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Thread: Mission requirements review - N19WT rebuild

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    bwilson4web's Avatar
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    Mission requirements review - N19WT rebuild

    INTRODUCTION

    NASA and the DoD have system engineering processes that include a requirements review. This includes what has been done, what needs to be accomplished, and architecture. System engineering is a flexible process that allows feedback, tweaking, to accomplish the goals and objectives. The purpose of a review is to encourage such early feedback. So please enjoy and feel free to add comments. The general outline:
    • Background - what has been done in the past with emphasis on 'lessons learned'
    • Requirements - what we want to accomplish
    • Architecture - one or more proposed solutions
    • Risks - area where special, early investigation are needed
    • Costs - the expected budget
    So enjoy and we'll discuss the N19WT rebuild project.

    BACKGROUND

    In 1976, I bought a Cherokee 140 and hired an instructor to teach me to fly. Four years later I had to choose between wife, balanced check book, and airplane . . . I still have the wife and my checks are good. But owning that Cherokee for 320 hours taught a lot of valuable lessons:
    • 8 gallons/hour and 50 gallon tanks - the 150 hp engine burned 8 gallons/hour pretty much every hour it flew. This gave a 5+ hour cruise but this was 'bladder limited' after I met my wife.
    • For every 100 hours - 92 hours were pure VFR with multiple landing sites easily found; 6 hours were marginal with the only emergency landing sites being directly below the plane, and; 2 hours were unplanned and not.
    • VFR every 100 hours - ~70 hours were day VFR and ~30 hours were night VFR.
    • Terrain for every 100 hours - ~20 hours were over forested hills and mountains and ~2 hours were over bays where due to airspace limitations, one or the other shore was outside of engine-out, glide distance.
    • 100 mph IAS - climb prop limited to support small airfield operation.
    • 65 mph IAS approach speed - any slower and the controls became mushy. Below 60 mph, the controls were more 'suggestions' than positive, operators.
    • two people with luggage - but too often the luggage included stuff that was never needed nor appropriate for the duration of the flight. But they sure required someone to load it in the airplane, take it out at the destination, reload it when returning, and unloading when back at home base.
    • four people - like loading all four seats in a VW Beetle, it can be done but the folks in the rear seat suffer from noise and limited visibility. Worse, they often had no understanding of what was going on and could not appreciate the speed versus their discomfort.
    • Paradox of flight certified but outdated technology - a certified aircraft is all but immune from modern technology. The current certification system all but forces use of "flight qualified' hardware and denying modern, lower weight, higher performance, technology. Magnetos, carburetors, fixed pitch props and low compression engines . . . The aircraft owner, I was all but forbidden to do anything on my Cherokee 140.
    I don't really care if someone else's experience has been different. This is my experience in airplane ownership and based upon 3+ years of ownership and 320 hours. No sophistry can change the past reality.

    MISSION REQUIREMENTS
    • 4 gallons/hour and flight duration of 3.5 to 4 hours to match bladder capacity
    • Safe return to earth regardless of weather, terrain, or equipment failure . . . night VFR over hostile terrain included
    • two people with 'val-pack' luggage - everyone sits on their luggage and no more extra stuff
    • 65 mph approach speed - with little or no risk of stall-spin
    • engine
      • compression ratio that uses 100LL octane
      • electronic ignition with sensible spark advance
      • hot spark to blast through 'crap in the gap'
      • automatic mixture to avoid 'crap in the gap'
      • eliminate carburetor ice risk
    • instruments
      • GPS with track recording to replace VOR
      • engine and system detailed data recording for post flight analysis
      • LED lighting to reduce electrical overhead
      • circuit breakers for 'one reset' operation in flight
      • light weight MEMs based instruments
    • propeller
      • in-flight adjustable to optimize engine and airframe performance
    ARCHITECTURE

    It turns out that the Viking Dragonfly meets these requirements and in particular, N19WT:

    These are metrics from the original, N19WT, flight tests using a 60 HP, 1835 cc, HAPI engine. Given the standard 170 lb pilot and passenger weight, 340 lbs, this is a two people plane with 26 lbs for her purse.

    This aircraft is a canard design:

    As long as the plane weight and balance is correct, they are difficult to stall-spin. If pulled back at low speed, the canard stalls first automatically lowering the nose. There are numerous anecdotal stories of these types of planes going into canard bobble yet remaining in control by the pilot and passenger surviving the subsequent crash. Furthermore, the pilot and passenger are located between the canard and wing which serve as crush structures to protect the cabin.

