Yellow Aircraft. I use a 9204 servo running on 6 volts on each elevator half and it flys GREAT !!!!
I have flown my F/A-18 at many world comps and haven't ballanced my stabs. To date it has been flown at The JWM 2001, TopGun 2001, the British Nationals and the Scottish Nationals. I would agree with Yellow Aircraft. I use a 9204 servo running on 6 volts on each elevator half and it flys GREAT !!!!
12-18-2002, 11:56 PM
#37
Building YELLOW F-18 Twin
Originally posted by sideshow
Crashmaster where can you find that gear?
Crashmaster where can you find that gear?
12-19-2002, 07:18 AM
#40
Senior Member
Building YELLOW F-18 Twin
Yeah...
They aint' cheap. Each set is scratched and requires about 100 - 140 hours to produce and this is not my profession. Never really intended on doing more than one set. If there is a demand, I may be molding many of the parts, which would reduce building time and final cost. But for now, think of them more as jewelry than landing gear. Definately for the ignant scale consumed eccentric who would have nothing less and will go to extremes to make sure of it. I would love nothing more than to provide these at an extremely affordable price. Nothing would please me more than to see every single Yellow twin sportin' em'. Anyone who knows me will tell you that it's not about the money. The F/A-18 gear in the picture were done as a favor for a friend. I'm about to start on another set real soon. I'll keep you posted.
They aint' cheap. Each set is scratched and requires about 100 - 140 hours to produce and this is not my profession. Never really intended on doing more than one set. If there is a demand, I may be molding many of the parts, which would reduce building time and final cost. But for now, think of them more as jewelry than landing gear. Definately for the ignant scale consumed eccentric who would have nothing less and will go to extremes to make sure of it. I would love nothing more than to provide these at an extremely affordable price. Nothing would please me more than to see every single Yellow twin sportin' em'. Anyone who knows me will tell you that it's not about the money. The F/A-18 gear in the picture were done as a favor for a friend. I'm about to start on another set real soon. I'll keep you posted.
12-20-2002, 05:47 PM
#44
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F18
What mounting modifications did you do for a single turbine set up? Here is a pic that was emailed to me of one way to mount the turbine. What other modifications have you guys done? And lets see some installation pics.
thanks David
thanks David
12-20-2002, 09:11 PM
#45
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f18
Do not want to suggest something that is not appropriate, however comapring the full scall F-18 to the model is a bit of a stretch. With regards to the stabs that is. I do belive that balancing them will allow one to use a lower output servo. Without balancing and with turbine performance and speed i belive you should use atleast 200 oz's on each side.
Food for thought.
JB
Food for thought.
JB
12-20-2002, 10:03 PM
#46
Building YELLOW F-18 Twin
Diceman,
Thank you for your input. I was beginning to think nobody else wanted to chime in on that subject. I'm very interested to hear from people what they think about that.
You said you think it's a stretch to make a comparison between the model and the full-size job. Why? True, there are enough differences that you couldn't necessarily say that what one does, so must the other, but don't they use models in tests to design and build the big ones? Specifically, if the model's stabs were similar enough to the full-scale's (i.e.; location of the pivot, relative size and shape of the surface, airfoil shape, etc.), wouldn't a comparison of their characteristics in flight be valid? I'm not suggesting that they are so similar, but if they were....
While I don't know the reasons why the horizontal stabs on most modern jets with full-flying stabs droop down toward the trailing edge when the plane isn't 'on', I notice that they do. Some of these planes have horizontal stabs that are actuated by electric servos, so wouldn't they also want to have as little strain on the servo as possible? In that sense, I think a comparison between the model and the full-scale is valid.
Anyway, I'm still hoping to get a chime-in from some of you engineer-types or even the many full-scale jet pilots out there. I'd like to get to the bottom of this!
Thank you for your input. I was beginning to think nobody else wanted to chime in on that subject. I'm very interested to hear from people what they think about that.
You said you think it's a stretch to make a comparison between the model and the full-size job. Why? True, there are enough differences that you couldn't necessarily say that what one does, so must the other, but don't they use models in tests to design and build the big ones? Specifically, if the model's stabs were similar enough to the full-scale's (i.e.; location of the pivot, relative size and shape of the surface, airfoil shape, etc.), wouldn't a comparison of their characteristics in flight be valid? I'm not suggesting that they are so similar, but if they were....
While I don't know the reasons why the horizontal stabs on most modern jets with full-flying stabs droop down toward the trailing edge when the plane isn't 'on', I notice that they do. Some of these planes have horizontal stabs that are actuated by electric servos, so wouldn't they also want to have as little strain on the servo as possible? In that sense, I think a comparison between the model and the full-scale is valid.
Anyway, I'm still hoping to get a chime-in from some of you engineer-types or even the many full-scale jet pilots out there. I'd like to get to the bottom of this!
12-20-2002, 10:33 PM
#47
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I believe all the control surfaces on most of the modern jet fighters are computer controlled. And, they have very very powerfull actuators on these surfaces. Much greater than anything we use on models (with respect to scale).
JB
JB
12-21-2002, 12:11 AM
#49
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We don't really know where the full-scale stabs balance. It is reasonable to believe that the bearing and operating system on the full scale bird is so good that even a minor amount of mass would cause the stabs to sag. Just as mine would with the paint mass behind the pivot point being the difference. We also don't know if the model has the same stab pivot point as the full scale ship.
Again, as I mentioned the full scale ship has computer controlled stabs and extremly powerfull actuators.
JB
Again, as I mentioned the full scale ship has computer controlled stabs and extremly powerfull actuators.
JB
12-21-2002, 12:34 AM
#50
I'm no engineer, I just play one at work.
1.) THe full size do indeed use basically "irreversible" actuators that effectively do not have slop or play when compared to ours, even adjusting for scale
2.) Supersonic flight moves the aerodynamic center back to about 50% MAC. THe full size guys have to take that into account.
I am no expert, but I think the static balancing is to help prevent a quick displacement from producing a torquing moment. IE if the CG of the stab is on the pivot, then a rapid displacement of the pivot (from a gust or vibration, whatever) will not tend to place a rotating force (or moment) on the stab. This is good for the servo, and prevents some sort of oscillation I would suspect.
My guess is that the reason some consider this bad, is that the balancing tends to increase the polar moment of inertia of the stab, and can cause oscillation with certain servos, again with bad effects.
The balancing of the stab's aerodynamic center to the stab pivot is very similar in concept to the placement of the plane's CG with respect to the plane's aerodynamic center in that is provides stability.
2.) Supersonic flight moves the aerodynamic center back to about 50% MAC. THe full size guys have to take that into account.
I am no expert, but I think the static balancing is to help prevent a quick displacement from producing a torquing moment. IE if the CG of the stab is on the pivot, then a rapid displacement of the pivot (from a gust or vibration, whatever) will not tend to place a rotating force (or moment) on the stab. This is good for the servo, and prevents some sort of oscillation I would suspect.
My guess is that the reason some consider this bad, is that the balancing tends to increase the polar moment of inertia of the stab, and can cause oscillation with certain servos, again with bad effects.
The balancing of the stab's aerodynamic center to the stab pivot is very similar in concept to the placement of the plane's CG with respect to the plane's aerodynamic center in that is provides stability.