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paul(at)eucleides.com
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 Posted: Thu Sep 11, 2008 10:42 am Post subject: engine selection |
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On Thu, September 11, 2008 10:18 am, JetPilot wrote:
| Quote: | I don't have the money and time to develop a new engine choice for Kitfoxes,
but I hope someone does. This could be the new light alternative we need to older
technology Rotax and Jabiru engines.
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This debate has been going for so long now, it would seem to me that what is needed is
an airframe design with heavier engines in mind. That is, one with a higher gross
weight such that a useful load for two standard people and 4-6 hours of fuel would be
perfect while still maintaining a relatively low stall speed and rugged enough for
some off-airport operations.
Maybe alter a couple of the existing designs for longer wings, slower flight and
thicker wings. I assume the gross weight is determined primarily by wing strength. I
never see designer/engineer's comments on this list or on usenet so I don't know what
limits gross weight for the airframe design. Maybe if one were to start the process,
you'd end up with a plane that uses one of the little Rotax two-strokes. IIRC, gross
weight is determined by wing strength and the loads go up with speed so reducing the
maximum speed to reduce maximum wing loading negates the need for a heavier engine and
we're back to the starting point.
| Quote: | Attached are a couple pictures.
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Mike, where were those pictures taken? The foliage looks like it could be around here
(western Washington) someplace.
--
Paul A. Franz, P.E.
PAF Consulting Engineers
Office 425.440.9505
Cell 425.241.1618
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Tom Beirne
Joined: 02 Apr 2008
Posts: 13
Location: Ireland
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| Posted: Sat Sep 13, 2008 3:58 am Post subject: Re: engine selection |
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| Quote: | | This debate has been going for so long now, it would seem to me that what is needed is an airframe design with heavier engines in mind. That is, one with a higher gross weight such that a useful load for two standard people and 4-6 hours of fuel would be perfect while still maintaining a relatively low stall speed and rugged enough for some off-airport operations. |
This is what the Series 5 and above were designed for.
In answer to your question regarding Gross weight, design gross weight can be limited by a number of factors;
Aerodynamic- stall speed, takeoff distance required, climb-rate etc. all increase with higher wing loading. Depending on the CG the stick load forces may be critical on larger designs (ie. when loaded to gross the distribution of the loads may such that they put the CG at either limit)
Legal - LSA limitations for example, minimum 1 engine-out climb performance on a multi-engine design etc.
Structural - aero structures are designed with typically a safety factor of 1.5 so the design loading increases very rapidly with airspeed. Turbulence is the designers enemy here and the structure has to be able to withstand a specified minimum up-gust loading at Vno (top of the green arc). Again at Vne (top of the Yellow arc - beyond here be dragons) there is another minimum up-gust to design for. That's why the yellow arc exists, only to be entered on smooth air conditions. On most aircraft the highest bending moments and shear loading will occur at the spar attachment points. On the Kitfox the spar attach and carry-through structure are light by necessity but very strong considering. We know that the aircraft will perform slightly overgross - it will climb from sea level and happily exceed Vne with a 582 in straight and level flight if you firewall the throttle. The problem is that one bump could be your undoing, it may not break the aircraft but may well bend bits that are important and once plastically deformed they are much weaker and will never stand up to the same kind of loading again.
Limiting Vne is one way in which to increase the Gross limit with no structural change but this can only be specified by the manufacturer and must be proven out at the design certification stage, in other words you cannot just placard your ASI thus moving the goalposts. Small redesigns of the structural layout are another method eg. there was a change made to the lift strut layout between the II and III that increased the Gross weight from 950 to 1050lbs
Taken from http://www.kitfoxaircraft.com/Model%20III.htm
| Quote: | | A larger vertical stabilizer and rudder were added, as well as larger, stronger lift struts and spar carry through tubes in the fuselage. The gross weight was increased to 1050 pounds, but the empty weight only went up by a few pounds. |
And again between the IV and Classic IV the Gross weight went from 1050 to 1200lbs
| Quote: | | The Classic 4 has heavier lift struts and gear legs, as well as beefed up carry through tubes in the fuselage, to allow for a higher gross weight than the earlier Model 4 kits. |
So a design engineer can pick the max gross weight that satisfies all of the criteria. If he is not happy with the limiting speeds he may choose to beef up the structure, thus increasing weight and effecting performance or exceeding certification criteria. Or maybe he chooses to limit the performance to meet his needs but in doing so effects the marketability and usefulness of the design. Its a bit of a tightrope walking act to get the optimum balance and create a good all round design.
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