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RV-8 Tire Pressure

jdhmotor

Member
Hi All,
I have been on the site for awhile getting all the information I could on a RV-8. I just sold my Pitts and purchased an 8 and was looking for the correct tire pressures for the mains. I did not get a answer from the previous owner. I search the forum but could not find it. If I missed it, I am sorry to bother anyone with silly questions. Thanks.
 
I'm running 34 at present. Not sure there is a "right answer". I asked my local RV8 guru and he told me what he uses....so I copied him....again.
 
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50 psi on my Goodyear Flight Custom II and Michelin Air Stop tubes too. If I recall correctly, under pressure on airplane tires is a bad thing due to flexing. I'm sorry, I can't remember where the article is, but it might be a Goodyear publication. Anybody else know?
 
I found that the rolling resistance, and hence takeoff distance, was significantly increased when experimenting with dropping from my normal 50 to 35 lbs (on my -4). I didn't notice it at first when flying by myself; however, I took a passenger and bags and was surprised at the increased length of the takeoff roll -- well beyond what I expected based purely on the increased weight. Actually, I found afterwards that my tire pressure had dropped a few pounds so I was in the low 30's. If you push your plane by hand on the ground with the tires at 50 and then again at 30 I think you'll find the increased rolling resistance very noticeable - I did. And once you're attuned to the issue you may also notice you're using more power to taxi at the same speed with lower pressure tires. I'm back up to 50 with no intention of going lower again. I now think tire pressure is an important factor to pay attention to, particularly if you are carrying a heavier load and dealing with a situation where runway length is critical on takeoff.
 
50 psi on my -8. I re-fill when it gets down to 40 which doesn't take very long on these little tires.
 
Shorter roll out

How does ground handling on landing differ with 50 vice say 35?

In my experience it's not as easy to notice the difference on landing as on takeoff, as there are more variables on landing, such as the fact that you might touch down at a higher or slower airspeed, which will obviously cause a longer or shorter roll out independent of tire rolling resistance factors. Nevertheless, I think its undeniable that your roll out (taking brake application out of the equation) will be shorter when you are running at a lower pressure with more rolling resistance than vice versa, and my experience has shown that to be the case. You don't need as much braking to slow down.

Of course in the real world we apply brakes if needed and I'm sure there are experts who would be capable of saying how braking effectiveness, for example in an emergency situation calling for maximum braking, varies with tire pressure, but that's a separate issue.
 
Another vote for 50

Another vote for 50 psi. I use Condor tires and Michelin Air Stops. Nine years ago, I was told to use 50 psi by Jay Pratt and I always do what Jay says :D. I believe that's what Michelin recommends also. 50 makes it easier to push around the hangar among other advantages. I refill about every 4-6 months depending on my flight activity.
 
I use 38 PSI but I like to fly into grass strips and wanted a little more give to soften the roll out. Having said that, I think 50 PSI makes a lot of sense for most guys, especially ones never leaving pavement. You'll see less wear for one thing. Or more centralized tread wear. I tend to see a bit more on the edges v when I run higher pressures, which tend the wear the middle quicker. Frankly, anything between 35 and 50 works fine.

As far as handling I've not noticed much difference between either settings. You might see slightly faster deceleration with the lower pressure, or a slightly softer feel to your full stall landings, but medium and low speed steering is about idiot proof in the RV-8. Higher pressure doesn't make it more squirrely. At least not in my experience. As tail draggers go its one of the easiest to keep straight.
 
RV8A won't slow down!

Hey Guys,

I've been reading all these tire pressure threads and I haven't seen my problem mentioned. This problem occurs on pavement (luckily I'm based on a grass strip). When I land it takes forever for the plane to slow down, and it will never go below about 25mph without brakes. This also occurs while taxiing from a stop. It slowly accelerates to about 25mph unless I apply brakes. I'm currently using about 35psi in the mains and 40 in the nosewheel. This is a pain because even if I put it down at 60mph on the numbers I use up lots of runway. I have the Idle set at 800. The plane is very light, so I'm sure that exacerbates the problem. I'm tempted to lower the tire pressure to 25 (or even less), but most responses seem to like higher numbers. Any suggestions?

