Scale model tests

On average, about half of the effort of the study was the design and test of scale models, initially to understand the mechanics of meshing and tilting, then progressively to explore flight related issues.

Static tests

The static tests concentrated on preparation for helicopter mode testing: checking lift capability in and out of ground effect, comparing static control torques for pitch, roll and yaw.

One surprise was that the rotors′ downwash covered much less of the inboard wings than expected.

Different methods of achieving yaw control were investigated: differential collective, differential tilt, and the use of inboard wing surfaces. Where results were hard to reconcile with estimates, then smoke visualization, anemometer probe or other simple techniques were used to attempt an understanding. One surprise was that the rotors′ downwash covered much less of the inboard wings than expected.

Helicopter mode tests using 1/20th scale model

Tethered testing of the 1/20th scale model proved problematical until fly-bar stabilization was used. This implied that the initial weight shift control strategy was flawed; it had insufficient bandwidth for rate gyro stabilization. However once the fly- bars had been introduced, the model proved flyable, see embedded video below.

The obvious way forward was to upgrade to full cyclic control.

The tests were flown by Paul Heckles (Ref. 14), a model airplane and model helicopter pilot and instructor, with experience of test flying of experimental designs. The longest flight was 8 minutes, the flying weight being 2.55 kg. Varying the fly-bar mass, confirmed that the model stability was dependent on the fly-bars.

The obvious way forward was to upgrade to full cyclic control, dispensing with the weight shift approach. A strip check of the transmission showed that the model still had many hours of operating life left. However there was insufficient room to install the required cyclic servo system, so the decision was taken to go to a new model.

Preparation for 1/10th scale model

The objective of the 1/10th scale model is to investigate conversion in flight. The first step is planned to prove the airplane mode, then the helicopter mode and finally to investigate conversion. Parametric studies have been made to choose an initial layout, see Figure 15, by varying the principal component locations, total wing area and ratio of tail to main, payload, rotor cant and overlap. The program computes aircraft centres of lift and gravity for helicopter and airplane modes.

fig15
Figure 15. 3D view from parametric study of plan form options for the proposed 1/10th scale model.

A mock-up for the airplane mode has been built and flown using a pusher prop in place of the intermeshing rotors, see below - enjoy the flight!

Figure 17 shows a drawing of the bevels location. The lower axle is part of the cross-shaft that drives both rotors. The upper shaft is one of the rotor shafts. It is canted 11° from the XZ plane on the airframe centre-line and the ratio of the pair of bevels has been chosen to provide good blade meshing over the full range of rotor tilting.

Planning the flight test programme for 1/10th scale model

The objective is full conversion between rotary and fixed wing modes: however a robust flight envelope needs to be proven before conversion is attempted.

The proposed plan has 4 phases

  1. VTOL with bare rotor
  2. VTOL with wings present
  3. Winged flight from runway
  4. Conversion
    1. Fixed
    2. Fully variable

The first 3 test phases are to provide confidence to move to phase 4 where conversion will be investigated.

Phase 4a will be CTOL from the runway used in 3, but the rotors will be fixed for the whole test flight at positions representative of part way through conversion. It is intended that 4a will give a first understanding of the conversion corridor.

Phase 4b will start when the test pilot is satisfied that 4a has investigated the conversion envelope sufficiently.

Phase 1: Progress to Sept. 2011 on VTOL the 1/10th scale model with bare rotor

system, "the Hover Rig"

he meshing and one-at-a-time tilting functions of the rotors/transmission were bench tested at mid range rpm. This video shows the "hover rig", ie no wings but all the rotors/transmission/motor/controls/basic airframe etc, being bench tested through the full one-at-a-time conversion tilting process, rotors fully meshing, constant rpm.

The next video shows the hover rig being tested in July 2011.

