Wood Gear Placement Analysis

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Axel placement Updated 2013: The axel placement advice by Chris Bobka (below) has been on my web page for a long time but may not be the answer for Pietenpols with brakes. Here is why, there was an excellent article by Ryan Mueller and William Wynne published in the January 2011 Brodhead Pietenpol newsletter. It wasn't the focus of their article but in reference to correct landing gear placement in the fourth paragraph of the article it says, "The late model plans specify that the axle should only be 0.5" behind the leading edge".  I knew this is not on the plans so it made be dig even deeper into my archives where I found an 8.5"x11" piece of paper I received with my plans set but forgot I had it. This sheet shows the weight and balance for a "1966 Pietenpol Air Camper Powered with a 110-66 Corvair Engine".  Right there is says "D is 0.5 inches back of weighing point" and D is the distance from the main wheel to the datum (leading edge). The fuselage measurement and vintage suggest this is a long fuselage.

Also, when I received my plans, I received a 6 page document entitled “Converting the Corvair Engine”. On page 1 Mr. Pietenpol lists the empty weight of the airplane using this modified Corvair engine at 622 pounds, which is the same weight as listed on the weight and balance sheet mentioned above.  On page two Mr. Pietenpol list the modifications to this plane. The important ones for this discussion are:
-fuselage lengthened 9 inches (the genesis of the LONG fuselage)  
-Wings slanted back 3 inches
-wheels moved forward 7 inches.

Using a little math, if the split axel landing gear legs were built per the plans the axel would be 19 inches from the long fuselage firewall (17 inches for the standard plans fuselage plus 2 inches more for the LONG fuselage extension of the first bay).  Then moving the axel forward 7 inches would put it at 19-7= 12 inches back from the firewall.  The wing was at 7.5 inches on the standard fuselage plus 2 inches (extension of the first bay) puts the wing at 9.5 inches behind the firewall plus the 3 inch slant puts the wing at 12.5 inches from the firewall.  This brings us back to the 0.5 inch measurement but in this case its axel in front of the wing.  This modified airplane Mr. Pietenpol speaks of must be the same as the "1966 Pietenpol Air Camper Powered with a 110-66 Corvair Engine" airplane shown on the weight and balance sheet mentioned above. However, Mr. Pietenpol goes on to say on page 2 that 7 inches was too much.  He recommends splitting the difference which would mean the axel should be at 12+3.5=15.5 inches behind the firewall or 3 inches behind the leading edge of this aircrafts wing. 

More background research on landing gear placement shows:

-The 1933-34 "Improved Air Camper" plans, no brakes: show the axel to be 17 inches behind the firewall and the wing 7.5 inches behind the firewall. This puts the axel 9.5 inches behind the wings leading edge.

-The LONG fuselage adds 2 inches to the first bay so if using the landing gear from the 1933-34 plans the axel would be at 19 inches behind the firewall and the wing should be at 7.5+2 or 9.5 inches from the firewall. 

-The fuselage drawing with the sample weight and balance shown on the supplemental plan sheet (also showing the tube fuselage) appears to be a 1933-34 plans fuselage but with brakes. Here the axle is shown at 16.5 inches behind the firewall and 5.25 inches behind the leading edge of the wing.

-There is no advice for the axel placement on the supplemental plans for the LONG fuselage without brakes.  Chris Bobka did some research (see below) and guesswork to figure it should be at 21 inches behind the firewall with no brakes. He did the analysis to help Greg and Dale with their LONG fuselage Pietenpol.  Chris reported during flight tests that the placement was right on.  Also, he reports that Frank Pavliga started with his axle at 17 inches but moved it back when adding the C-65 to his long fuselage (no brakes) wings back 2 inches. 

- The 4th Quarter 1984 Brodhead Pietenpol newsletter quotes Mr. Pietenpol as saying, the plane is OK with the gear as far forward as it is now, with a note saying he was talking about the last two Corvair powered ships. But unfortunately it does not say where the axel is. 

Lastly, in an article about landing gear design published in Sport Aviation by Ladislao Pazmany, he states "The main gear should contact the ground at least 15 degrees ahead of the most forward center of gravity with the aircraft in level attitude." This center of gravity is the CG of the plane and on a parasol plane it is somewhere below the wing.  Unfortunately, I don't know where this point is on a Pietenpol. Some have suggested it is about the center of the instrument panel but that is just a guess. As an aside I also found in "Aeronautical Engineering and Airplane Design" published in 1918, the landing gear should be at 13 degrees 10 minutes. It also assumes the CG is the same height as the propeller

Reported on the Pietenpol discussion list:

Chuck Ganzer , NX770CG, has a short fuselage, with the axel at 17 aft of the firewall, wing back 3.5 inches from vertical. If you have ever seen him stand on the brakes and spin, you would not think the wheels are to far back.