    Fortunately this airplane has over 100 flight hours and 150 hours on the Hobbs meter. It is the classic airplane in a barn having spent the past 25 years in hangers. The second owner bought a Mooney and this plane was left alone.

    RISKS

    Searching the National Transportation Safety Board reports for the Dragonfly:

    So there have been roughly three sources of crashes:
    • landing - the original canard tip wings were sensitive to hard landing induced oscillations but they have also been seen in the Mark II style with landing gear mounted 1/3d of the way out the canard.
    • engine failures - not just the HAPI but other modified VW and modified engines have failed in flight.
    • pilot skill and handling - pilot skills and training remains a challenge regardless of aircraft.
    Landing Risks

    Although the Mark II reduced the frequency, it did not eliminate the risk. Close examination of the gear reveals:

    The wheel is held by a very stiff, fiberglass strut. Any landing force compresses the tires which work like an air-spring to rebound the airplane back into the air. There are no energy absorbing struts or bundle of shock cords to absorb the vertical landing load. In contrast, my Cherokee had oleo struts that absorb landing energy to prevent tossing the plane back into the air.

    Mitigation
    • steel spring gear - provides more travel to absorb part of the landing load
    • custom oleo or shock cord bundle gear - some design effort required
    Engine Failures

    Fuel contamination from debris in the fuel tank is one source but there were several instances where the engine failed from fractured crankshafts at the propeller hub. One using a vehicle control system acts as if it had gone into a 'limp home mode' . . . in flight. But the systemic problem seems to be ground vehicle based crankshafts being coupled to a propeller load with undersized bearings for flight loads and metal fatigue failure. Sad to say the 1980s HAPI engines taught us valuable lessons about the risks so that engine is gone:


    Mitigation
    • Perform a trade-study to identify a replacement engine
    Pilot Skills

    The human factor remains a common risk regardless of airplane. When I sold my Cherokee 140, I stopped flying. I knew that if I didn't fly at least every three weeks, my skills and judgement would atrophy making me an unsafe pilot until I did some practice landings and radio work. But I also payed close attention to accident reports and analyzed my own performance. The combination of marginal VFR, flight over hostile terrain, and night flight remains a pattern that realistically is going to happen again.

    Mitigation
    • Ballistic Recovery System - whole aircraft parachute system so when the pilot gets in a place that can not be flown out, bring the plane and passenger down safely.
    COSTS

    These are my budget numbers.

    Already expended:
    • $4,500 - airframe
    • $1,700 - trailer, $900 salvage value
    • $2,000 - hardware and supplies
    • $500 - tools
    Expected major expenses:
    • $12,500 - estimated engine replacement, 0 hours, aircraft design
    • $6,000 - BSR system (two vendors)
    • $3,000 - flight adjustable propeller
    • $500 - landing gear, steel or a trade study
    • $5,000 - GPS, Mode-C transponder, ELT
    • $2,500 - refurbish or replace existing turn-and-bank, altimeter, e.t.c.
    • $3,000 - pilot retraining
    • $1,000 - tools
    Recurring costs:
    • $125/mo. - work shed rental, one mile from home
    CONCLUSION

    Flying isn't cheap and there are no guarantees but sometimes things come together in a way that allows us to enjoy our time on earth. This is my plan and God is laughing.

    Bob Wilson
    Last edited by bwilson4web; 03-19-2012 at 10:35 PM.

  2. #2

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    Are the Mk II Dragonflys with the hoop style gear noted for being bouncy as well?

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    bwilson4web's Avatar
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    Quote Originally Posted by martymayes View Post
    Are the Mk II Dragonflys with the hoop style gear noted for being bouncy as well?
    I don't know. There is a fly-in at St. Elmo AL this weekend and I plan to be there. I'll ask.

    Bob Wilson

  4. #4
    bwilson4web's Avatar
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    Quote Originally Posted by martymayes View Post
    Are the Mk II Dragonflys with the hoop style gear noted for being bouncy as well?
    Actually you brought up a good point. All known gear configurations should be part of a "trade study". This includes tricycle, bungie cord, conventional hoop, and any others that might have some practical use. Heck, I remember once seeing dual-hoop gear making a four wheel configuration on a light airplane but I don't remember it being for the Dragonfly. I think it was for some sort of roadable car/plane.

    A 'trade study' is how we narrow down the options to come up with practical solutions for more detailed study.

    Thanks, Bob Wilson

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