Date: Fri, 12 Feb 1999 01:17:08 -0800
From: "LCDR James D. Newman" <infaero@flash.net>
Subject: COZY: Re: [c-a] Tire air pressure

Hi Curt and All,

> Curt Smith wrote:
> Gang,
> A warning about tire pressure.
> Several months ago, we had a thread going regarding tire pressure. A recommendation was made here
> that the "proper" pressure for 5.00/5's and the nosegear tire were 40 and 70 psi, respectively.

    I wrote this.  While I don't know much about landing gear and tires, I will try and clarify
myself, so please bare with me.
    The tire pressures are not just a recommendation.  If you will all VERY carefully re-read my
post (that took me 6+ hours to research, calculate and write) dated 09/23/98, "Subject:  Re:  Flat
Tire", which should be in the archives (if it's not, or you can't find it, let me know and I'll send
it to you), the published tire pressures come from the tire manufacturer for *their* tires - not
recommended by me or any one else - and I quoted my sources (as usual).
    The Tire and Rim Association states, and all the tire manufacturers state, that 50 PSI is the
*normal* tire pressure for a 5.00 x 5 aircraft tire.  The 40 PSI commented by Bill Theeringer in his
post was told to Bill and I by a Michelin engineer during a phone conversation years ago that for a
5.00 x 5 tire not normally flown at maximum weights of 1285 lbs. per tire (2570 lbs. total), one can
*let* the tire pressure *get / be* as low as 40 PSI when normally flying at lower weights, such as a
Long-EZ (LE) at 1700 lbs.  But he said they still state 50 PSI officially.

> I put those pressures in my Long's tires and, while the 40 psi in the mains seems fine, I just had
> the nose wheel tire fail and the failure appears to be due to overpressure. A 3" section of tread
> separated, but did not depart, from the tire carcass, causing a large "bump" on the tire that
> interfered with the fork. <snip>

    Since the *tire* manufacturer states 70 PSI for their tire used as a nose tire for the Long-EZ
(LE), there are probably many other factors that will/did cause this tire to fail.
    For example, the age of the tire.  How many times did tire pressure drop below 40 PSI (40 PSI
seems to be the magic minimum PSI for this particular tire before the tube will slip on the rim
cutting the stem and other damage happens)?  When this occurs, as we all know if we run our car
tires too low (even 3 to 5 PSI), damage WILL occur to the tread and tire side wall for running the
tire at too low a pressure, particularly when flying at heavier than design weights and the
recommended GW.  The rim can begin to cut into the side wall; the tire can 'roll' from side to side
on the rim damaging the tire and rim; the tire will get hot; etc.; etc.
    As I said in my September post, the maximum 3 point aircraft landing weight for this 8" diameter
tire used on the Long-EZ is only about 1600 lbs. (with a Limit Load Factor of 3) with no safety
factor (as we all know, Rutan designed and published a gross weight for the Long-EZ with this
landing gear and nose tire to be 1325 lbs., which allows for a safety factor), which will exceed the
Bottom Load limit of this tire in a 3 point landing causing an eventual (or immediate) failure.  A
hard 3 point landing, heavy braking, landing on the nose first, running over FOD, letting the nose
fall through hard on landing, etc., all damage a low pressure tire.
    I know of many EZ's flying with 50, 60 and 70 PSI in their nose tires all the time with no
problems (if they are not too heavy or land to hard, etc.).  It sounds like the higher 70 PSI
pressure in this nose tire in question just caused the damage to the tire from long time low
pressures to fail the tire a little sooner, and/or the tire was defective from the "get go" and the
higher pressure caused the defect to manifest itself sooner.  Fortunately, you were just taxing out
:-) .

> Anyway, 70 psi is way too much in the nose gear tire, . . .

    No, it is not too much pressure (see above).  Guys, just make sure it never gets below 40 PSI,
or the tube WILL slip inside the tire, as I said before, and history has proven.  Also, just don't
land too hard at heavier weights (above 1625 lbs.) and/or too fast, nose fall through hard, all the
stuff I've mentioned above, etc.  Go around if you have to.


> . . . which doesn't carry more than 2-3 hundred pounds, at most.

    Yes, you're right, but no - that's just static weight.  A normal nose wheel, tire and strut will
be designed to carry 10 to 20% of the aircraft Gross Weight (GW), then the tire will be sized and
chosen accordingly.  The Long-EZ nose strut and tire static weight is about 12 to 15%.  This means a
static (just sitting on the ground) nose tire and nose strut carrying weight of 159 to 199 lbs. for
a 1325 lb. GW LE.  A 1625 lbs. GW LE would be a static load of 195 to 244 lbs. on the nose strut and
tire (close to what Curt was trying to say).
    What one needs to worry about is exceeding the Bottom Load limit for the tire (see discussion
above).  This particular 8" tire used for the LE nose tires' Bottom Load limit is ~1171 lbs.  A
Long-EZ landing weight of ~1625 lbs. will exceed the Bottom Load limit for this tire, with NO safety
margin, if you land at the FAR design Limit Load Factor (kinda hard).


