Balancing like the big boys...
03-10-2015, 06:13 AM
#126
That 160g (plus the hot glue) was based entirely on a visual inspection. That is, I propped the model up on the cones at the 124mm mark and added weight to the tail until it was level. Once I bring back the more precise HF digital scale I will weigh everything again, including getting a more precise total weight. What I hope to find is that the calculated numbers correspond. If they don't, then I will trust the visual inspection and just go fly the thing.
Haven't tried this either. I've done a lot of playing around with variables like this on RealFlight. Yeah, I know, I know. But the one thing is shows you is that you can go pretty extreme in the nose-heavy direction and still fly the thing. But go just a bit too far rearwards and it suddenly becomes a nightmare.
Rafael
03-10-2015, 07:12 AM
#127
Funny you should suggest this. Last Sunday at the field I did indeed look at the CG on the smaller model. As far as I can judge eyeballing the two models they are the same. The smaller one is balanced at the recommended CG which looks very close to the 28% MAC numbers using on the larger P-56. It flies very well at this CG.
I haven't tried this. How might I expect a nose-heavy model to fly?
As long as you don't go to extremes, you won't notice much of a change. The plane will probably seem a little less sensitive to elevator input. If you can spin the model now, it may not be so willing to spin with the nose weight added. If you put too much weight in the nose you may have trouble with nosing over on landing. The tail may come off the ground a little sooner on takeoff.
Haven't tried this either. I've done a lot of playing around with variables like this on RealFlight. Yeah, I know, I know. But the one thing is shows you is that you can go pretty extreme in the nose-heavy direction and still fly the thing. But go just a bit too far rearwards and it suddenly becomes a nightmare.
I haven't tried this. How might I expect a nose-heavy model to fly?
As long as you don't go to extremes, you won't notice much of a change. The plane will probably seem a little less sensitive to elevator input. If you can spin the model now, it may not be so willing to spin with the nose weight added. If you put too much weight in the nose you may have trouble with nosing over on landing. The tail may come off the ground a little sooner on takeoff.
Haven't tried this either. I've done a lot of playing around with variables like this on RealFlight. Yeah, I know, I know. But the one thing is shows you is that you can go pretty extreme in the nose-heavy direction and still fly the thing. But go just a bit too far rearwards and it suddenly becomes a nightmare.
I don't know anything about the details of RealFlight, but doubt that it has the mechanization to truly represent subtle changes in CG. But the fact that you see changes in the simulated behavior when you adjust the CG suggests that the designers at least attempted to include CG effects. In the Aft CG direction the model has the ability to achieve higher angles of attack. That's where nonlinearities in aerodynamics will show up and even multi million dollar computer simulations are hard pressed to be accurate in that regime.
Dick
03-10-2015, 09:17 AM
#128
otrcman's idea is good. Balance is something that can be established in a small model and should translate to the larger one, assuming same proportions. The CG depends on the neutral point, and that will stay the same proportionally regardless of scale. So you can play around with the CG on the smaller model and what works well on it should work well on the larger one.
The balance point has to be ahead of the neutral point in order for a plane to be stable in pitch. As you move it back closer to the neutral point the following things happen: 1. the plane will have a flatter glide because it won't require as much down force on the stab, i.e., you will find you can adjust the trim down more; 2. the trim will not change as much between low power and high power because there will be less difference in angle between the wing and the stab/elevator; 3. the plane will not recover automatically from a dive, but instead, once put in a dive and elevator neutralized, it will keep right on going till you pull out or it hits the ground; 4. elevator will be more responsive, may become too touchy especially on landing; you can reach a point where elevator control doesn't feel "solid" anymore, which is the main reason people often keep the balance point forward.. The problem with shifting the balance point back comes if you get too close to the neutral point, or actually behind it. Then the plane becomes unstable in pitch, i.e., it will diverge from control inputs, so it will tend to porpoise, which usually ends badly.
As you shift the balance point forward, the tendencies above reverse, but there is no clear line that you cross as there is when you shift backwards and cross the neutral point. This fits with your experience on the sim. You can reach a point with the balance too far forward where it comes impossible to flare out on landing unless you dive to build up speed first and bleed it off close to the ground. Because you have to carry more up trim, the plane will tend to climb too much when you advance the throttle, assuming it is trimmed first for level cruise. Conversely, it will fall quickly when you retard the throttle.