Thanks, Lance
 
Lower RPM

Hi Lance
With my metal fixed pitch, IO-360 I have the static RPM just below 600. At a full stop the engine will quit if the ball is all the way back.
I taxi at 650 RPM no brakes
Tire pressure 46.7 :)
This saves my brakes, is quieter, plus slows us down on final.
Inflight idle RPM is higher of course.
 
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Sounds good, I'll give that a try. I had a wood prop originally, and it didn't like a lower rpm, but I switched to a carbon fiber that's 4 lbs heavier, so maybe I can slow it down.

Thanks, Lance
 
You gents running 50psi must be flying from very smooth southern runways. I tried 42 here on Colorado's bumpy runways and it wasn't a pleasant ride. Asked several RV8 guys here in town and all are in the low to mid 30's as well. I'm back at 34psi. Stock vans tires, Michelin Airstop tubes and grove gear.
 
You gents running 50psi must be flying from very smooth southern runways.

Ride quality was never a consideration. I would suggest a sharp pavement lip is even more reason to set correct pressure, as are most hangar tracks.

The Tire & Rim Association: 500-5 6 Ply - 50lbs psi
(Dresser site, page bottom) http://www.vansaircrafttires.com/pages/Articles{47}FAQ.html

Michelin Tire Databook: 50-5 6 ply 50 psi
http://www.airmichelin.com/generalcontent.aspx?id=219

Goodyear Aircraft Tire Databook: 500-5 6 ply 50 psi
http://www.goodyearaviation.com/resources/tiredatabook.html

The Neal Willford landing gear design spreadsheet is S409. It should still be available for download at the EAA site. Plug in the values for your RV-8 and look at the drop test tire compression for various tire pressures.

As for "they say", well.....

 
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Wow Dan, you're pretty sure your way is the only way aren't you? I tend to put more emphasis on field data than spreadsheets and in this case several thousand flight hours in the same plane, on the same runways with actual tire wear data accumulated over years...is more relevant to me than a spreadsheet.

Monkey see monkey do is also called learning from experience. Still, I am as capable of falling back to a chart as the next guy and the chart on Desser.com says the maximum tire pressure for Vans issued tire is 50psi. Running a tire at its maximum psi when not supporting its maximum weight is unnecessary.

As in all forms of inflated tire from race cars to over the road trucking, the tire provides a good chunk of the suspension give. Harsh ride may not be a consideration, but it does tell you what the tire is doing if you pay attention. Having my tires absorb and roll over that sharp pavement edge while staying in contact with the pavement rather than bounce off of it like a brick and skip certainly merits consideration when that pavement is often wet, snowy or very cold as well. Proper inflation depends on weight and unless your RV8 weighs 2570 lbs (Desser chart), 50psi isn't necessary. The surface you land on will greatly affect the proper pressure.
 
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gents running 50psi must be flying from very smooth southern runways

I have tried two full seasons at 35 psi and two full seasons at 45-50 psi. flipped the tires with the pressure change. All the same RV-8 and living on a short field grass runway (that is not a golfing fairway) and a majority of destination being the same. Typical runway conditions is wet in the morning and dry in the afternoon. When it's cold, it's raw and cold. I don't plow so when it's snowing, I'm not flying.

My personal experience is that ground handling is easier at the higher pressure. Take off has been a bit better at the higher pressure. Landing has been easier at the higher pressure.

This is just one pilot's data ... and yes, I'm now from the south :)
 
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I bought the larger AeroClassic 380x150-5 6 ply.
Should these run at the same pressure as the standard tire (tyre ;)), or lower, due to the considerably larger footprint?
Paul.
 
Wow Dan, you're pretty sure your way is the only way aren't you?

Not at all. My way changes as I learn. I am pretty sure there is great value in studying the databooks and fundamental mechanics, and only then considering tribal knowledge.

Let's lighten up and have some fun. Are you running the standard 199-102 Cleveland wheel and brake kit on a -3 through -9? Most are, so consider this puzzle...

The technically correct maximum inflation pressure for the 199-102 kit is 31 psi per Cleveland drawing 50-76.

