The following analysis is an attempt to determine the maximum safe range
of MY aircraft under normal flying conditions at its 'brisk' cruise
setting, whilst following a previously prepared flight route. As a
consequence of this, 'single leg' destinations in excess of this
distance are ruled out, preventing ambition getting the better of good
sense !
I realise some will not find this analysis relevant and apologise for
the waste of bandwidth !. However, from those who do find it interesting
and have the patience to read through to the end, I welcome any
constructive criticism, off-forum. I'm no aeronautics specialist, though
I've tried to be as rigorous and dispassionate as possible. The question
"How far can I safely fly in a single hop ?" is I think, relevant and is
subtly different from "How long can I fly for ?".
I own a monowheel Europa 'Classic' 912 with plenum cold air modification
and speed kit fitted. In most respects it is typical of type, though the
fuel tank is larger than normal, (83 litres) and the standard 3 blade
Warp Drive, ground adjustable propeller is slightly coarse. (Giving a
maximum climb RPM of around 5100). All estimates assume the aircraft is
loaded to near its limit of 1300 lb (591 kg). However, save for climb
rate, I haven't yet detected any significant effect of loading on
performance.
I've chosen to tackle this issue in two, different ways:
A/ Calculation of the range limit based on known aircraft and
environmental constraints.
B/ Practical measurement of average performance over statistically
representative longer trips.
Given that I have a tendency to fly everywhere at approximately the same
cruise setting, I would expect close agreeement on average speed, if the
number of trips considered in B/ is quite high.
A/ Range Calculation.
In order to determine a realistic maximum range for my aircraft, I need
to consider:
1/ Fuel burn rate --- Not greater than 17 - 18 litres per hour at
around 5200 RPM (Flydat) and circa 1500ft AGL.
2/ Fuel tank capacity --- 83 litres to the top of fuel filler
pipe, giving about 69 litres useable on main.
3/ Estimated ground speed when following the track. --- NOT the
straight line performance in zero wind.
---From the above, I have come to expect that my aircraft will fly safely
for 3hrs 50 mins at 127 kts TAS at the brisk cruise value stated above,
with a minimum of 15 mins flying time remaining in the main tank and 25
mins in reserve.
---From a practical standpoint, 'Estimated ground speed' (3) is highly
variable. For range calculations, the lowest normal figure should be
used. The following components have an effect. Mostly, they reduce
ground speed.
1/ Headwind or tailwind ?
The headwind or tailwind component of airflow, relative to direction of
travel. This is typically the largest single variable. Around half of
the time, the net effect is beneficial.
It's my practice to deduct around 10 knots from straight line
performance figures for pre-prepared range estimation. Most plans are
conceived before wind direction and strength are known. From personal
observation, the range appears to be up to +-12 knots on typical flying
days in the UK (light breeze at ground level). Obviously, estimates may
be amended on the day of the flight when transit weather becomes known,
though this remains the true 'wild card' for predictive purposes. I've
also assumed transit and destination weather have been checked and are
similar.
2/ Crosswind component retarding effect on track flown.
At Europa speeds, this is much smaller than headwind/tailwind effects
and very nearly gets discounted. Usually reduces progress by a maximum
of one knot/hr, frequently significantly less.
3/ Altitude changes in transit.
Climbing and descending en-route as per flight plan. Also influenced by
ATC instruction, traffic avoidance, terrain and local weather factors.
Fuel burn may temporarily increase to 23 litres/hr or more and airspeed
may decrease to 80 knots or less. Fuel use will correspondingly decrease
when the subsequent descent is initiated, though the extra fuel
used/speed lost will not be fully reclaimed. I have costed this
behaviour at around 3 knots/hr, using my calculator and a bit of
practical experience. Obviously, it will vary and this must be bourne in
mind.
4/ Enforced diversions from ATC for traffic avoidance and weather
diversions.