This was a little rambling; hope it makes some sense.
Jim
The balance point has to be ahead of the neutral point in order for a plane to be stable in pitch. As you move it back closer to the neutral point the following things happen: 1. the plane will have a flatter glide because it won't require as much down force on the stab, i.e., you will find you can adjust the trim down more; 2. the trim will not change as much between low power and high power because there will be less difference in angle between the wing and the stab/elevator; 3. the plane will not recover automatically from a dive, but instead, once put in a dive and elevator neutralized, it will keep right on going till you pull out or it hits the ground; 4. elevator will be more responsive, may become too touchy especially on landing; you can reach a point where elevator control doesn't feel "solid" anymore, which is the main reason people often keep the balance point forward.. The problem with shifting the balance point back comes if you get too close to the neutral point, or actually behind it. Then the plane becomes unstable in pitch, i.e., it will diverge from control inputs, so it will tend to porpoise, which usually ends badly.
As you shift the balance point forward, the tendencies above reverse, but there is no clear line that you cross as there is when you shift backwards and cross the neutral point. This fits with your experience on the sim. You can reach a point with the balance too far forward where it comes impossible to flare out on landing unless you dive to build up speed first and bleed it off close to the ground. Because you have to carry more up trim, the plane will tend to climb too much when you advance the throttle, assuming it is trimmed first for level cruise. Conversely, it will fall quickly when you retard the throttle.
This was a little rambling; hope it makes some sense.
Jim
03-10-2015, 09:45 AM
#129
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For example - we have an aft fuel tank that holds 2250 lbs of fuel - this places the CG further back, requiring less downforce from the horizontal stabiliser - (this results in less drag)
Fuel schedule is to burn this aft fuel in the last 90 minutes of a flight. If we used it during the first 60 minutes it would reduce range & endurance by around 500 miles and 1 hour.
Last edited by Rob2160; 03-10-2015 at 10:01 AM.
03-25-2015, 07:51 AM
#130
Thread Starter
12-07-2016, 01:47 PM
#133
I also have a question for you fella's using this method. I purchased three identical digital scales and this method works great. However, i am finishing up a Ziroli Hellcat. Per the plans the plane is to be balanced with the gear in the up/retracted position and the plans indicate that location. (The Hellcat uses a retract that rotates rearward). Many other planes have this same type of retract design. Is there a calculation or a way to determine where the CG should be in the down/forward position? If not it makes it virtually impossible to use this method unless i am missing something.
12-07-2016, 02:27 PM
#134
OSG,
Yes, you can account for the change in CG due to gear retraction on your Hellcat. But you need to know the weight of the landing gear and the CG of the movable part of the gear. Just weighing and checking CG of one gear leg/wheel will be sufficient. It sounds complicated, but once you do it you will realize it's quite simple.
Dick
Procedure goes like this:
1. Weigh one gear leg and wheel (all the movable parts, not the fixed portion of the retract unit).
2. Lay the movable part of the gear out in the retracted position and check the CG of that assembly. You don't have to be too exact, as the gear retraction is a small percentage of the total airplane.
3. Measure the distance from the pivot point of the gear to the balance point.
4. Multiply the distance (from item 3.) by the weight (from item 1.). That's the "Moment" due to a single gear retraction.
5. Double the answer found in item 4. That's the Moment Change due to both main gear retracting.
6. Now, divide the Moment Change (item 5.) by the weight of the airplane. Your answer is the number of inches that the CG has moved due to gear retraction. It'll probably be a fraction of an inch.
7. Add the CG Change (item 6.) to the Gear Down CG and you now have the Gear Up CG.
8. You can do the same thing for a retractable tailwheel, but the effect is probably negligible. If it's a nose gear airplane, you need to account for that in the same way that you did for the main gear. Just remember that you may need to add or subtract, depending on whether the gear retracts forward or rearward. My example on Steps 1 - 7 assumes a rearward retracting main gear as on the Hellcat.
