Fuel Flow high

ghrasko

I am testing v0.3.0 and fuel flow seems to be high:

ALT: 7000 ft
OAT: 1 deg C (standard atmosphere)

Based on best economy power setting table in ODM page 12
Power: 55% (110 BPH)
Prop: 2200 RPM
MP: 22.1 inHg
FF: 10.9 GPH instead of 8.5 GPH (ODM page 16)
TAS: 125 kn instead of 120 kn

I will check in other ranges as well. Do you experience similar discrepancies?

BernieV

Are you having problems leaning to 8.5 GPH or is it when you do the speed is wrong for 55% power at that altitude?

ghrasko

@BernieV, I am not leaning to fuel flow, but to Manifold pressure, based on the performance table. But I will try. Maybe then FF and speed will be ok, but manifold pressure not.

BernieV

@ghrasko power is a function of RPM, manifold pressure, density altitude, and fuel flow. You need to set the appropriate fuel using the mixture control. Don't forget it's got a lock on it which can prevent you from restricting fuel flow below a certain value

ghrasko

@BernieV are you sure? Prop is handling RPM, throttle is handling manifold pressure. Mixture should be set to peek EGT (economy) or 100F rich of peek EGT (max performance). I use the official performance rables.

ghrasko

Basically what I wrote, is not contradicting with what you had written. I mean, I follow the performance table settings.

ghrasko

@BernieV, I will try your approach tomorrow:

set RPM with prop
set MP with throttle
set fuel flow (instead of EGT) with mixture...

... and I check speed.

I learnt sometime ago that it is better to set mixture based on EGT and not based on FF.

set3times

I believe the peak EGT is reached at a too rich mixture. Leaning for the correct best economy fuel flow gives you an approx 100F LEAN of peak, instead of peak egt, which is incorrect. That might be a msfs limitation or the simulated pa28 one, I am not sure.

BernieV

I also suspect that EGT is not being modeled or displayed correctly. Try using fuel flow and see if you get to the expected speed for the altitude you were testing. Don't forget most speed graphs are based on testing at gross weight.

set3times

Yes speed is pretty close to ODM values at max gross weight and fuel leaned to fuel flow and not egt, so the issue seems to be the EGT indication and peak.