Hi Todd,
Keep in mind this is all generalization but the breaking point of a shaft
is pretty much related to torque, not HP. Torque is pretty much
directly related to manifold pressure. 45" MAP is 50% higher than a NA
engine at sea level.
Can I predict whether your welded shaft will take the added torque?
Short answer is 'no', so all I can do is err on the side of caution.
Have gotten a raft of questions from various builders regarding the issue
of why the shaft was done this or that way so I'll give a brief thumb nail
sketch of how the shaft design evolved.
The first flying proof of concept prototype used a pressed together
gear/shaft assembly with an interference fit. This is a reliable method
when the interference is controller properly. This was also used on
the next production prototype but I noticed that the ID of the Ford sun
gears varied over a significant range when I got the first batch of
gears in for production. A worst case gear and shaft combination
was assembled and flown. It flew great for about 20 hours when I noticed
what sounded like an intermittent engine miss at full throttle. Turned out
to be the shaft occasionally breaking loose and friction welding itself
back together again. (Be alert for this symptom)
That's when the move to welded gears was made (prior to shipping any
production drives). Welding was good, fast and inexpensive (if the
proper welding rod was used) but I noticed that the better the gear was
welded, the more distortion it sustained. Engineering rock & hard
spot. A hopefully happy medium was reached and tested and all was well.
About the time the C drive was put into production, the drilled &
pinned method was adopted in an effort to avoid the slight distortion of the sun
gear due to welding. This was made possible by my "discovery" of
micro-grain solid carbide drill bits. Till then, I had no idea you could
drill holes in heat treated gear alloy with anything short of EDM.
Only down side is when a $35 drill bit is ruined by a chipped edge on the
very first hole : (
I had no reason to believe that the welded version posed any problem
until Dave Leonard's turbo engine broke one. Was it an abnormal
backfire that broke it? Don't know and never will since I lack
NASA's research budget to investigate it. I did examine the shaft and
found that it had weld penetration at the very bottom of the range
used. Recently, Steve Brooks turbo engine's shaft started to slip
and it was found to have welds in the middle of the weld penetration
range. The service alert was issued immediately after this
observation.
Why was the pinned version not used from the very beginning?
Don't know how to answer that. Same reason you find things in the last
place you look, I guess. Next rhetorical question: Are you acting as
a Beta tester for some of the stuff I build? You bet! It's the
nature of this game we choose to play.
Tracy
Hi Tracy;
I
have the RD-1A on my turbo 13B which while it is not recommended for turbo
applications is rated for up to 200hp. Since my turbo is used only for
normalization purposes (mountain flying) and take off performance (during
which I use an absolute maximum of 45"MAP, but usually <40"), I doubt that
I ever see 200 hp and since the RV9 is only rated for 160 hp I never intend to
attempt to reach 200 hp.
Does the RD-1A have a welded
or pinned input shaft? And should I be concerned about it?
Todd
(too busy to put any stress on any of my airplane's parts)
I will publish a service alert later today on the website (
www.rotaryaviation.com) on B drives
used on turbo charged 13Bs and 20B engines.
Decided to do this after reviewing Steve Brooks input shaft
problem. His input shaft was previously used for well over 100 hours
on Ed Anderson's drive (fairly early drive) with no problems. It used
the welded style sun gear attachment to the input shaft. Steve's turbo
engine apparently had enough torque to break the weld in the same way that
Dave Leonard's (another turbo) did.
Shortly before starting C drive production (4-23-03) we changed
over to a drilled and pinned input shaft. This service alert requests
(strongly) that any B drive used on a turbo charged 13B or 20B engine
delivered before this date be inspected for presence of a pinned input
shaft. If the sun gear is welded instead of pinned, the input shaft
should be returned to RWS to have it drilled and pinned prior to
any flight. This service will be performed for only the cost of
materials and shipping. No problems have been encountered on RD-1,
RD-1A or RD-1B drives used on normally aspirated engines but if you wish,
this update can be done for a nominal service charge ( parts &
shipping + shop time TBD).