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wheel pants question

3stepdrop

3stepdrop

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talk to me like I'm 12 and help me understand why, when setting up your wheel pants, do I need to raise the plane to take weight off the main before getting things plumb and square? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains. Hopefully this question makes sense. Thanks in advance!
 
The angle of the wheel pants can affect your speed, & trim (Level Flight). The only way to simulate flying conditions is being essentially weightless. When you do lift your plane off the ground you will be shocked how much they move. This also goes for gear leg fairings. Think of them like little ailerons or elevators affecting the airflow around your plane. That is what was at least told to me.
 
I once bought a RV6 that at the time had the wheel pants off. Later I found out why…
Apparently he fit the pants while it was sitting on the ground- so, if installed, they would have about a 10 degree pitch down stance as well not being parallel in flight. Once I knew this, I tested how far I could throw a wheel pant across the ramp to the garbage bin.
 
3stepdrop said:
talk to me like I'm 12 and help me understand why, when setting up your wheel pants, do I need to raise the plane to take weight off the main before getting things plumb and square? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains. Hopefully this question makes sense. Thanks in advance!
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The airplane needs to be off the ground so the gear relaxes and you can orient the wheel pants and fairings to the in-flight airstream for maximum performance.
 
3stepdrop said:
talk to me like I'm 12 and help me understand why, when setting up your wheel pants, do I need to raise the plane to take weight off the main before getting things plumb and square? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains. Hopefully this question makes sense. Thanks in advance!
Click to expand...
I don’t recall lifting the plane to fit the wheel pants. Only to install the landing gear. The later installation of the wheels and brakes will dictate the positioning of the plate which the pants attach to. As bt3 noted the orientation of the pants if misaligned could influence flying characteristics. IIRC I put the pants on both sides while ensuring the aft edges were even at 6.5” above ground. Also I put a cushion around the upper half of the tire to maintain separation. I then eyeballed the angle between pant centerline vs. tire centerline. It was very close and I only had to shim one section of the plate to avoid distorting the shape of the pants. I think it turned out well. Plane rolls nice and straight with never a shimmy. I was pleasantly suprised at how close the pants fit. After 14 years they get beat up.
 
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JimWoo50 said:
I don’t recall lifting the plane to fit the wheel pants. Only to install the landing gear. The later installation of the wheels and brakes will dictate the positioning of the plate which the pants attach to. As bt3 noted the orientation of the pants if misaligned could influence flying characteristics. IIRC I put the pants on both sides while ensuring the aft edges were even at 6.5” above ground. Also I put a cushion around the upper half of the tire to maintain separation. I then eyeballed the angle between pant centerline vs. tire centerline. It was very close and I only had to shim one section of the plate to avoid distorting the shape of the pants. I think it turned out well. Plane rolls nice and straight with never a shimmy. I was pleasantly suprised at how close the pants fit. After 14 years they get beat up.
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Mis aligned wheel pants will rarely produce shimmy. Their purpose is speed. So the question is, what is your cruise speed? Is it at or close to Vans' numbers? And, going from no wheel/gear leg fairings to all wheel and leg fairings, did the ball change position?
 
3stepdrop said:
? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains.
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that would make sense if you cared about drag while on the ground. Given that we care about drag while in the air, you want to align everything with the wheels as they will be in the air.
 
BobTurner said:
Mis aligned wheel pants will rarely produce shimmy. Their purpose is speed. So the question is, what is your cruise speed? Is it at or close to Vans' numbers? And, going from no wheel/gear leg fairings to all wheel and leg fairings, did the ball change position?
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I mention shimmy to show the gear legs were installed accurately which helped with the fitting of the pants. I think misaligning the legs would cause shimmy. My cruise speed at 2400 rpm is 155kts. I have to throttle back to stay out of the yellow arc. I have never flown without the fairings. The ball is very close to being centered and I can center it perfectly by resting my foot against the right rudder pedal.
 
3stepdrop said:
talk to me like I'm 12 and help me understand why, when setting up your wheel pants, do I need to raise the plane to take weight off the main before getting things plumb and square? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains. Hopefully this question makes sense. Thanks in advance!
Click to expand...
The proper Dad response for a 12 year-old is, "Because I said so." ;)

I think the others have explained why, and that's mainly to eliminate the flex and potential misalignment in the gear when the weight is on it. Without rehashing all all of that, it's in your best interest to align the fairings properly as Van's has outlined. Other than figuring out the method to get the plane safely off the ground and stable, it's really not a big deal. As I recall, it took more time figuring out how to do it than it did to actually do the alignment. You might get lucky and get everything square with it on the ground, but why risk all the potential rework down the road. I did my RV-7 as outlined in the procedures and it flies level with the ball centered, feet flat on the floor.
 
