Weight & Balance Whisper Express

Questions you would like answered about the Whisper Motorglider.

Weight & Balance Whisper Express

Postby PeterB » Wed Apr 11, 2007 12:47 pm

On your Spec page you have the all up weight at 720kg and the empty weight at 450kg.That leaves a total of 270kg for fuel pax and luggage.If you have full tanks that leaves 185kg for passengers and luggage.I weigh in at 105kg so that does not leave much for my pax and luggage? In other words are these figures very accurate and to be obeyed?
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Specs allowance

Postby jonimike » Wed Apr 11, 2007 1:10 pm

PeterB,

The MAUW is not to be abused, for obvious reasons.

However, look at the fuel consumption, translating into 80 Liters of usable fuel.

That gives you a realistic, (remember higher consumption on take off and climb) endurance of 6 hours, at a range of approx. 1080 km, with a small reserve.

Surely, if you are to compromise on weight related issues, it makes sense to put in less fuel, and plan your flight accordingly.

Six hours on end in a light aircraft cockpit, is a very, very long time, and IMO not advisable.

Air above the inversion layer is very dry, and you will need to constantly hydrate. With the obvious result.

Your bladder will force you to obey the mentioned suggestion of less fuel after your first attempt at a six hour endurance flight. :roll:
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Body Hydration v Engine Hydration

Postby PeterB » Wed Apr 11, 2007 2:23 pm

I get your point but my feelings are that the 6 Cylinder Jab will propably only get to around 19l/hour @ 75% power as per Len which tells me 6 hours endurance will be a little on the optimistic side.Also when you worked out the specs what weight did you allow for the engine as the Jab 6 Cylinder comes in around 81Kg.Will the Manufacturer work out an exact weight and balance for each Whisper they make?
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Postby jonimike » Wed Apr 11, 2007 8:50 pm

The 6cyl Jabiru engine max output is rated at 120hp.

You dont need to run at 75% rated power to make 180km/h. you can run at substantially less, as the trade off of excessive power does not equal a similar sized increase in airspeed, but the opposite is true, in that similar rated performance/rated power output, should produce the same airspeed i.e.

75% of 80hp= 60hp (Aerovee or Limbach engine options)

75% of 120 hp=90hp(Jabiru 6cyl)

50% of 120hp= 60hp (Jabiru 6cyl)

To have approx similar performance, bar some minor issues about dissimilar configuration, the equation will hold reasonably true, that with the Jabiru running at 50% rated power, you should make 180km/h.

So, the fuel consumption at the same airspeed (180 for both engine options) will be very much less than 19L/hr for the Jabiru, because of the lower power setting, and suddenly 5+ hours endurance is not unrealistic.

To come back to the earlier statement of setting 75% of the Jabiru to produce 90hp, this setting is 50% higher than the other engine option at the same power setting, but will definitely not produce an increase of 50% in airspeed.

And yes, the builder will work out the mass and balance according to the engine and options fitted, it is part of the registration process to provide proof of doing this, prior to obtaining ATF.

Your option of fitting a jabiru 6cyl is a beautiful combination. Do not worry yourself to death about retaining all other aspects of the specifications as listed. Put in a little less fuel to allow a pax and sandwiches.

Build the plane, keep it within the MAUW and fly it. Good luck!
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