Yes Achemha I’m talking about the remarkably aerodynamicly sophisticated cellulose composite masterpiece from Dijon that out performs all other aircraft in its class and not the aerodynamicly chalened over engined mistake from Toulouse that is out performed by all the other aircraft in its class.
There are two reasons for climb performance being critical, the first is to get the aircraft to the best cruise altitude ASAP and the second (with the DR400) to avoid a step climb due to oil temp issues.
Are you sure you’re talking about the DR400 — a simple wooden aeroclub plane — and not an A340? 
Tomjnx
The leg distance is part of the equation, with the DR400 you need to be traveling a leg 100 NM to make it worth climbing to 5000ft in terms of aircraft cruise performance.
There are two reasons for climb performance being critical, the first is to get the aircraft to the best cruise altitude ASAP and the second (with the DR400) to avoid a step climb due to oil temp issues.
My normal mission for the aircraft is UK South Midlands to the French/Spanish border non stop so getting the optimum cruise is important because the cruise forms so much of the flight, there are a number of other factors involved when selecting a cruise alititude but the instant real time fuel info that the Miniflo will give will be valuable for tactical decisions about the altitude to be at.
Operation at Vbg should in theory give the best endurance and climb performance but this does not take into account the best airspeed for the propeller with the DR400-180 the best L/D speed is 70Kt but the best rate of climb is 92Kt.
Performance loss with increasing weight is proportional to the payload % increase over the empty weight
That is more or less true for climb and acceleration, thus takeoff performance.
it is the climb performance that is the critical … in … cruise
That surely cannot be true, unless you operate the Robin close to Vbg. I cannot remember having seen any Robin operated like that, but then again I never flew one myself.
The effect an increased climb duration has surely largely depends on the overall leg distance (and final cruising altitude).
It is an EASA requirement that fuel flow devices indicate the same units as the fuel gauges.
I’ve never heard of such requirement. E.g. my TB20 had 0, 1/4, 1/2, 3/4 and 1 markings on each tank’s fuel gauge while fuel flow at totalizer was set to l/h. It was precise up to 2-3 liters per full tank (326 l – 86 USG) measured at least 20 times. However, my DA42 has built-in totalizer and somewhat more precise fuel gauges but the difference between totalizer and the amount of fuel topped-up on refueling is 2-3 gallons per full tank (289 l – 76 USG).
Achimha
Performance loss with increasing weight is proportional to the payload % increase over the empty weight, as the DR400 is a light airframe the payload represents a higher proportion of the MTOW than most GA aircraft.
I would agree with you that the change in IAS with weight is negligible, it is the climb performance that is the critical to the over all performance plot both in runway performance and cruise performance.
The DR400 has a unique wing form that works very well at high ( for an SEP !) TAS so if you are going any distance the DR 400 needs to climb to get to the highest cruise altitude that you can get as soon as you can and this is a function of power over weight.
Another DR400 issue is engine cooling, part of the remarkable performance of this aircraft is due to the reduction of the engine cooling drag to the minimum, the knock on effect of this is that in a long climb in a southern European summer the engine oil temp will redline in the prolonged climb, the action you take is to level off and select cruise power until the oil temp drops to normal and then you can resume your climb to the optimum cruise altitude.
To put it in a nutshell it is the ability to get quickly to the optimum cruise altitude that gets the best point or point performance and that is a function of power vs weight.
I could talk about using the DR400’s AUX tank fuel to keep the aircraft aft loaded to reduce trim drag as the use of this technique also requires you to know the exact location of the fuel to avoid driving the C of G out of the rear limit.
Peter
It is an EASA requirement that fuel flow devices indicate the same units as the fuel gauges.
We’ve just rewritten all the performance and operational checklists to reflect this, so we can live with it. I’d rather have the fuel gauges themselfs reset to either USG or Liters, but that mod does not exist.
The Shadin should be settable to litres or USG or even lb. What model is it? I have most of the manuals.
The gauges in my old Cessna 150 were quite unreliable. They were ok in the lower half but basically stayed on full too long and then went to half real quick.
On the Mooney, I was quite astonished to find out that the gauges actually work pretty well. The only issue I have with those is that they are in lb while the quantity indication on the fuel tank caps are in USG and theShadin fuel flow/ totaliser is in liters. All from before my time and not much can be done about it. I use the recommended fuel schedule which is run first hour on one tank, then switch to the other until it runs dry and the go back to the first tank.
When taking off from airports at say 6000 ft DA you may need to carefully consider the effect of weight on takeoff performance and obstacle clearance, assuming you don’t want to die. In the subsequent reduced rate climb you may also have an issue with clearing still higher terrain, deviating, circling etc, but it’s true that the additional fuel burn integrated over the extended climb period does not greatly affect cruise range in a subsequent long distance flight.
Not everybody lives where the land is flat for miles in every direction 
On some shorter local flights in mountainous terrain, time en route is more affected by rate of climb than by a substantial difference in speed. A 120 kt Pitts biplane can seem very ‘fast’… in the sense of getting there first.
That aside, I stick my tanks on one plane, with great accuracy, and then fly time on tanks. On the other I stick the tanks when possible (only possible if they are relatively full), now use an accurate fuel flow totalizer, and also fly time on tanks. On neither plane do I pay great attention to the gauges, and would only do so if I noticed them reading empty unexpectedly.
