Parkzone's New Habu 2 EDF - Page 8 - RC Groups
Apr 17, 2012, 11:22 PM
Registered User
It happens, but I think not in this case.

The seesaw analogy is good, because it shows that moving a person requires that person to be of less mass to maintain the same CG. In the case of the Habu the CG has in practical terms not been changed at all, only the support point has (nosewheel). Considering that the moment from the CG remains unchanged, increasing the arm means the weight on that arm is reduced.

The principle can be demonstrated by making an arm with two supports; one at the center, one an end, and the other end the fulcrum. Use a scale to check the weight at both the center and end.

Quote:
 Originally Posted by flydown Sorry but you are wrong. A good analogy is the playground seesaw. The center of the seesaw being the CG if you you have 2 people of equal weight sitting the same distance from the CG it will balance. However as soon as one person moves further back from the CG that side of the seesaw goes down thus exerting more downward pressure towards the ground. It is no different than the front wheel being further out from the CG.
Apr 17, 2012, 11:28 PM
Flying Full Size & Fun Size :)
Quote:
 Originally Posted by flydown Sorry but you are wrong. A good analogy is the playground seesaw. The center of the seesaw being the CG if you you have 2 people of equal weight sitting the same distance from the CG it will balance. However as soon as one person moves further back from the CG that side of the seesaw goes down thus exerting more downward pressure towards the ground. It is no different than the front wheel being further out from the CG.
This is true, if what you're moving is the weight. However, that isn't the case. Here, we're talking about moving the point where that mass is supported. If the distance from the CG to the nose gear is short, the nose Gear will carry more of the weight of the plane. If the distance is longer, it will carry less, and the main gear will carry more. You can try it yourself with a heavy plane (with light planes, its tough to feel the difference) or the teeter totter. With either one, apply a force halfway between the pivot point and the end to raise it. Then try the same thing, except this time apply the force at the end. You should feel a difference between the two.
J
 Apr 17, 2012, 11:29 PM Flying Full Size & Fun Size :) Fish, you obv type faster than me
 Apr 18, 2012, 12:01 AM Defender of the park sky You guys might be right on the ground but isn't it the opposite in the air? The further you move weight up the nose, that same weight makes the plane even more nose heavy? The further from CG the greater the effect? So the effect in the air is moving weight to the nose in flight by dropping the gear from the retracted position and everything dropping forward. This means stability and less tip stalling, like an arrow head, I imagine. Obviously on the ground the cg is out, it's the placement of where the back gear is in relation to where the front gear is. Looks like the mains are near the cord, pretty far up. The habu has a more classic gear position in the wings vs. coming from the fuse farther back. Bring that front gear back to far to the cord also and maybe it would be too squirrelly when trying to turn on the ground? I think instead of looking at it from an forward/aft stability issue it might be more for the side to side, making that triangle of stability bigger. Keeping the wing tips off the dirt. Could be just reading to much into it I suppose though. Perhaps just funny looking lol. Last edited by Ispintechno; Apr 18, 2012 at 12:20 PM.
 Apr 18, 2012, 10:21 PM Registered User The reason the gear retracts to the rear is obvious when looking at the manual, the strut has to be bent back for the wheel to fit into the fuselage. There would not be enough room for it if it retracted forward. Additionally it would be unusual to have the gear bent forward, I have seen f 16s this way, but no other aircraft. My Habu 32 has the gear retracting the same way and the Tamjet struts are bent back to allow the wheel to fit fully into the fuselage. I agree the gear seems to be too far forward, but it is probably because they did not want to redesign the foam.
Apr 19, 2012, 08:04 AM
Registered User
Quote:
 Originally Posted by wpotter ...Additionally it would be unusual to have the gear bent forward, I have seen f 16s this way, but no other aircraft...
You are wrong. The F-16 nosegear retracts rearward!
.

But many other military jets' nosegear retracts forward!

F-5

A-10

F-14

F-15

F-18

F-22

F-35

And the list goes on and on and...

Last edited by MartinT; Apr 19, 2012 at 08:16 AM.
 Apr 19, 2012, 10:06 AM Warbird & Jet Lover Guys, it's simple. The wheel fit better into the nose this way, along with the nosewheel steering servo. Ground handling had nothing to do with it.
Apr 19, 2012, 11:59 AM
Registered User
Quote:
 Originally Posted by Dave Eichstedt Guys, it's simple. The wheel fit better into the nose this way, along with the nosewheel steering servo. Ground handling had nothing to do with it.

And to add to the discussion of the 'see-saw' effect there are a few different things happening with a retractible nose gear and moment arms that affect aircraft performance.

