Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

CG / Landing gear relationship

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Old 01-25-2010, 06:46 AM
  #1
F86_SABRE
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Default CG / Landing gear relationship

Hi,

I would like to enquire whether there is any form of calculator that caters for the optimum landing gear position in relation with the aircraft's CG.
At the moment I am designing a Pilatus PC-21 and would like to properly locate the best location for its landing gear. As you may be aware the PC-21 landing gear is of the L type (as I call it) as the wheel shaft is rearward to the main strut. In my case, the wheel's shaft centre is about 2.5" (circa 63mm) behind the centre of the vertical main strut. Consequently, should the main strut be on the CG since the main wheel is rearward by the quoted amount?

Any assistance will be appreciated.

With thanks

Reuben
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Old 01-25-2010, 07:57 AM
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Default RE: CG / Landing gear relationship

Bottom line is................

Where the rubber meets the road (runway) is the important location.

Figure the angles from where the CG actually is as seen from the side to where the tire contact points are. How those footprints get to where they are doesn't matter much at all.
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Old 01-25-2010, 08:06 AM
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Default RE: CG / Landing gear relationship

Here is a design layout that's used by lots and lots of people. It's a scan from Andy Lennon's Basics of R/C Model Aircraft Design.

Notice that Leon shows the CG where it actually is, not where it's vertical axis bisects the wing. You can really screw up if you assume it's located within the wing.
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Old 01-25-2010, 10:18 AM
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Default RE: CG / Landing gear relationship

Lots of other setups also used
for muddy fields a more forward gear setup may save nosing over
BUT for best arrangement on smooth hard fields the plane takes of more easily and is more apt to land and stay down better - if the gear is close to vertical CG location (first sketch) just like airfoil ideas -
all a compromise for a particular setup

No one design fits all.
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Old 01-25-2010, 10:54 AM
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Default RE: CG / Landing gear relationship


Quote:
ORIGINAL: F86_SABRE
Hi,
I would like to enquire whether there is any form of calculator that caters for the optimum landing gear position in relation with the aircraft's CG.
At the moment I am designing a Pilatus PC-21 and would like to properly locate the best location for its landing gear. As you may be aware the PC-21 landing gear is of the L type (as I call it) as the wheel shaft is rearward to the main strut. In my case, the wheel's shaft centre is about 2.5'' (circa 63mm) behind the centre of the vertical main strut. Consequently, should the main strut be on the CG since the main wheel is rearward by the quoted amount?
Any assistance will be appreciated.
With thanks
Reuben
I do not know about the PC-21. I have observed a few Pilatus aircraft so from that and your description, I take it the landing gear is a tricycle gear. Now since you seem to be willing to have the mainstrut located for active flying rather than exact scale, here are some suggestions from many years experience.

TRI-Gear: The wheel axle should be behind the wing center of gravity (actually the aircraft CG as per da Rock) by a small portion of the chord. If you get it really back, like half way between the CG and the TE of the wing, you will have difficulty during the rotation part of the take-off. That will cause the take-off to be like a model that sets nose-low. It will take too much speed to rotate and when the wing goes from negative lift to positive lift Angle-of-Attack the model just leaps into the air - not good - better is a slow rotation to a lift-off at good flying speed.

Tail-Dragger Gear: The best choice for the wheel axle is just about the leading edge of the wing or very slightly forward. Way back many years ago, modelers wanted the gear well forward. Two things: 1. The airplane flies off too slow (high AOA) because the speed is not yet sufficient to lift the tail up. Generally that causes the proverbial "tip-stall". 2. The model hits a slight bump (especially on landing) and all that weight behind the wheel axle-line rolls over, i.e. a nose over. Kind of like landing with the brakes applied - not good!
Finally modelers realized that a more aft gear position was far better and both take-offs and landings smoothed out.

No scientific formulas here , just simple "That Works."
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Old 01-25-2010, 01:46 PM
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Default RE: CG / Landing gear relationship

Hi guys,
Thanks for the input. Very interesting. Please excuse my ommission as I failed to mention that the plane has actually a tricycle type landing gear setup (sorry Horrace!). Indeed DA ROCK's quotation seems to be VERY relevant in my opinion "Where the rubber meets the road (runway) is the important location" and judging by what has been stated in the drawing its should not be more than 5% aft of CG.

Thanks for your input!!

Reuben
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Old 01-27-2010, 09:30 AM
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Default RE: CG / Landing gear relationship

A picture is worth a thousand words. I think the term you were looking for is Trailing Link Landing Gear. I'm sorry I can't help you with placement.
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Old 01-28-2010, 10:59 AM
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Default RE: CG / Landing gear relationship

used to overfly malta in f-86d on the way to tripoli. 1958-1960.
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Old 01-28-2010, 12:04 PM
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Default RE: CG / Landing gear relationship

Indeed!! Some of the F-86's used to land here....was still very young then You were based at Wheelus Airbase? I believe our Aviation Museum have some fotos of the F-86's in Malta...my friend used to work on them too.

Reuben
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Old 01-28-2010, 01:11 PM
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Default RE: CG / Landing gear relationship

Reuben,

Just a clarification:


Quote:
ORIGINAL: F86_SABRE
''Where the rubber meets the road (runway) is the important location'' and judging by what has been stated in the drawing its should not be more than 5% aft of CG.
That 5% is measured with the tail touching the ground.
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Old 01-28-2010, 03:16 PM
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Default RE: CG / Landing gear relationship

Hi!
thanks for the information.... I thought that the plane had to be straight and level. Guess I thought wrong. What if in the case (as is the case) of a tricycle landing gear? Would that statement still apply too?

Meanwhile, I added some more stuff to the drawing...have the formers added.

Reuben
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Old 01-28-2010, 05:16 PM
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Default RE: CG / Landing gear relationship

Yes, Reuben, that is the case for the tricycle landing gear.

The theory supporting this is that, at landing time, when the tank is empty, the CG is in the closest to the tail position.
Then the position of the back wheels should avoid that the plane could end up in a nose up attitude, with the tail on the ground and unable to return to level by itself (due to a gust or extreme flaring).

This could happen only if the vertical line coming down from the CG touches the ground behind the wheels for that attitude.
The 0.05MAC distance is a safety factor that places the wheels beyond that critical point.

On the other hand, if that distance between the wheels and the CG is made too big, that will make the take-off rotation of the model around the back wheels too hard on the elevator, when the fuel tank is full and the CG is located toward the nose.

Note that is important to have the horizontal-vertical location of the CG.

Now, for normal horizontal attitude, it is recommended to make the nose to point down about 3 degree, in order to avoid any bouncing tendency during landing.
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Old 01-29-2010, 12:10 PM
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Default RE: CG / Landing gear relationship

Thanks! Very informative....will keep this in mind
Thanks again

Reuben
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Old 01-29-2010, 12:32 PM
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Default RE: CG / Landing gear relationship

You are very welcome.

That is just to complement the schematic of Post #3 above.
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Old 01-29-2010, 03:20 PM
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Default RE: CG / Landing gear relationship

Yes I have saved both pictures. Whilst at it, I noticed that you mentioned a particular book by Andy Leon. My guess is that the correct author's name is Andy Lennon. I am saying this because re-reading your post joggled my memory as I know I had a book with such a title by that author but it is likely that I might have either lent it or given it away. Must make a search!

Thanks

Reuben
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Old 01-29-2010, 06:03 PM
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Default RE: CG / Landing gear relationship

Yes, Andy Lennon, Basics of R/C Model Aircraft Design.
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