>>>>>Would you be so kind to describe how I would change the>Cdo on the 737 in FSX please.<<<<>>First, make sure you have an up to date .ini file for AirEd.>You can obtain one at: > ..........>Now you

>>>>Gee Whiz. I see my name at the top of REC 1101.<<<

Thanks Doug for the assistance. I changed the value to 62 as well as the TSFC to 0.5. Let;s see how that works. I will also experiment with the parasite_drag_scalar as Ron suggested to see how that works as well. Cheers to you bothRichard

> JSBSim allows one to set drag and lift somewhat as above. I>suggested Reynold's number be added, though it's>'polymorphism' allows one to set most anything as a function>of anything else in the XML data files.This is something I've wanted to do for a while. Unfortunately, I can't find a good reference to calculate it using our atmosphere model.Jon

>> JSBSim allows one to set drag and lift somewhat as above. >>I suggested Reynold's number be added, though it's>>'polymorphism' allows one to set most anything as a function>>of anything else in the XML data files.>>This is something I've wanted to do for a while.>Unfortunately, I can't find a good reference to calculate it>using our atmosphere model.>Jon It seems that Reynolds Number depends in part on 'air' viscosity and density. That ratio may be nearly constant at normal flight levels. The other part varies with V/l. I replied more on this in the AC and Panel Forum. Ron

Reynolds Number is given by:Re = ρ * vs * L / ν where:vs - mean velocity, L - characteristic length, μ - (absolute) dynamic viscosity, ν - kinematic viscosity: ν = μ / ρ, ρ - fluid densityIt basically represents the ratio between inertial force and viscous forces. In theory, aircraft with the same Reynolds number will have the same aerodynamic characteristics and as the Reynolds number changes so the will airfow. One of the significant effect is that below a certain Reynolds Number airflow will tend to be lamina, and turbulent above it.It's of interest in the design of aircraft but I'm not sure of its applicability to FS. There can be alsmost step changes as the flow goes from laminar to turbulent so, although Reynolds Number may indicate where that occurs, I don't think it will be of much help in modelling that effect at the level of detail of FS.

>Reynolds Number is given by:>>Re = ρ * vs * L / ν>the Reynolds number changes so the will airfow. One of the>significant effect is that below a certain Reynolds Number>airflow will tend to be lamina, and turbulent above it.>>It's of interest in the design of aircraft but I'm not sure of>its applicability to FS. There can be alsmost step changes as>the flow goes from laminar to turbulent so, although Reynolds>Number may indicate where that occurs, I don't think it will>be of much help in modelling that effect at the level of>detail of FS. Re affects skin drag, which increases at lower speeds. Thus, I've been increasing TBL 430, Cdm, at lower Mach values. While 'TAS' vs Mach isn't constant, adding some extra drag in TBL 430 does approximate the increase in zero lift drag at lower speeds. CL vs AoA also changes at lower Reynolds Numbers. CL may peak, decrease, then peak again at 35 degrees or so. RAF

> Re affects skin drag, which increases at lower speeds. >Thus, I've been increasing TBL 430, Cdm, at lower Mach values.It seems to me that making aerodynamic lookup tables with the coefficient versus Re would be preferable to coefficient value versus both velocity and altitude. I don't know if that's a logical comparison, but I had figured that indexing via Re would be preferable.Jon

>> Re affects skin drag, which increases at lower speeds. >>Thus, I've been increasing TBL 430, Cdm, at lower Mach>values.>>It seems to me that making aerodynamic lookup tables with the>coefficient versus Re would be preferable to coefficient value>versus both velocity and altitude. I don't know if that's a>logical comparison, but I had figured that indexing via Re>would be preferable.>Jon Section 5.8 of Olson's Handbook has some info on Re and Skin Drag. He mentions using 'Sutherland's Law' to calculate 'u', viscosity, as a function of temperature. Then, gives a formula for a Reynolds Number Index. A function of Theta^2, Delta, and T. "Then, RN= (7.101E+6)⋅M⋅l⋅RNI" So, Re is a function of Mach, Theta^2, Delta and T. Which should be easy to get from atmospheric and speed variables. "The drag coefficient due to skin friction is typically as much as 70 percent of minimum drag coefficient and is a significant factor in the corrections to the drag polar. It is typical that the Reynolds number correction is on the order of 1 drag count (0.0001 D C ) per 2,000 feet of pressure altitude. This is also a function of temperature, which cannot be ignored. For 10 degrees K off standard day, typically, a 1-drag count effect can be encountered. " In that case, zero lift drag coefficient decreases by about 0.7 * 0.001 going from SL to 20,000 ft. -0.0007. Cdo is typically 0.018 in jet transports at cruise, so one might figure an increase to 0.0187 at low flight levels. That may assume constant TAS. Since TAS increases in a constant IAS climb, the end effect would be quite a bit larger. Maybe near 10% in Cdp, and 7% in CD. Which certainly makes a difference in climb and descent rates, also in specific range. Ron

Thanks to all above who have participated in this discussion.I have been unable to find an accurate TSFC or SFC for the CFM56-7B engines used in this aircraft. I decided to search the SDK and the aircraft.cfg file for all factors that might affect fuel consumption of the default Boeing 737-800 in FSX. After numerous tests of various modifications, I have found that the following change to the aircraft.cfg file produces acceptable results for me.I decided against modifying either fuel_flow_gain or ThrustSpecificFuelConsumption in the Turbine Engine Data section.[GeneralEngineData] fuel_flow_scalar = 0.6500 // DEFAULT IS 1.000 At cruise I now get 6252 lbs/hr (934 gals/hr) which is likely still not technically correct, but closer to reality.Cheers ....... Wayne