Landing gear Rake

[kevin king]

Can somebody give me a refresher on how to arrive at the correct landing Gear Rake? All I remember was it's measured relative to the CG and the rake was either 15 or 20 degrees, one for grass and one for pavement??? I would like to set the rake between the grass and pavement setting. It's wing mounted gear for a 1996 Urtnowski/Adamusko Spitfire. Thanks. Kevin King.

[Dan McEntee]

  I believe 15 degrees in front of the balance point is a generally good spot and a little further forward for grass if necessary.
  Type at you later,
   Dan McEntee

[kevin king]

And how do I measure it? Draw a vertical line staight down from the CG, Then  the 2nd line? From the landing exit point on the wing or??

[FLOYD CARTER]

I've found an easier way.  Sight straight down from above the plane.  The wheel axles should be just behind the wing leading edge.

[Trostle]

And how do I measure it? Draw a vertical line staight down from the CG, Then  the 2nd line? From the landing exit point on the wing or??

Your plans should show you were the balance point should be.  For a conventional landing gear, if the plans do not do this, then make an estimate where the the balance point should be about 25% of the MAC.  (If no flaps, maybe slightly forward of that approaching 20% MAC.)  (Using the average chord is close enough unless the wing has a lot of taper.)  Then, to have some degree of accuracy, make an estimate about where the vertical CG is along this balance point.  It will not necessarily be in the middle of the wing unless the wing is on or near the CL of the model.  Yes, draw a vertical line down from this point.  Then, use a protractor to measure 15^o in front of that.  That will show where the wheels should touch the ground.  The axle should be directly above that point.  Then, the LG leg goes from there to the gear mount, wherever that may be.

That 15^o parameter in front of the vertical CG is pretty much a time proven factor.  Bill Netzeband mentioned this in several of his articles and columns over the years.  It even works for full scale aircraft with a conventional gear. 

Keith

[Dwayne Donnelly]

Good post, can I dig a bit deeper and ask why? I've always just eyeballed it, is there a reason you want to be precise?

[PJ Rowland]

The more precise the better in my opinion.

Its also dependent on wheel diameter.  A 1.75" wheel can accept a forward rake of up to 18deg, whereas a 2.25" wheel requires the lower 15deg, however 15.5 can also work.

The other consideration to make is the "frontal undercart number"  of ( F.U.N ) for short, is the rake inward . This angle  has been 90deg down to 60deg, there is still much conjecture surrounding the optimum angles of all these factors.

[Trostle]

Good post, can I dig a bit deeper and ask why? I've always just eyeballed it, is there a reason you want to be precise?

With the LG 15^o in front of the CG, it is easier to get smoother landings and takeoffs.  Even smoother if the LG is slightly less than that 15^o but takes practice and a soft touch at the handle, not so good if greater than 15^o.  Takeoffs and landings are maneuvers that should get or get close to that 40 point maneuver score, regardless of skill category.

Keith

[kevin king]

Good post, can I dig a bit deeper and ask why? I've always just eyeballed it, is there a reason you want to be precise?

Many reasons Dwayne. I will just list a few, I want to minimize the
 chances of the plane nosing over, and the paint on the cowl getting scuffed,  or dirt getting in the engine. The other is I don't want to make a set of gear for pavement and a set of gear for grass, thus  the decision to go in between the two. I would much rather do this in my shop where it's nice and cool and I have everything I need to accurately make any adjustments. Much better than doing it at the field. Last but not least is seeing the plane greasing every landing. Kevin.

[BillLee]

What Keith said........


Or,...

Balance the model on the wheels. Bend the gear forward or backward until the model is balanced with a 15 degree nose-down attitude.

[Dave_Trible]

I've found an easier way.  Sight straight down from above the plane.  The wheel axles should be just behind the wing leading edge.

I've found this simple way to be adequate in most cases.  I fly almost entirely off lumpy grass.  Having the axle right under the wing leading edge seems to solve any nose over issues for full size flapped stunt airplanes.  Smaller, lighter models will need them further forward in the grass to prevent (most) flipovers.  If you fly on pavement the location can be a little behind the leading edge for smoothest departures and arrivals. 

Dave

[Chris Wilson]

Bamm, end thread.


Motorman

Not really when the original question asked for precise angle measured in degrees, and then Keith shows workings for the answer.
It kinda ended then.
Chris.