The maximum inflation pressure for a 199-156 kit is 50 psi, again per Cleveland 50-76.

The two kits use the exact same wheel halves, bearings, bolts, etc. The only difference between the kits is the brake disk....so why the difference in inflation pressure?

I bought the larger AeroClassic 380x150-5 6 ply.
Should these run at the same pressure as the standard tire (tyre ;)), or lower, due to the considerably larger footprint?

Paul, please look up the rated pressure and weight capacity in one of the databooks previously linked.
 
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Lance, I've known more than a few 8 pilots running PSI down at 28. Nothing wrong with that other than that you will find youself having to add air more often. Handling is fine that low and on bumpy fields its not altogether a bad thing to have lower pressures. But as others have stated, your idle RPM is too high.

As far as having to use brakes...eh, in the grand scheme they are cheap enough, so don't be afraid to use em. I am sure some braniac has done a spread sheet on replacing brake pads every year or so versus the extra fuel used in a long taxi back to the hanger (proving that some folks take SPORT aviaton WAY too seriously, lol) but in the end, do what you gotta do to get the bird stopped where you want it. There is really no right answer on PSI. You have a usable range for a given aircraft weight and we are blessed with the ability to experiment within that range to see what works for what conditions. Too low a pressure and you turn your wheel pants into mud scoops. Too high and you make a brick out of every landing. Best advice, IGNORE all of our recommendations and find out what suits your individual RV the best for the kind of surfaces you use.
 
Thanks Scott!

That's kind of the direction I plan on heading. First I'll lower the idle, then play with the tire pressures. BTW, The idle is set there because I have the Rotec TBI and it acts up when it gets hot, and wants to starve the engine at idle. Changing to a carb tomorrow, so hopefully I can change idle to 600 or so.

Lance
 
Also, minimum aquaplane speed varies with the square root of tyre pressure, if you often land or taxi on hard surfaces with standing water.
 
All I have to say: Tubes+low pressure = Flat tire

I would be careful with blanket statements like that.

Over the past 15 years I have tried various tire pressures for my RV-6 with standard size tires. Low 20's, mid 20's, high 20's, mid 30's and higher. All are a compromise between rolling resistance, inhibiting shimmy and harsh landings. Trying various tire pressure is a valid way to tailor gear characteristics for a particular environment.

But in all this experimentation, I've never had a tire go flat (well, there was the time when I didn't tighten the valve core enough...). Flats do seem to be more prevalent according to anecdotal evidence on the nose tires, maybe due to the lighter weight construction of the tires and tubes (?).
 
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Over the past 15 years I have tried various tire pressures for my RV-6 with standard size tires. Low 20's, mid 20's, high 20's, mid 30's and higher......Trying various tire pressure is a valid way to tailor gear characteristics for a particular environment.
But in all this experimentation, I've never had a tire go flat.....

That's because you're a good pilot. No leg pulling; the facts say so.

I just happen to have an RV-6 leg in my gear design files. I was using it as a comparison sample of a well proven tapered gear when I was doing one for a biplane. This is one of Neal Willford's spreadsheets.

Assume a 6.5" wheel diameter per the Michelin databook for a 500-5. Further assume the tire is smashed flat and pinching the tube at 1/4" additional radius (think two layers of rubber, sidewall and tread, sandwiched between rim and asphalt). Last, assume a 3.1G hit, equal to a vertical drop of about 14". The minimum tire pressure to meet this criteria is 38 psi.



At 25 psi the tire is pinched hard against the rim at about 2G, less in a one-wheel stab at the ground on a windy day, with a bit of side load.

No flats? You're good ;)
 
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No flats? You're good ;)

Naw...just lucky, I guess. I can assure you my poor ol' RV-6 has seen its share of bounced landings......the bottom of the wheel pants can attest to that.....

I air the tires to 35 and pump them back up when they start looking a little "low" which is 26-28.

I'm not presenting an argument to prove or recommend any particular pressure-- just wanted to counter the statement that lower than rated pressure will guarantee a flat tire. My experience since 1999 has shown me otherwise.
 
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Naw...just lucky, I guess. I can assure you my poor ol' RV-6 has seen its share of bounced landings......the bottom of the wheel pants can attest to that.....

