Hi.
The US-Army indeed did a study on the effects of blade tape application on the OH-6 Cayuse in the early 70's which had interesting results. The values of degradation were in the order of 4%, for tape.
The reason for the performance loss may not be fully explained in 2D studies but is rather evident when evaluated in 3D experimentation.
(IMHO):
A simple experiment can be conducted by just reviewing the wind down time of modified v unmodified blades. From normal Nr, close the throttle and time the decay period to reach a given Nr. Note that the application of blad tape increases total inertia by a small amount, (as is also increases radial shear, flapping and feathering inertial moments). Notwithstanding that "increase in inertia" from adding tape, the time interval,
tau, will be shorter for the blade with a normal blade tape added. This test is moderately sensitive to environmental conditions, and control position, so ensure you maintain a repeatable position of the controls.
Take away point: If your response time was short before in the event of a power loss to achieve autorotative conditions, it has just been eroded further by conventional blade tape.
performance shifts of L/D will affect the following issues:
- Tau, response time in the event of power loss
- Nr recovery in flare
- Nr decay rate post flare
- torque demand through the power train
- TR authority
- ROD in autorotation
Does erosion protection improve the life of R22/44 blades? Open question, and the discussion on this forum has covered the issues quite well. Erosion of the PPS will allow moisture ingress if the structure was not well manufactured... and that remains the basic issue that the applicable AD refers to.
Coning Angles: The R22/44 blade bending loads are not significantly altered by coning angles per se, the hinge relieves bending loads. The greatest contributor of bending loads are associate with span wise non uniform random excitation, occurring in high speed forward flight primarily and during high rate manoeuvring, or through BVI and similar unsteady effects. (not the case for the B-206 and similar configurations... where low Nr will result in high bending loads at the root that are not relieved).
I think Shawn may be referring to my work re blade tape....
I have been conducting flight tests for over a year of a blade tape system that does improve the L/D of the blade, significantly so. It results in lower torque requirement for a given performance, and improves TR control authority, autorotation. The tape is protective of the leading edge, and is clear, allowing visual inspection of the blade surface. Erosion protection is moderate in sand and good in rain. certified performance values of the helicopter will not be changed, but the operator will note a change in power demand. It is the intent to add safety margin, not utility, and to reduce maintenance costs associated with erosion. The modification is complete in experimentation phase and is now in certification phase.
For any party that needs convincing that lift and drag performance can be improved through simple mechanisms, feel free to PM me for a reading list of numerical and experimental studies on the matter.
The question of demand for a solution to the R22/44 and other helicopter erosion issues is rather open, and given the industry's experience with tape performance that is not surprising. Any parties that may be interested in protection of their fleet MR & TR's as discussed are welcome to PM.
Specifically for the R22, one configuration trialled resulted in low Nr blade stall at 68%Nr v 83% nominal, and 80% control. The configuration for certification is not quite that radical, it does reduce blade stall Nr though slightly, to around 76%Nr. Torque requirement is also reduced by approximately 9% for a given condition, and autorotation ROD is reduced by a similar margin. Operationally, the only notable change is that in the autorotation flare the Nr recovery/g is greater, and collective needs to be used to maintain Nr within upper limits. This makes the flare/landing somewhat more comfortable than baseline. The time constant for engine failure is still short, but does appear to be slightly kinder than the baseline. It is still not a fun activity, and all of Frank's safety notices about response to low Nr are applicable as they are in any rotary aircraft.
Costing is proposed to be low-moderate for the STC, and the tape is expected to be field replaceable for the outer section at least. Payback period for the STC will be in the order of no more than 1.5 maintenance visits for blade protection costs alone.
cheers FDR
Reference:
F.Y.M Wan Flapping Response of Lifting Rotor Blades to Spanwise nonuniform Random Excitation Journal of Engineering Mathematics 1980 vol. 14 (4) pp. 1-21
T Lee, D Matteescu experimental and numerical investigation of 2D backward facing step flow, Canada Journal of Fluids and Structures 1998 vol. 12 pp. 703-716
Technical Bulletin, Rotor Blade Erosion Protection Dept of the Army 31 Dec 1991 TB-1-1615-351-23
L Pfledderer, M Pepi, "Sand Erosion Test Method for DOD Unique Environments", Army Research Laboratory Aberdeen, MD. ARL-RP-229 September 2008 (reprint from the Proceedings of the 2007 Tri-Service Corrosion Conference, paper 1783, Denver, CO, 3–6 December 2007).