The flying qualities provided by full cyclic control are a huge improvement over the 1/20th scale model′s "weight shift" approach. Nevertheless the test pilot, Paul Heckles, was unhappy with a strong tendency to dig in on turns, a problem that was resolved in later tests. All the initial Phase 1 testing was made without flybars or any artificial stabilisation so the tests would reveal the basic behaviour. Here is the video, the rods with the coloured balls are called training gear: very useful during experimental testing to assist the test pilot see aircraft attitude easily in flight:

A series of tests followed with considerable improvements in handling: one aspect was the wide tolerance of (symmetric) tilting of the rotors in hover. The next video clip show the rotors tilted forward, and then tilted back.

Note that the rotors stay horizontal and that the airframe has to do the tilting!

The next four clips are from the last flight of test session 39. The flying weight was about 6.2 kg and the flight lasted just 5 minutes.

Take-off!

Trim in hover, then cameraman loses sight of it after fast fly past.

Found the aircraft again, just before it dives to a sedate hover.

And back to the pits after a very successful flight.

The final clip is from the last flight of Test session 41. Yaw damping has been introduced and is easing the task of the test pilot (Paul Heckles) who decided that it was time to try flying with no training gear: the sticks and coloured balls needed to show attitude to the pilot have been removed.

There is more flight testing to be done on motor speed control, on inceptor positions, on setting up yaw damping and installation layout of electronics before Phase 1 is complete.

Phases 2, 3, 4.

These phases await completion of the winged airframe.

Enabling technologies

The study, in describing the proposed Escort, has identified design features important to the aircraft concept. Freeing up the design of the wings is achieved by moving the proprotors to the fuselage. The forward field of view and field of fire is improved by tilting the proprotors back to pusher-prop position for the airplane mode. Tilting them one-at-a-time gives safe conversion between helicopter and airplane modes. These enabling technologies should allow the Escort to meet and exceed the specification and performance proposed in Table 2.

Proprotors

In choosing the tiltrotor physics for the Escort the study assumed 0.75 proprotor efficiency, and aircraft L/D of 9, in cruise. Then in Table 5 the proprotor efficiency is downgraded to 0.65 for untwisted blades. The disadvantage was addressed by targeting that the aircraft L/D to increase to 11.

fig17
Figure 17. This shows part of the 1/10th
scale model′s transimission.

Another solution needs assessing: varying twist during flight. The study was aware but unable to take account of progress by different teams that were researching and demonstrating variable blade twist for proprotors and other rotorcraft applications.

The high overlap of the intermeshing proprotors used in the Escort determines the effective disk and blade loadings that are fundamental to the rotors′ performance.

However the amount of overlap is also a useful method of improving efficiencies by balancing disk loading across the rotors. Figure 18 shows an assessment in hover, of how overlap could spread disk loading, at least along the lateral axis through the two proprotor hubs.

The figure assumes local loading is proportional to radial distance squared, includes losses at blade tip and root, but assumes uniform induced velocity, and uniform untwisted blades.

fig18 Figure 18. Local disk loading as a % of maximum; y if the lateral distance , left to right, through the rotor hubs; D the diameter of an individual disk. The plot is for disks that overlap by 0.3 D between hubs.

Maximum local loading will still occur near where blade tips pass at the longitudinal axis. Elsewhere, particularly near the lateral axis, the peaks are halved and the areas of zero lift are eliminated.

Wings

Once the wing design has played its part in achieving speed and range, further improvements in L/D must take lower priority than other wing issues: hover down-wash from rotors; location of universal wing store stations; wing fold for compact stowage; wing fuel cells; and booms to support the empennage.

As mentioned in discussing take-off performance in Table 4, the Escort needs greater articulation of wing surfaces than MV-22 to achieve equivalent low blockage of rotor down wash. This needs investigating.

Raising the tilt axis

Figure 4 showed the configuration chosen for the studies: twin booms to support the empennage and give room for the tilting proprotors.