Dick Navratil, I re- measured mine today to confirm. My short fuse has axle 19" back from FW and axle is 3.5" aft of LE. The new plane has long fuse has axle 21" back and wing isn't mounted yet. My short fuse has a CG with min fuel and me at 19.05 and flies perfect. You are on the right track. dick N.

Don Emch, NX899DE has long fuselage (1966 model), Wing is slanted 4" aft of plans, A-65 engine mount is 1" longer than plans (just a little extra without losing 'the look', Axle is 1" forward of steel gear plans.

Dick N.I built the long fuselage with an A65 (mount extended about 1 3/4" to anticipate my bodily weight of 215") And had to move the wing back 3 inches. I used the split gear plans supplied. Flies fine

 

 

 

Wood Landing Gear Placement Analysis

By Chris Bobka (from the Pietenpol List)

 

Rick,

The following is related to your issue and I hope becomes a classic posting that

is referenced by many for years to come. I wrote it 6.5 years ago almost to the

day in response to a question posed by Greg Cardinal. It was about 5 hours

of research and headscratching. Of course, it will prompt some controversy.

This was supposed to have been published in MacLaren's BPA newsletter. I sent

it to him but he never came out with the next edition.

 

It appears that the 1932 fuselage has the wood gear axle about 13.5 inches aft

of the firewall. The 1933 Improved Air Camper has the axle 17 inches aft of the

firewall. The weight and balance sheet I have from Don Pietenpol shows the

axle on the "1937 Air Camper with Corvair engine" (metal split axle gear on a

163 inch fuselage) at 16.5 inches aft of the firewall. The question Dale is

asking is how far aft of the firewall should the axle be on the longEST fuselage

for which we have no help from the drawings?

 

The 1932 fuselage (Hoopman drawings and 1932 Flying and Glider Manual) is 161

inches long. The 1933 Improved Air Camper fuselage is 163 inches long. The

longEST fuselage is 172.375 inches long. This is the one I understand you

have built.

 

It appears that the intersection of the first truss verticals with the lower longeron

on the 1932 fuselage is 8.375 or 8.5 inches aft of the firewall, depending

on which set of plans you look at. On the 1933 Improved fuselage, it is 10

inches, and on the longEST fuselage, it is 12 inches. This would mean

that the wood gear, unmodified from the 1932 plans and as mounted on the longEST

fuselage, would put the axle 12-8.5 or 3.5 inches farther aft on the longEST

fuselage than on the 1932 fuselage. It would be at 13.5 (see paragraph 1

sentence 1) + 3.5 or 17 inches aft of the firewall. Is this a good place for

it? Frank Pavliga. who did this to his longEST fuselage, said it was too far forward

at 17 inches aft of the firewall so he moved it aft when he did an engine switch

from the Model A to the A-65.

 

A better indicator of proper gear position is comparing it to the rear seat back

position in the particular fuselage since this indicates the shift aft of the

CG position as the fuselages have been stretched. The rear seat back (at the

top longeron) in the 1932 fuselage is 70.5 inches aft of the firewall. The

rear seat back in the 1933 Improved fuselage is 72.25 inches aft of the firewall.

The rear seat back of the longEST fuselage is 76.25 inches aft of the

firewall. This is a substantial shift aft in the position of the CG versus

the axle position as the fuselage is stretched.

 

Therefore, the axle on the 1932 fuselage is 70.5 -13.5 or 57 inches forward of

the rear seat back. The axle on the 1933 Improved is 72.25 -17 or 55.25 inches

forward of the rear seat back. Let us ignore the value from the 1932 fuselage

for reasons to be discussed later. Using the number from 1933 and applying

this to the longEST fuselage, we should have the axle at 76.25 - 55.25 or

21 inches aft of the firewall. Two paragraphs ago we determined that it will

actually wind up at 17 inches aft of the firewall with the wood gear, unmodified,

and Frank P. says this is too far forward. Therefore, it appears that we

need to redesign the gear so that the axle will sit farther aft in the V to the

tune of about 21 -17 or 4 inches.