I air the tires to 35 and pump them back up when they start looking a little "low" which is 26-28.

I'm not presenting an argument to prove or recommend any particular pressure-- just wanted to counter the statement that lower than rated pressure will guarantee a flat tire. My experience since 1999 has shown me otherwise.


You are far from alone. If 38psi were needed to keep our tires from pinch flatting with typical loaded weight, there would be a flat tire epidemic. That spreadsheet just doesn't pass the smell test. There are many RV's running significantly less than 38 and we can't all be perfect pilots. Mine was at 28psi through half of my initial flight testing and some of my landings were positively comical...with no flats.
 
Hi Guys,

This discussion prompted me to try a higher tyre pressure after installing new tyres and tubes.

I have been running 35psi in the past with no issues and decided to see what 50psi was like.

Sadly for me this introduced wheel shimmy for the first time ever and it was quite disconcerting, is it's back to low to mid 30s for me.

Cheers
 
You are far from alone. If 38psi were needed to keep our tires from pinch flatting with typical loaded weight, there would be a flat tire epidemic. That spreadsheet just doesn't pass the smell test. There are many RV's running significantly less than 38 and we can't all be perfect pilots. Mine was at 28psi through half of my initial flight testing and some of my landings were positively comical...with no flats.

No one is offering a "one hard landing = one flat" rule. I'm sure Walt is thinking long term, as is Sam, and direct evidence will be hard to find either way. For empirical evidence, set 25 psi and check back in 2018.

I actually pulled up the spreadsheet to see if its tire compression predictions matched the values in the Michelin databook tables. They do. Please note the stated G forces; 38 psi minimum at 3.1G They're based in Part 23 requirements, and intended to to keep Freddie Fivehoursayear out of trouble. We've all seen Freddie stall a Cessna at 5 AGL.

As several have stated, tire pressure is a compromise. The trick is really understanding the various factors being balanced. Sometimes the factors are not so obvious.

Which bring us back to an illustrative puzzle. Single and two-place RVs are supplied with a 199-102 Cleveland wheel and brake kit. Cleveland says 31 psi max, so all of us inflating in excess of 31 are, in theory, wrong. However, they allow 50 psi with a brake rotor swap. Why?

Hint: It only takes a moment to understand.
 
380x150-5

Regarding the 380's, looks like 45 PSI inflation, and they have about 25% higher rated load/max bottoming load. (compared to the 500-5 6ply Flight Custom III at 50 PSI).
Be some time before mine get used in anger though!
 
?...
Which bring us back to an illustrative puzzle. Single and two-place RVs are supplied with a 199-102 Cleveland wheel and brake kit. Cleveland says 31 psi max, so all of us inflating in excess of 31 are, in theory, wrong. However, they allow 50 psi with a brake rotor swap. Why?

Hint: It only takes a moment to understand.

Hummm, increased rolling tires speed needs a heavier brake rotor? Just guessing but sounds like a heavier rotor increases the heat dissipation.
 
Which bring us back to an illustrative puzzle. Single and two-place RVs are supplied with a 199-102 Cleveland wheel and brake kit. Cleveland says 31 psi max said:
wrong[/I]. However, they allow 50 psi with a brake rotor swap. Why?

Hint: It only takes a moment to understand.

Swap out for a larger diameter rotor gives you a longer "Moment" (hint) on the brake force applied from the disc...giving you more brake for the fast and taller full inflated tires? (That answer rather right or wrong took more than a moment.)
 
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Very good gentlemen. You're both close.

Our standard 199-109 wheel and brake kit (RV-3 through RV-9) comes with the very light (read "thin") #164-017 brake disk. The small disk mass doesn't allow much heat storage, which the FARs and brake literature call "kinetic energy capacity". The Cleveland-rated kinetic energy capacity of an 199-109 kit w/ a 164-017 lightweight disk is 117,500 ft-lbs.

There is, of course, a standard equation for the kinetic energy required to stop the airplane; it's found in FAR 23.735 or all brake manufacturer's literature:

(0.0443 x aircraft weight x velocity in knots^2) / # of brakes = KE

Turn it around to see how many knots your brakes will handle....

sqrt (rating x 2 / weight x .0443) = knots at start of braking

Go ahead, pick up a calculator. You'll find it's close to the accepted minimum (Vso or V1) for the 1800 lb gross models...one stop from 54 knots.