Yes, you can account for the change in CG due to gear retraction on your Hellcat. But you need to know the weight of the landing gear and the CG of the movable part of the gear. Just weighing and checking CG of one gear leg/wheel will be sufficient. It sounds complicated, but once you do it you will realize it's quite simple.
Dick
Procedure goes like this:
1. Weigh one gear leg and wheel (all the movable parts, not the fixed portion of the retract unit).
2. Lay the movable part of the gear out in the retracted position and check the CG of that assembly. You don't have to be too exact, as the gear retraction is a small percentage of the total airplane.
3. Measure the distance from the pivot point of the gear to the balance point.
4. Multiply the distance (from item 3.) by the weight (from item 1.). That's the "Moment" due to a single gear retraction.
5. Double the answer found in item 4. That's the Moment Change due to both main gear retracting.
6. Now, divide the Moment Change (item 5.) by the weight of the airplane. Your answer is the number of inches that the CG has moved due to gear retraction. It'll probably be a fraction of an inch.
7. Add the CG Change (item 6.) to the Gear Down CG and you now have the Gear Up CG.
8. You can do the same thing for a retractable tailwheel, but the effect is probably negligible. If it's a nose gear airplane, you need to account for that in the same way that you did for the main gear. Just remember that you may need to add or subtract, depending on whether the gear retracts forward or rearward. My example on Steps 1 - 7 assumes a rearward retracting main gear as on the Hellcat.
Last edited by otrcman; 12-07-2016 at 02:56 PM. Reason: spelling
12-07-2016, 04:39 PM
#135
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Go to YouTube and look at "Balancing Your RC Airplane. A complete explanation of the process, including calculations. I just purchased some scales off eBay and plan to give the process a try myself.
12-08-2016, 09:58 AM
#136
gear is an easy point to weigh. You can build a jack to hold the plane up on the scale. Big planes usually have two different W/B calculation sheets. One on gear and one on jacks. Position is not as critical as correct measurement.
12-09-2016, 11:53 AM
#138
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I would use round-nosed balsa blocks that are longer than the gear struts under each wing. The radius of the "point" would be determined by the strength of the wood in the area - the sharper the better so you can accurately measure the point of contact for the moment arm measurement. Place these blocks on your scales and zero them out (tare weight). Then place one scale/block combo under the wings where they would be clear of the gear-swing path (take care to make sure they are at the exact same axial location). This way you can still do the moment-weight method and swing your gear to check CG in both conditions.
12-16-2016, 11:08 AM
#140
Mate...Plug in wing models are a walk in the park. Wings off, mark the CG position on the fuselage/wing root section the wings plug into, drill a small hole. Two pieces of piano wire with 90 deg bend at one end and something to hold onto at the other. Slot in the wings, position the piano wire hangers in each hole, push the wings home. Stand at the nose facing the aircraft and pick it up with the wires. People have done very big aircraft this way for years.
12-17-2016, 06:30 AM
#142
Steve... Judging by the length of this thread, neither have the majority.
If you're model has a wing tube or aluminium joining spars like the old Byrons, there a 99% chance that the front one will, due the the max aerofoil thickness, be mysteriously on or very close to the GC;-) Just check the plans. I just tie some fishing line around the tube a lift it up. Decent nose down attitude will indicate a balance point at the front of the tube, tail down, the opposite. 30 seconds from start to finish will give you a very accurate starting point. Then fly and adjust as required.
If your going spars are away from the GC, drill a hole as mentioned previously.
The model referred to by the tread starter will have an extensive CG range, no need for even the above method, just finger it.
If you're model has a wing tube or aluminium joining spars like the old Byrons, there a 99% chance that the front one will, due the the max aerofoil thickness, be mysteriously on or very close to the GC;-) Just check the plans. I just tie some fishing line around the tube a lift it up. Decent nose down attitude will indicate a balance point at the front of the tube, tail down, the opposite. 30 seconds from start to finish will give you a very accurate starting point. Then fly and adjust as required.
If your going spars are away from the GC, drill a hole as mentioned previously.
The model referred to by the tread starter will have an extensive CG range, no need for even the above method, just finger it.
