Question on Helicopter Impact on Ground

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MSI
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Question on Helicopter Impact on Ground

Post by MSI »

Q: Posted to INCR, "I've been asked to calculate/estimate the forward speed of a helicopter on impact with the ground. Briefly, it landed with forward airspeed on its
skids and slid for 9.6 metres then rolled due to a drop off in the terrain. has anyone derived a drag factor?

A: We normally don't cover helicopter crashes, however the following may be of assistance/use:
A 1987 research study we did with Calspan includes discussions of drag forces in various soils and can be found at: (These reports contain proprietary HVOSM program w/extensions and refinements provided by McHenry & McHenry)
One of the references in those reports:
  • Avery, JP Larson, S "Analysis of Army Fixed-Wing Cargo Restraint Design Criteria" USAAVLABS Technical report 67-3 PB#651219, Jan 1967
Included in that report was research on the motion resistance offered by clay-sand soil which was measured and a mathematical model was postulated as follows:
  • FT = f*FN + v*AS*Xdot
Where
  • FT = resultant tangential force, lbs
    F = nominal friction coeffcient
    FN = normal load, lbs
    V = viscous damping term, lb-sec/ft**3 (soil shear strength)
    Xdot = velocity ft/sec
    AS = total contact area, f**2
The model of motion resistance in soil presented by the authors offers both a simplistic approach as well as inclusion of the viscous damping term which is intuitively correct
(i.e. one would assume that plowing forces associated with 5 MPH and 50 MPH under similar conditions of sinkage, etc would be different)
See the full Calspan report for addition references (Bekker, Schuring, etc) related to motion resistance of soils.