If, however, the aircraft dynamics and aerodynamics would allow the tilt axis to be raised, and the rotors to be separated a little more, then extending the fuselage as a single boom through the lower "meshing gap" could be very attractive, broadly as Chris Gladwin′s suggestion (Ref. 15) for the 1/10th scale test model of Figure 19.

For the full scale Escort, the advantages sought would be reduced aircraft empty weight; simplified wing, fuselage, empennage design and manufacture; to achieve rolling take- offs and/or CTOL for higher payloads or higher altitudes; simpler deck handling; simpler wing fold; and simpler conversion and meshing systems.

fig19
Figure 19. This is a suggestion for a single boom configuration for the 1/10th scale model flight tests.

Flat-rated engine

At sea level, assuming that the engine has power in hand, the flight envelope of a rotorcraft is limited by the torque capacity of the transmission. In effect the pilot, torque limiter or equivalent device, flat-rates the engine for these low altitude conditions.

The selection of a suitable flat-rated engine would extend the Escorts performance to higher altitudes, potentially to the limits of its blade loading capabilities.

Manoeuvre envelope

If the engine is generously flat-rated, then the Escort is able to manoeuvre to the transmission torque limits. Again this leaves blade loading as a potential limit.

Lowering the blade loading would enable the rotors to pull higher g-forces at the maximum blade stall limited loading. If higher manoeuvrability is essential, then some increase in effective rotor solidity and/or in blade tip speeds may need to be considered.

Conclusions

  • The success of the MV-22 has created the opportunity for a gunship escort
    • The escort needs to be compact, agile, longer range and as fast as MV-22
  • The solution proposed is a compact tiltrotor
    • Place the rotors on the fuselage: this gives an efficient wing, and a compact and agile design
    • Tilt the rotors backwards for a wide field of view and fire
    • The configuration was granted US patent 7784923 in 2009
    • These studies show that it has excellent potential as a gunship escort for the MV- 22 Osprey
  • The next steps are proposed to be feasibility studies as a precursor to proposals for full scale flight demonstration.

References

1. Prouty, W. R., "Military Helicopter Design Technology", published by Janes' Defence Data, 1989.

2. Wernicke, K. G., "Mission Potential of Derivatives of the XV-15 Tilt Rotor Research Aircraft." AGARD Paper No. 19, Paris, France, April 6-9, 1981.

3. Bensahel, N ... [et al.]. "After Saddam: pre-war planning and the occupation of Iraq", published by Rand, 2008, ISBN 978-0-8330-4458-7.

4. Trask, J. T., "The Special Osprey: Impact on Special Operations Doctrine", Thesis presented to School of Advanced Airpower Studies, June 1996.

5. Whittle, R., "Marines want companion for Osprey--A tilt- rotor gunship could be boon for Bell Helicopter", The Dallas Morning News, July 5, 2004.

6. Fejtek, I. G., "Navier-Stokes Flowfield Computation of Wing/Rotor Interaction for a Tilt Rotor Aircraft in Hover", NASA Contractor Report 4532, July 1993.

7. McVeigh, K. A., Liu, J., O'Toole, S., "V-22 Osprey aerodynamic development - a progress review", The Aeronautical Journal, June/July issue 1997.

8. Poland, M., Nathan, S., "Q&A - Designing the helicopter of the future", The Engineer, 12 October 2009.

9. Piasecki, J. W., "Vectored Thrust Ducted Propeller (VTDP) Compound Helicopter Technology", SBIR Topic Number N91-317 (NAVAIR)

10. Brand, A., Kisor, R., Blyth, R., Mason, D., Host, C., "V- 22 High Rate of Descent (HROD) Test Procedures and Long Record Analysis", American Helicopter Society 60th Annual Forum, June 2004.

11. Prouty, R. W., "Helicopter Performance, Stability, and Control", published by PWS, 1986.

12. Leishman, G. J., "Principles of Helicopter Aerodynamics", 2nd edition, published by Cambridge University Press, 2006.

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