> I'm back to 40 psi in the new one.

    Good luck  [ maybe at least 50+ PSI - give yourself a little more margin above that historical
40 PSI number  :-)  ].


    I hope this helps and clears things up.


Infinity's Forever,

        JD

Date: Fri, 12 Feb 1999 08:40:11 -0600
From: Curt Smith <csmith@siue.edu>
Subject: Re: COZY: Re: [c-a] Tire air pressure

JD wrote:

>    For example, the age of the tire.  How many times did tire pressure
drop below 40 PSI (40 PSI
>seems to be the magic minimum PSI for this particular tire before the tube
will slip on the rim
>cutting the stem and other damage happens)?  

Thanks, JD, good discussion. The tire was new as of last April, so age was
not a factor. And, pressure never went below 40 psi in the interim.
Actually, 45 is what I have in the new one.

I previously flew a Varieze for several hundred hours over a period of
several years, used 40 psi or so, and never had a problem. I still think 70
psi is what caused the tire to fail and just wanted to save someone else
from the same experience.

Your point about using at least 40psi to avoid slippage is an important one.

Curt
LE N86CS




*****************************************************************
Curtis A. Smith, Ph.D.
Associate Professor
Southern Illinois University at Edwardsville
Box 1125 
Edwardsville, IL 62026-1125

Phone: 618/650-3970
Fax:   618/650-3359

Date: Fri, 12 Feb 1999 13:16:07 -0800
From: "LCDR James D. Newman" <infaero@flash.net>
Subject: COZY: Re: [c-a] Tire air pressure

Hi Curt and All,

> Curt Smith wrote:
> Thanks, JD, good discussion.

    You're very welcome (I hope others got something out of it), and thanks - lotsa work when I
write those short novels :-) .  But I strongly feel we all need to know as much about our planes as
possible concerning every aspect that we can find out.
    Data like this and the data like what I posted in my September post are all part of the
information in my Infinity 1 Flight Manual and Pocket Check List.  Yes, I know it is not published
as part of a typical Spam Can Owners Manual, but those pilots are not the manufacturer of that
aircraft (and they usually don't care).

> The tire was new as of last April, so age was not a factor. And, pressure never went below 40 psi
in the interim.  Actually, 45 is what I have in the new one.

    Info:  this tire is also used as a pneumatic tail wheel and for nose wheels on gyroplanes.
    It would be interesting to have it examined to see if it had an internal defect, and/or what
actually caused it to fail.  But that would probably cost too much and be of little interest to
most, since the nose tire problems like this and of the past are at most a rare nuisance, so we end
up speculating like we are.
    Now if it was a Lamb tire (which I use for my oleo nose strut), I would ask you to send it to me
and I would try to have it thoroughly examined to find out what happened.  I also know the designer
of the Lamb tire (he's one of my stick grip customers).

    I thought of something else this morning - tire gages are notorious for being many pounds off
(for example, I NEVER trust a tire gage that is connected to the end of a air hose at 7-11).  Over
pressurization is a problem too.  Try to get and always use a calibrated tire gage.

> I previously flew a Varieze for several hundred hours over a period of several years, used 40 psi
or so, and never had a problem.

    Yeap.  The Vari-EZE GW is much less than what guys fly the Long-EZ at, so the worst case
scenarios I addressed of reaching anything close to the tires Bottom Load Limits would never occur.
So 40+ PSI was evidently at least adequate (barely) for that aircraft.
    This tire Bottom Load Limits is all based on 70 PSI.  With less tire pressure, the Bottom Load
Limit is obviously less.

> I still think 70 psi is what caused the tire to fail and just wanted to save someone else from the
same experience.

    Hmmm.  I considered the reference I have for the published specs for that tire was a typo, but I

found the same PSI rating for that tire in other references.  Same goes for the Cozy MK-IV tire and
the Lamb tire - several references publish 70 PSI for these 2 tires used by canards too.

> Your point about using at least 40 psi to avoid slippage is an important one.

    Very important - NEVER let that nose tire PSI get too low, particularly below 40 PSI.  I hope
all keep a little more than that for, like I said, the Bottom Load Limit for that tire is based on
70 PSI.


    Thanks Curt.  I love these brain storming, research and learning sessions :-) .


Infinity's Forever,

        JD

Date: Fri, 12 Feb 1999 16:23:32 -0800
From: Curt Smith <csmith@siue.edu>
Subject: Re: COZY: Re: [c-a] Tire air pressure

Thanks again, JD. You're quite right, this is how we all learn and this
forum is invaluable.

Ok, I think I'll crank up the pressure to 55 psi. You are very persuasive
and from all the responses, no one else seems to have had the problem I
did, so perhaps it was an anomaly.

Curt
LE N86CS
Cozy III in progress

**********************************************************************
Curtis A. Smith, Ph.D.
Associate Professor
Department of Educational Leadership
Southern Illinois University at Edwardsville
Box 1125 
Edwardsville, IL 62026-1125

Phone: 618-650-3970
FAX: 618-650-3359
************************************************************************

