WIND TUNNEL TESTS OF R/C AUTOGYRO ROTORS

Conducted by:

Lester W. Garber, Ph.D.

Bill Friedlander, Ph.D

Note:  The basics of this Wind Tunnel Test project has recently been published in the August 2001 Issue of RCModeler magazine and is highly recommended reading for those of you interested in the technicalities of rotor blade airfoils.

In February 2000 the above gentlemen conducted numerous wind tunnel tests of model Autogyro rotor blades.
Seven selected airfoils [Clark Y, Horton HU13-3s, SG6043, SG6042, SAB (Sig airfoil blanks), NACA H813, and BTE C4 (Reflexed beveled trailing edge), were constructed of vacuum-bagged epoxy glassed foam cores, and standard monokote covered balsa.
Blade widths (chord) ranged from 1.5" to 2.31", with a thickness of 10 to 13.7% if their respective chord widths.
Rotor hubs holding 2, 3, and 4 blades were utilized. All rotors tested had a diameter of about 44".

A total of 35 experiments of 14 rotor sets were conducted, with measurements of ROTOR RPM, ROTOR LIFT recorded in tunnel velocities ranging from 12 to 28 MPH. Since the complete report (60 plus pages) contains considerable technical data and is impossible to post here, I have extracted the following highlights, with personal comments for our reader interest:

(1)  Rotors containing 4 blades perform significantly better than those with 2 or 3 blades. 3 blades are much better than 2, with 4 being slightly better than 4. [ Interestingly, the RPM of the respective sets was relatively the same. Previously we have always thought the more the blades, the slower/lower the RPM ].

(2)  Aspect ratios of 8:1 were noted to perform well. [ Previously we have found that ratios from 8 to 12 were satisfactory, thus we need to pay more attention to the lower ratio of 8 in the future].

(3)  The SG6042, an airfoil with a slight undercamber in the aft lower trailing edge, performed superbly. This airfoil is slightly similar to the Clark W, but has the undercamber. The undercamber outperforming the reflexed is significant. [ Technically it should be much easier for the average model builder, who does not have vac-bagging equipment, to hand-make the SG6042 airfoiled blade versus any reflex such as the H813 ].

(4)  The thinner airfoils (10%) outperformed the thicker (13%) ones. [ Thin is in…. Thick is sick… Many of us have known this for some time, and this is the proof. This why we have advocated the "flat bottomed" Clark Y (Clark W), versus the standard Clark Y, which is far too thick. ]


You may click on an image for a full size view

(5)   The exceptionally smooth vac-bagged glassed blades far outperformed the standard Monokoted balsa blades, both in lift and RPM. [ This is something we have always generally understood. The key point here is that "smoothness" is essential, and while the vac-bagged blade is best, a least a plastic covered blade is much better than any plain un-covered wood/balsa blade ].

(6)  The "slightest" change in blade incidence will significantly alter performance. Positive incidences produce the greatest lift, but are more difficult to achieve rapid acceleration. Lower incidences (negative) will accelerate more rapidly, but produce less lift. [ Use the least positive incidence that will still allow the rotor to accelerate within reason. Begin at zero, and change ever so slightly if necessary ].

(7)  The use of narrow ("turbulating") trips at 20% aft of the leading edge on the top surface, improved performance enough to warrant their use and further experimentation. These "trips" consisted of 1/8" wide sticky-back strips of automobile trim tapes. [ This is something all gyro modelers should consider trying. I have been using a similar turbulator at 33%, but will now move them forward to the 20% position and make note of any improved performance. ]

(8)  In regard to blade incidence angles, it would be best to quote the following from the report verbatim:

"Test T-34 (upper curve and .08 deg higher blade incidence angle) produced greater rotor lift forces than did test T-13 (lower curve) at air velocities between 16 and 26 MPH. Below 16 MPH these effects are reversed; the blades with the lower incidence angle (T-13) produced greater lift forces. [ It is not difficult to reason/understand that a blade with more positive incidence will certainly lift better at the higher speeds, but can we now assume that a blade with the lower in incidence will allow the model to "hover" better? ]

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GENERAL TECHNICAL CONCLUSIONS as presented by Bill Friedlander:

1. The Airfoil used has a significant influence on the lift/rpm performance of the rotor.    Thus far the SG6042 appears to be the airfoil of choice.
   However, because the blades experience dramatic differences in Reynolds numbers from root to tip,  it may be possible to develop blades with airfoils that perform best at low Re for the root sections then blending into airfoils that perform better at the Reynolds numbers the blade tips experience in the range of 150-250,000.

2.  Wider,   rather than narrow blades appear to produce greater lift and rpm.    However, aspect ratios of much less than 8-10 are likely to be less efficient.  ( This latter statement is based more on anecdotal experience amongst our group of "Gyronuts" and needs to be tested quantitatively. )

3.  Rotors with more blades provide greater lift   (but not greater rpm)   than rotors with fewer blades.

4.  Adjusting the Angle Of Incidence  (AOI)  of rotor blades on the rotor hub for optimum rpm vs ease of start-up results in greater lift for the rotor.

5.  Trips on the blades can benefit certain rotor configurations.     If a rotor is performing erratically,   it is such an easy modification to apply trips,  that it is worth the effort !

If you would like discuss this report with Bill Friedlander,   click here...


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rev: 07-12-01..jb