TSO C26 requires slowing the aircraft at a minimum rate of 10 ft/sec^2 without exceeding the kinetic energy rating. The KE rating is a measure of energy storage capacity, while brake torque is a measure of stopping power. Torque means arm x force, i.e. a moment (thus the hint).

The thin disk heats very quickly. In general, as the disk temperature increases, so does the required pad pressure in order to maintain the same brake torque. Don't want to increase line pressure or caliper piston diameter? Well then, just reduce the arm of the rotating assembly (tire rolling radius) by lowering the max tire pressure.



Doesn't add weight, which is how Cleveland keeps the 10.8 lb sales position for the 199-102 wheel and brake kit. The catch is technically, tire pressure is restricted to 31 psi if you want to meet TSO C26. Best I can tell they bury that detail in drawing 50-76, which is not readily available on the Parker/Cleveland web site.

You can return to 50 psi and meet the TSO by installing a thicker disk, which doesn't get so hot soaking up the same kinetic energy. In our case it is conversion kit 199-200, a thicker disk (164-099), a caliper spacer, and two longer caliper bolts. You get an increased KE rating in the deal, to 155,000 ft-lbs

Or you can just run 50 psi and accept longer stopping distances, which is the considered compromise I currently accept. Eventually the disks will need replacing, and I'll install thicker 164-099's. I've installed them before, and under hard braking there is a noticeable reduction in required brake pressure near the end of the rollout.
 
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Dan,
Everytime I read one of your posts, I learn something... Thanks.
Do you have a "build site" that chronicled your RV-8?

DM
 
28-30 pounds on AirHawks on my RV-4

I tried Goodyear III's and just don't like the feel of the harder/heavier tire. They last a long time, but feel stiff. I routinely fly from a 2000' strip and like to use 3-4 seconds of moderate braking and turn off mid field. I was taught to get a taildragger slowed quickly and I only do 3-point landings.
600 hours, brakes are original and just put on a third set of tires.
YMMV :)
Ron
N8ZD
 
RV-8 tire pressure 199-102 chrome disk kit-31PSI max

Reference:
Cleveland Product Reference Memo (Data Sheet shipped with my wheels and brakes 'around' 2005)
PRM62 Rev. A July 1, 1995

Disk Kit: 199-102 chrome
Tire Size and Ply rating: 5.00-5 6ply
Tire Inflation: 31 PSI maximum (note 5)
Static Load Capacity: 1260 lbs

Note 5: Maximum Tire Inflation Pressure:
Over inflation may result in explosive failure or reduced wheel service life. Inflation pressure may be adjusted down to obtain desired flotation for specific aircraft weight and operating environment.
=======

Vince
RV8 N8432 Flying
 
Reference:
Cleveland Product Reference Memo (Data Sheet shipped with my wheels and brakes 'around' 2005)
PRM62 Rev. A July 1, 1995

Disk Kit: 199-102 chrome
Tire Size and Ply rating: 5.00-5 6ply
Tire Inflation: 31 PSI maximum (note 5)
Static Load Capacity: 1260 lbs

Note 5: Maximum Tire Inflation Pressure:
Over inflation may result in explosive failure or reduced wheel service life. Inflation pressure may be adjusted down to obtain desired flotation for specific aircraft weight and operating environment.

See posts 23 and 38....and then use whatever pressure you wish.
 
Hi Guys,

This discussion prompted me to try a higher tyre pressure after installing new tyres and tubes.

I have been running 35psi in the past with no issues and decided to see what 50psi was like.

Sadly for me this introduced wheel shimmy for the first time ever and it was quite disconcerting, is it's back to low to mid 30s for me.

Cheers

I needed to top up my tires recently so I also tried raising the pressure to 44psi, I found the same problem you had... A very weird shimmy or bunny hop at around 40 mph on take off. After two flights I reduced the pressure to 34 psi and everything is back to normal again.

Mid 30's for me now...
 
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