Wood Gear Placement Analysis
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Wood Gear Placement Analysis
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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.
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