fairlea(at)amtelecom.net
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 Posted: Sat Sep 13, 2008 11:54 am Post subject: HR 2 - any ideas on where to start looking to make my HR2 f |
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Andrew
Both of my HRIIs with stock engines were 200 knot airplanes. This
is what John Harmon advertises and my numbers match his. I have spent a lot
of work on the cowling over the years, on a total of seven rockets, and I
can tell you that the stock cowling is pretty efficient. The main thing
that can be done to improve cooling is the proper shape and size of the
inlets. This has been mentioned by previous posters and it is quite
important. Obviously no leaks from the upper to lower plenum is a given. I
have found that the outlet is an area that can use some work. Often I see
outlets that have been cut off and this allows more air for cooling. More
cooling air is directly related to increased drag. Particularly if you cut
the cowling off to a point where the air flows directly down the firewall
and enters the free air stream at an acute angle. My cowl lip extends aft
of the firewall at least three inches. Note that my comments regarding the
cowling mainly relate to cooling. I have been able to make a lot of
adjustments to aid in cooling but I have yet to see an increase in total
speed from cowling modifications. You can make is slower, but increasing
speed, has been elusive.
Weight kills speed and performance.
Upper and lower gear leg farings also can have an impact on speed.
Here you need a tight fit and a generous fillet aft of the faring.
The choice of propellers can make about as much difference as
anything you can do. The MT prop, although smooth is going to cost you
speed. The new two blade blended airfoil Hartzell is quite a bit faster and
much smoother than the old paddle blade Hartzell.
There is a dip in the fuselage at the lower firewall. If you look
at a F1 rocket it does not have as much of a dip at that location. I
estimate that dip costs an average of three knots.
The main limitation to increasing speed on a HRII, or the standard
wing F1, is the wing. Although in many ways it is the perfect wing for this
airplane it does limit top speed. Keep in mind that there are only a
handful of planes of any kind that are true 200 knot airplanes.
Currently I am flying a F1 with the EVO wing. It is my fastest
rocket to date and I have been making a series of minor improvements over
the last three flying seasons. My original MT prop was limiting my speed
and the new Hartzell has been a real improvement. In the AirVenture cup
race this year my average speed over 405 nautical miles was 232 knots. I
believe this to be the fastest recorded rocket speed in an official race.
Yes I did have some nice tail winds but this plane is showing some good
numbers now. My data leads me to believe that my 8000 foot TAS is
approaching 215 knots.
This winter it goes back in the shop for some more clean up and a
new primer paint scheme, the original is getting a bit scruffy. If you are
interested in speed, do not paint your airplane as it will definitely limit
your will to make modifications.
Tom Martin
EVO I 280 hours.
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