First of all, has anyone tried balancing a model with the gear down and then put the gear up? Unless the gear is attached by a multi-point hinge, all the weight from the gear remains on the hinge-point, so no change to moment arms or cg. Thus, the plane will have the same cg with gear up or down and any change in the plane's flying characteristics comes from the aerodynamic forces of the gear hanging out in the wind. Counter-intuitive but true.

Next up is the effect on performance of the plane because of cg location. This one is more technical and involves more brain cells to understand the concept of a "static margin". What was mentioned upthread about ground vehicle handling being different depending on the wheelbase also applies to aircraft. This is because of the effects of forces and lever arms (called 'moments' in engineering/physics). Regardless of it being a rudder, elevator, aileron or wheel, when it is moved it generates a force that acts to change an orientation. What is interesting is that it does not always change that orientation directly about the cg, a very common misconception.

For a "perfectly" balanced car or motorcycle the force generate by steering generates a moment about the center of the wheelbase system (halfway between the wheels of a motorcycle and between the non-steering wheels of a car). and the distance between that steering center and the cg combined with the mass of the vehicle is what makes a car or motorcycle more or less easy to steer. That is right, cars do not rotate about their center, they rotate about their non-steering wheels! So a short bike is likely to steer faster than a long bike because, in addition to being lighter, there is a shorter lever arm between the center of the bike and the cg. Steering is easier if there is a lower ratio of distance between center of steering and cg divided by the mass - A low static margin is highly maneuverable.

An airplane has a different mechanism, but uses the same theory. The movement of a control surface causes a force to be applied against the center of pressure (very similar to the center of gravity which is the mathematical sum of gravity acting on all parts of the aircraft, cp is the mathematical center of all aerodynamic forces acting on the aircraft and yes the cp moves with most of the control surfaces, but that is just too complicated to discuss here. So does cg, but we ignore that for most practical purposes.). The distance between the cg and cp divided by the mass of the aircraft is referred to as the "static margin" and the greater the value the more stable (and less maneuverable) the plane is likely to be. So, a shorter aircraft is likely to have a lower static margin and be more maneuverable.
 Apr 19, 2012, 12:09 PM QuAd FaNaTiC lol Dave answered the question can we all just let it go now or is this pissing contest to continue?
Apr 19, 2012, 01:24 PM
Registered User
No--You are wrong---He meant that the gear has a forward RAKE---All he could come up with for an example was the F-16...The F-86 Sabre also has forward rake to the nose strut as well as many others...I can say this---With the nose gear strut on the Habu II in it's forward position it is IDEAL for tracking as all you need to do is to put a slight bend on the strut at the wheel to make the nose wheel "caster"...This also ensures that the nose wheel is fully into the fuselage to allow the nose gear door to close all of the way...

Kevin

Quote:
 Originally Posted by MartinT You are wrong. The F-16 nosegear retracts rearward! . But many other military jets' nosegear retracts forward! F-5 A-10 F-14 F-15 F-18 F-22 F-35 And the list goes on and on and...
 Apr 19, 2012, 03:53 PM fly and then fly some more! Dave, can you please answer if the esc is still mounted in the same place or not?
 Apr 19, 2012, 04:33 PM Warbird & Jet Lover Should be same location.
 Apr 19, 2012, 04:49 PM Trash Hauler emeritus Also: has there been anything changed to counteract the effect of the nose-retract weight on CG? Because even without retracts and using a 3300 4S battery, I have to put lead in the tail of my Habu for balance. And that's even after I carved foam to shift the battery back some. Retracts will only increase the nose-heavy problem. Unless the designed battery bay or something else has been changed to compensate.
Apr 19, 2012, 04:52 PM
Registered User
Quote:
 Originally Posted by Kevin Greene No--You are wrong---He meant that the gear has a forward RAKE---All he could come up with for an example was the F-16....
After rereading it I think you are right!
Apr 19, 2012, 04:54 PM
Registered User
Quote:
 Originally Posted by martimer Good answer! First of all, has anyone tried balancing a model with the gear down and then put the gear up? Unless the gear is attached by a multi-point hinge, all the weight from the gear remains on the hinge-point, so no change to moment arms or cg. Thus, the plane will have the same cg with gear up or down and any change in the plane's flying characteristics comes from the aerodynamic forces of the gear hanging out in the wind. Counter-intuitive but true. .
provoking thought but i donīt thing is right. Once the gear is up, there is torque to hold it up. and that torque is making the airplane turn around the cg. the effect is the same.

Remember physics force diagrams, torques had an effect independently of their position on the plane.

fun discussion, lets get back to the habu..... which by the way looks really cool in this paint scheme!!!!

love the foamy habu. The 32 is a real airplane with all the worries and limitations (i have my electra for that) but the foamy is just pure fun... love my old one and will like it better now that I know I can replace it

Jack