Difficult to foresee and in practice I find this happens rarely. I have
not made any allowance for this in my standard range limit calculation
due to the sporadic nature of its occurrence. In practice, I would
either be forced to dip into reserves if overall progress was poor, or
forced to divert to an intermediate airfield. (on longer journeys to
Europe, Customs requirements would then become a consideration)
5/ Unintended drift from track.
Oh dear ! From past experience, I normally add up to 5% for this (around
6 knots). Clearly with a good deal more concentration and perhaps the
judicious use of my wing leveller/autopilot, I might usefully regain a
couple of knots.
6/ Cruising altitude selected.
I've read that airspeed increases approximately 1.5% for every thousand
feet. Unfortunately, altitude compensating carburettors aside, a
normally aspirated engine runs richer, and less efficiently at altitude.
That said, my limited high altitude flying in France (8-10000ft)
appeared to return a measurable speed gain without any attendant fuel
consumption penalty. However for most purposes, it isn't practical to go
as high as this, so I've assumed the 1-3000 ft AGL range for my
calculations. I'd be interested in others' experience of high altitude
flying and may investigate further, when next appropriate.
7/ Anticipated overheads at start and end of flight.
In addition to allotting a realistic fuel quota for taxi (at both ends
!!), power checks, climb, descent and normal circuit protocol, there
needs to be a contingency for the unexpected.
Our variety of quiet farm strip flying generates a complacency which is
often shattered when there is a requirement to 'fit in' with the needs
of others. Often after start-up, there are air traffic delays due to
congestion, both before take-off and prior to joining the destination
airfield circuit. Occasionally, I've even had problems locating the
destination airfield. (typically, well disguised farm strips, I might
add !) Also, at large airports, the taxi protocol can be lengthy.
I suspect the best solution to this problem is to factor in a set amount
of extra time depending on familiarity with the journey in question, or
known constraints. I wouldn't want to arrive at my destination with less
than the previously stated reserves available. Therefore, I suspect that
5 - 15 mins (sometimes more!) may need to be deducted. Consequently, I
will reduce my range expectations by 20 nautical miles (around 10 mins
at normal cruise) to account for 'normal' delays.
Taking all of this into account, I reckon my 'rate of progress' for
range purposes, along a track of known length (Overhead A to overhead B,
where A and B are more than 100 miles apart) is predicted as follows:
Stated cruise minus (headwind/crosswind + climb/decend time + drift):
127 - (11 +3 + 6) gives 107 knots at just under 18litres/hr.
Therefore, at this setting, the theoretical maximum range for my
airplane against a regular stiff headwind is around 410 nautical miles.
This is automatically reduced to 390 Nm (450 statute miles) due to the
considerations in item 7/. Notwithstanding all of this, on occasion, I
would still expect to dip into fuel reserves.
However, using the same calculation, I could still expect an average
rate of progress of around 116 knots for multiple journeys. This is
based on the fact that airflow is typically only slightly retarding
(2kts estimated).
B/ Practical rate of progress, drawn from my log book and records.
Eight distinct and distant destinations were chosen as representative.
For all of these I had recorded overhead to overhead flight times, the
weather was fair, and from memory a track was followed reasonably
accurately. The times were added and averaged to produce ... 115 knots,
with upper and lower limits of 107 kts and 120 kts respectively.
Yes, in all the cases above, I used more or less same cruise setting !!!
In many of these cases, recorded engine on-off time was around 10
minutes greater, re-enforcing the arguments put forward in consideration
of 7/.
Destinations considered were: Rayne Farm to Sandown, Le Touquet, York,
Guernsey, Jersey, Bradley, Sorbie, Toussus-Le-Noble (Paris).
Conclusion.
My two figures agree fairly well. I'm not so surprised. Under these
circumstances, I shouldn't normally consider destinations separated by a
route of more that 450 statute miles unless the overall effects of
airflow are known and factored, or I am prepared to calculate the effect
of reduced RPM on range.
Finally, I'm aware of the straightforward observation that if I LIVE A
LITTLE and reduce engine RPM I will of course, extend range. Now that's
a whole new argument......
Perhaps more figures and an Excel spreadsheet would be in order !
Alan
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