 

As we noted above, if you look at the sweep of the V in the 1932 plans, you will

note that the front attach of the V is at 8.5 inches aft of the firewall. We

know that the axle is about 13.5 inches aft of the firewall. Therefore, the

sweep is 13.5 - 8.5 or 5 inches for the wood gear. Doing the same analysis for

the 1933 Improved Air Camper, we know the front attach of the V is at 10 inches

aft of the firewall and the axle is at 17 inches aft of the firewall. Therefore

the sweep is 17 -10 or 7 inches for the split axle gear. The next sentence

is important. If you put the 1932 wood gear on a 1933 Improved fuselage, you

would have an axle that will be 7 - 5 or 2 inches forward of where it would

have been if you had used the split axle gear!!!! So the gears are not necessarily

interchangeable!!! Logic says that it does not matter which style gear

you use. The axle should always be in the same relative position. I see this

as an admission by BP that the original 1932 axle was too far forward by 2 inches.

And now we know what Frank P. was talking about!!!!

 

It is obvious that BP saw fit, when designing the 1933 Improved Air Camper, that

if he lengthened the fuselage from 161 to 163 inches and moved the pilot's rear

seat back aft by 1.75 inches, then he must move the axle aft by 17 -13.5 -

2 or 1.5 inches. (Consider 2 of the 3.5 inch difference between 17 and 13.5

as a design correction and the remaining 1.5 of the 3.5 inches to be an adjustment

for the new fuselage length and movement aft of the rear seat back.) So

what would BP do if he made the fuselage 172.375 inches long (a whopping 9.375

inches longer) and moved the rear seat back aft yet another 76.25 -72.25 or 4

inches?

 

As mentioned before, I propose a redesign of the V to allow the axle to sit 4 inches

further aft of where it sits in the 1932 V. This makes sense. As just

discussed, the original sweep was 5 inches. The new proposed sweep is 5 + 4 or

9 inches. This is reasonable compared to BP's increase of sweep in the split

axle gear to 7 inches for a slightly longer fuselage with a repositioned pilot.

Adding even more length for the longEST fuselage and moving the pilot

an astounding 4 more inches aft over the 1933 fuselage and 5.75 more inches aft

over the 1932 fuselage definitely calls for an adjustment in the sweep of the

wood V.

 

As I see it, after BP's design correction is factored in, if you want to use a

wood straight axle gear in the short 161 inch long 1932 fuselage, then the axle

should be at 13.5+2 (the 2 inch correction) or 15.5 inches aft of the firewall

(the V has a 7 inch sweep). If you want to use a wood straight axle gear in

the 163 inch long Improved Air Camper fuselage, then the axle should be at the

same location as in the split axle gear, 17 inches aft of the firewall (the

V has a 7 inch sweep). If you want to use a wood straight axle gear in the 172.375

inch long longEST fuselage, then the axle should be 21 inches aft

of the firewall (the V should have a 9 inch sweep).

 

Frank P. started with 5 inch sweep and ended up with 7 inch sweep after his engine

swap and gear modification. I propose that a 9 inch sweep is optimal but you

could probably get by just fine with the 7 inch sweep. Either way, I would

not use the 1932 gear as it is on the drawings for the longEST fuselage. Does this

help?

 

I rest my case and am going to bed.

Chris Bobka

 

Followup after Greg and Dale's Pietenpol Flew

 

Here is a portion of Chris Bobka's report on the first flight of Greg and Dale's Pietenpol:

 

... was flown in five flights this morning and afternoon. Wind was 1/2 to full quartering right headwind at a steady 10 kts. Field conditions were dry grass. Location was Stanton Field, near Northfield, Minnesota. The ship has spoked motorcycle wheels and tires rolling on bronze bushings with no brakes and a tail skid. A straight axle and wrapped bungies provided suspension. The first flight was 45 minutes, second flight was about 20 minutes, third flight was about 25 minutes, fourth flight was 25 minutes and the fifth flight was 45 minutes.

 

A few years ago I wrote a long dissertation on how to select the proper axle location with the 1929 style wooden gear legs installed on the long "Corvair" fuselage (See Above). I was right on the money in the analysis because at the aft CG loading that we had, the ship would perform flawlessly on the grass. Traveling 90 degrees to the 10 knot wind, I could turn into the wind by stick aft, windward rudder, and a burst of power and I could turn away from the wind by stick forward, lee rudder, and a burst of power. I was comfortable taxiing next to buildings and other aircraft with very little practice...

