Peter wrote:
So you lose 1kt for 1 person, basically.That’s not implausible but also is so little that most pilots won’t notice it.
In the TB20 the loss is less – probably under 1/3 of that. No idea why.
I’m willing to bet that’s @ SL or at least under say 5,000’.
As we learned earlier in this thread, the angle of attack is what changes with weight and air density. So whilst you may see only a slight loss in TAS with additional weight, that increases significantly as the IAS decreases and the AoA increases.
Then there’s the climb performance penalty which means you’re burning more gas and spending more time at lower less efficient altitudes.
So you lose 1kt for 1 person, basically.
That’s not implausible but also is so little that most pilots won’t notice it.
In the TB20 the loss is less – probably under 1/3 of that. No idea why.
A late datapoint for the discussion:
Cessa revised the POH for the C172S last year. The new version gives performance figures for cruise flight with less than maximum mass.
At power settings of 65% and above, cruise speed will increase by 1 kt/150 lbs reduction in mass.
At power settings below 65%, cruise speed will increase by 1 kt/125 lbs reduction in mass.
This agrees rather well with bookworm’s formula at the beginning of the thread.
which persistently does 138kt IAS (low level e.g. 3000ft) at 11.3usg/hr, 2400rpm>
Like the 11.3usg/hr = say 43 liters per hour.
Specifying the type of gallons (imp. or us) leaves no doubts.
Temp was -4
To convert to TAS I would need the temperature also.
At ISA i.e. -7C (unlikely for Croatia right now) I make that 136 TAS which is in the right ballpark. At say 0C it would be another 2kt or so.
To be doing close to 140 TAS on a genuine 9.0 USG/hr is very good going! Assuming say 6USG is burnt in the climb, 86 usable, that gives you a zero-fuel range of ~1300 NM which is, hey, just about all of Europe 
I have seen 1350nm showing as the fuel range on the GPS, from the top of the climb, in very light winds, so we are similar.
maybe during the weekend 3000 ft will be VFR
Unfortunatelly it wasn't, so weekend flying was IFR and I recorded performance only at F110: - peak EGT 2300/19.5 (full throttle) 9 gal/h 115 KIAS - best power 2300/19.5 (full throttle) 10.5 gal/h 120 KIAS
BW,
Enjoyed that post.
P,
Re CofG the further rearward less downward lift is required to be in balance and therefore the wing has to deliver less lift to keep one's jallopy flying along and the excess power is used to motor along faster rather than being afloat.
That is also one of the reasons why a tailwheel version aeroplane will go faster than its tri gear equivalent.
However having read on another post the problems with Champion spark plugs, could these be a factor?
One would think that there is no way spark plugs could cause a transient loss of power, without it showing up as a transient variation in EGT.
At a constant RPM and IAS, the airflow into the engine will be constant and the fuel flow is determined by the airflow - regardless of whether the engine is burning the stuff or not. So these may not tell you anything.
But plugs which are bad enough to affect performance should IMHO affect the EGT in a very obvious way, if the variation is transient.
Some very interesting comments and I will follow up. However having read on another post the problems with Champion spark plugs, could these be a factor? the engine is a Continental IO360 and I have to say that until I read the piece about them I had no idea. I wonder how many maintenance organisations are aware. Will the effect of the faulty plugs be as noticeable in a normally aspirated engine?