3stepdrop said:
talk to me like I'm 12 and help me understand why, when setting up your wheel pants, do I need to raise the plane to take weight off the main before getting things plumb and square? seems like to me the better idea would be to have the bottom of the wheel pants parallel to the ground while sitting on the mains as opposed to parallel to the ground while the weight is off the mains. Hopefully this question makes sense. Thanks in advance!
Click to expand...

If you plan to fly with the wheels on the ground, do it that way. If you plan to fly the plane, in the air, the way the manufacturer expected you to, do it according to the directions.

Or get those fancy carbon fiber things that dont need the plane jacked up to install!
 
Do you recall your primary training when they taught you about the 4 (actually 5) forces involved in flight?

-- Thrust, Weight/Gravity, Lift, Drag, (& Money)

The alignment of the wheel pants and fairings affect DRAG. Generally, we want the in-flight regime to be the most efficient, therefore we align the wheel pants and fairings in a configuration and attitude that most closely resembles in flight conditions; aligned with the slip stream and relaxed/unloaded,

Less Drag means less Thrust is required to achieve a particular speed and altitude, and less Thrust means less Money needs to be spent on gas and maintenance -- Win Win.
 
When you lift the plane to fit wheel pants (yes, you have to...) check your tires. Ours got way out of round (not sure if from sitting?) and caused a main gear shimmy. I thought it was the nose gear until someone saw me land when the shimmy happened (it was not all the time) and they said the left main was hopping up and down.
 
bjdecker said:
Do you recall your primary training when they taught you about the 4 (actually 5) forces involved in flight?

-- Thrust, Weight/Gravity, Lift, Drag, (& Money)

The alignment of the wheel pants and fairings affect DRAG. Generally, we want the in-flight regime to be the most efficient, therefore we align the wheel pants and fairings in a configuration and attitude that most closely resembles in flight conditions; aligned with the slip stream and relaxed/unloaded,

Less Drag means less Thrust is required to achieve a particular speed and altitude, and less Thrust means less Money needs to be spent on gas and maintenance -- Win Win.
Click to expand...
100% understand and agree with what you said. The lingering question is this: as long as the longitudinal axis of the main pants is true, will the rotation of the pants matter? And does anybody have a picture of the wheel pants at ground level from the rear? Not being arguments, just want to understand.
 
3stepdrop said:
100% understand and agree with what you said. The lingering question is this: as long as the longitudinal axis of the main pants is true, will the rotation of the pants matter? And does anybody have a picture of the wheel pants at ground level from the rear? Not being arguments, just want to understand.
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The gear legs don’t bend laterally as they take up the load of the airplane on the ground. They bend aft and out. On taildraggers it’s a lot. Nose draggers not as much.
So if the wheels track straight (act a bit of toe in when weight on wheels) when on the ground, they move down and inwards as the weight comes off them - and therefore the plane of the tire twists significantly away from the aircraft centerline.
So you’ll have meaningful drag if you align to the weight on wheels alignment.
 
3stepdrop said:
100% understand and agree with what you said. The lingering question is this: as long as the longitudinal axis of the main pants is true, will the rotation of the pants matter? And does anybody have a picture of the wheel pants at ground level from the rear? Not being arguments, just want to understand.
Click to expand...

The Whitman Gear style gear tend to bend aft and out when loaded vs the straight legged ones (outboard). The fore/aft movement of the gear leg and axle moves the front/rear of the wheel pant up/down relative to the longitudinal axis of the wheel pant. Because the wheel pant is symmetrical about the longitudinal axis, displacement from that axis will create a different drag profile (not the correct term I'm sure -- the wetted area is going to change as the shape moves position relative to the slip stream).

So if I had an 6A, 7A, 8A, 9A, 10 or a TW 6,7,9 or 14 I would unload the gear to match the optimum "in flight" attitude... (A TW 8, 14A use flat slab gear legs that only displace in/out from the centerline when unloaded/loaded.)
 
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