 

From:  Christian Bobka

Subject: RE: Pietenpol-List: Landing Gear Placement

Date: Mon, 3 Feb 2003 17:00:28 -0600

 

Del,

The mounting points are the same. The mounting hardware is the same. The V

just changes shape as the axle is moved back. Looking at the wood gear

shown 1932 F & G Manual, the forward leg of the V is more vertical and

shorter than the aft leg of the V which is longer. As the axle moves back

the legs begin to equal out in length. This is needed on the long "corvair"

fuselage as used on sky gypsy because you moved the CG aft and you increased

vertical surface area aft.

 

There are three conditions:

 

1) low speed aircraft handling on ground with negligible total aerodynamic

effects.

2) high speed aircraft handling on the ground with considerable total

aerodynamic effects.

3) aircraft in flight. Landing gear has limited aerodynamic effects.

 

In case one, you are dealing with taxiing, speeding up for take off and

slowing down after landing. This is with little or no help from the rudder

due to slow speeds. The farther forward the gear is located on the

fuselage, the greater the distance between the CG and the gear axle. Also,

lowering the tail slides the gear even further forward. We are used to the

CG being defined as the point of rotation of all reactions but this only

applies in flight. If a wheel is touching the ground, then the wheel

contact point, which is just below the axle becomes the point of rotation

because that is where the airplane is "attached" to the ground. The greater

the distance between the CG and this ground "attachment" point, the harder

it is for the pilot to transition from ground mode to flight mode. Small

changes forward in axle placement greatly increase the tendency to ground

loop.

 

In addition, as the axle is moved forward, there is an increased side

surface area aft of the "attach" point of the aircraft with the ground.

With a forward axle, any crosswinds would cause the tail to swing right

around into the wind. Don't forget that any surface moved from in front to

behind the axle hurts you twice because forward of the axle it was helping

to counter what was behind the axle. Now it is adding to what is behind the

axle. Also, the distance from the axle to the end of the ship increases the

leverage that side surface area exerts in a crosswind. Granted, this may

increase rudder affectivity but we are dealing with a flying surface that is

trying to use air on the downwind side of the fuselage to move the fuselage

back toward the windward side. The downwind side of the fuselage is

essentially blanked out so the rudder has limited effectiveness.

In case two, a too far forward placement of the axle will cause increased

ground looping tendencies due to CG placement relative the axle and side

surface area relative the axle as noted in the previous paragraph. With

the axle too far forward, it will be virtually impossible to lift the tail

for takeoff until an abnormally high speed in ground roll is attained. A

tail wheel airplane is not designed for this. It will present a high drag

profile to the relative wind and the takeoff rolls will be too long. It

will also increase the structural loading on the tail with some pretty large

impact loads. You will be ripping the tail skid/wheel off on a regular

basis.

 

In case three, the too far forward placement of the gear would put vertical

surface area (the wheel sides) far forward of the CG. This will tend to

cancel the stabilizing effect of the vertical stabilizer. As cool as

covered spoked wheels look, this is a big reason why many that have used

spoked wheels with fabric covering have removed the fabric covering. The

two big vertical discs negated the stabilizing effects of the vertical

stabilizer.

 

In summary, the axle needs to placed as far aft as possible but with

consideration of limiting the tendency to nose over. Others posted messages

to this list indicating the degree angle that the axle needs to be forward

of the CG. This is do true but it truly needs to be understood. A picture

would really help to see what is meant. We tend to think of the CG as a

point on the wing where the ship balances. This is a valid CG location but

this is the longitudinal CG or the CG along the longitudinal axis. We need

to also consider the other axes. There is a CG about the vertical axis and

a CG about the lateral axis. Combine these three and you get a single point

about which all the mass of the aircraft is located. A guess on the

Pietenpol, with its high wing, is that it is located at or about the center

of the instrument panel. If you level the ship and then hang a plumb bob

from this point and then make the angle forward of this (someone else

mentioned it in an email a minute ago, was it 7 degrees?), your axle should

optimally lie on that line. This would be the axle location that is as far

back as possible yet far enough forward to keep you from nosing over.

I hope this helps.

 

Chris Bobka

 

 

From: Rick Holland

Subject: Re: Pietenpol-List: PhD Dissertation on axle placement for the longEST Piet fuselage

Date: Thu, 14 Jul 2005 07:24:06 -0700

 

Just one more variable in this landing gear design thing is deck angle and I

think Mike Cuy answered this one in a couple archive messages indicating

that building your gear to result in a top longeron deck angle of 12-13

degrees works well. I believe I will design to this.

 

Rick H.