20th Dec 2000, 23:04
I need some help in estimating some figures for part of my final year project. In order to calculate the range I need to know the specific fuel consumption per flight segment i.e. taxiing, t/o, climb, cruise, descent and landing using the following eqn:
SFC = kg / hp*hr
I have all the info to carry this out per flight segment except the hp. I need to know the % of max hp per flight segment. For example am I correct in thinking that cruise = 75% max hp
I will be doing this for a 90hp engine and a 120 hp engine.
Any help would be much appreciated.
21st Dec 2000, 00:35
Lady Heath, I'm afraid the answers to your questions will often be of the "it all depends on....." type.
As pilots, we tend to be rather conservative in spite of what might get written on PPRUNE. This is probably because we prefer the engines to stop after landing, rather than before, so fuel consumptions tend to always be rounded up.
To try and give some answers:
For 90hp and 120 hp aeroplanes a sensible cruise figure might be 75%, but the Pilots Operating Handbook (POH) for the individual aeroplane will often give fuel consumption figures for other settings. There will typically be tables of fuel consumption and Air Speed versus Manifold Pressure and rpm for different cruise altitudes and temperatures in the POH. The POH will also give the climb figures (normally only at gross weight). Incidently, 90hp and 120 hp aeroplanes would normally climb at full power.
These figures are for a new engine and aeroplane with a perfect pilot running the engine at the optimum mixture setting.
For taxi it is usual to allow a fixed amount. I allow 1 U/S gal in a light aircraft.
For a flight under instrument flight rules (IFR) we add 45 mins fuel for holding, plus enough fuel for a diversion to the alternate airfield. I also add 15 mins fuel for the approach and then add the whole lot up and add another 5% for contingency.
For a flight under Visual Flight Rules in good weather in a small light aircraft with fairly limited range (only 12 UK gals) I will sometimes fly the return leg of a flight knowing that I will only have 20 mins usable fuel left when overhead my home airfield. However, I have an alternate airfield only 5 mins flying time away. I am prepared to do this in the above circumstances, but others might not.
Good luck with your project !
21st Dec 2000, 01:05
Might I suggest you select a specific aircraft for each engine size then use the figures and graphs from the POH. Everything you need should be in there(somewhere).
Your next question is bound to be: "Where can I get a POH?" Perhaps try to befriend an owner and ask to borrow it? :)
21st Dec 2000, 01:22
For the engines I have figures for (160 hp and 200 hp) the SFC works out pretty close to 0.20 kg/hr/hp, at most flight power settings. It's slightly better at higher powers, though you'll do slightly worse in the climb itself as the mixture is usually set over-rich for cooling.
Typical numbers would be 100% for t/o, 80-100% for climb (max available power falls with altitude, e.g. 75% at about 7000 ft for normally aspirated), 55 to 75% for cruise, and something like 40-50% for a cruise descent. I haven't got a clue about taxi -- I'd guess a lot less than 20%.
Hope that helps.
21st Dec 2000, 03:08
If it helps:
I have a Lycoming 180 in a Robin DR400.
Max continuous rpm is is 2700. We run it in the cruise at 2500 rpm.
Our AVERAGE fuel burn incl taxiing and climb is 8.8 imp galls per hour. At 2100rpm it is 6.6 imp galls per hour.
21st Dec 2000, 20:24
Get hold of the the book Aircraft Design(A conceptual approach) by Daniel Raymer it gives you all the information you need.
21st Dec 2000, 22:15
Thanks guys for your replies,
The problem is, my project is a design study and performance comparison of installing a diesel engines into light aircraft. There are 7 manufacturers at the moment. Wilksch airmotive have successfully test flown a Europa with 120hp diesel. Diesel engines have many advantages over piston engines. I know the range is increased but I need to know by what extent. My two case studies involve:
-The 120hp diesel engine compared to the 80hp Rotax912 installed in the europa
-the zoche diesel engine into the islander.
It is a really interesting project however I cannot get a hold of a lot of info - esp on the diesel engines as the manufacturers have to be fairly secretive at this stage and therefore i am having to make intelligent guesses.