Performances penalties for loaded planes

[Francis marliere]

Gentlemen,

I understand that planes carrying loaded with ordnance have their speed, climb rate, roll rate and acceleration reduced by the load and the drag. However, I don't know the scale of the penalties (ie is climb rate reduced by 20% or 50% ?). Do you have an idea ?

Thanks for any help,

Francis Marliere

 

[Shortround6]

Unfortunately it depends on a lot of different things, speed is much less affected than climb by increased weight if the load is internal and causes NO increase in drag except increased incidence of the wing.

hanging things on the outside causes a lot more problems and because there is interference drag between external bombs/drop tanks ( not all 500lb bombs are even shaped the same) and wing bottoms, fuselages, engine nacelles, etc, none of which are shaped the same (or rarely so) the penalties change for different airplanes and even a bit for different speed ranges.

 

[Francis marliere]

Shortround, thanks for your reply. I understand that things are complicated and one can't have the exact numbers. However, what I am looking for is an approximation. I gess that speed loss for a loaded plane may be about 20-25%. I assume that the penalty is more important for climb rate (50% ?) and even more for roll rate, but I have absolutely no information to confirm. Do you have historical exemples ?

Thanks,

Francis

 

[FLYBOYJ]

In the back of many flight manuals there are charts that show how the aircraft will perform under loaded conditions.

 

[Ivan1GFP]

In some cases, the reduction in performance may be due to other limitations such as aerodynamic load limitations rather than a simple reduction that is calculated from the increased drag of external ordnance.

From reading about US testing of the the Mitsubishi J2M3 Raiden, the maximum speed was 417 mph but with a drop tank installed, speed dropped to 371 mph which is the figure quoted in a lot of books.

- Ivan.

 

[Reegor]

In the back of many flight manuals there are charts that show how the aircraft will perform under loaded conditions.

Yes. For example I have spent some time with B-25 performance data. There are tables for different weights, and also for external items with different drag. Their main focus is not on maximum speed, but on "economical" settings to achieve different speeds at different altitudes.

For fighter aircraft, the situation seems similar. I'm looking at a P-51 manual, and it has a lot of information about endurance/fuel consumption under different loads/altitudes. But I don't see any discussion of maximum performance, at least not directly.

 

[FLYBOYJ]

Yes. For example I have spent some time with B-25 performance data. There are tables for different weights, and also for external items with different drag. Their main focus is not on maximum speed, but on "economical" settings to achieve different speeds at different altitudes.

For fighter aircraft, the situation seems similar. I'm looking at a P-51 manual, and it has a lot of information about endurance/fuel consumption under different loads/altitudes. But I don't see any discussion of maximum performance, at least not directly.

I don't have a P-51 flight manual in front of me but you may have to "interpulate" some of the charts to get max performance numbers. Another reason why you might not see any maximum performance data with a full load is because it may be "assumed" you woun't operate the aircraft in that configuration (I'm guessing). There's at least 2 of our members who have flown -51s, hopefully they'll chime in on this.

 

[drgondog]

The Flight test reports will specify the deterioration of cruise speeds in level flight and in some cases reduction of climb speeds with various loadings, But -

Each external load represents two detriments to maximum performance. First is Parasite Drag which affects top speeds in level flight. Second is Weight which affects both Climb performance as well as increasing Induced Drag.

Very simplicly:

Total Drag = Parasite Drag + Form Drag + Induced Drag.

If the aircraft is capable of flying in Mach numbers > .5M, then the equation looks more like this. Total Drag=(Parasite Drag+Form Drag)*Cf+Induced Drag. Cf= Compressibility factor ranging from 1.0 in incompressible flow near .3M to 1.2-1.3 for regime near Compressible drag rise to 1.3 to 1.5 as shock waves form on classical WWII airfoils. The propeller also experiences compressible drag rise as tip speeds from combined rotational velocity and forward velocity and must be added to the equation.

Rate of Climb= (Thrust*Velocity-Drag*Velocity)/Weight for steady state climb angle.

So the external load impinges on Drag and Weight values.

If you are trying to 'Game' this realistically - good luck finding accurate drag values as they not only vary as a function of Reynolds number, but also as a function of angle of attack, variable density of the air. Ditto propeller thrust which varies for velocity, Hp, and prop efficiency.

Next challenge is to find the 'real' Parasite Drag numbers as analyzed by the manufacturer - most of which are not available - then plug in the chart values for RN, re-calculate the Velocity as a function of density and Mean Aerodynamic Chord.

In many cases, given enough flight test data which reports flight conditions of Gross Weight during speed runs at specific altitudes, assume the plotted HP is to manufacturer's specs (occasionally correct but usually within 2-3%), assume the contribution of exhaust velocity to Thrust is in the range of 0.1 to 0.13 of the Thrust Hp as calculated above, then and only then do you have the data to develop (Parasite Drag CDo)*Cf.

The Sea Level values are best place to start because the top speeds are usually below .50 to .55M and the Compressibility factor Cf is close to 1.0. Also the prop tips aren't usually M>=.98M.

Obviously the above assumptions are NOT correct for Reno Unlimited Racers. At 500mph for a Mustang at 5,000 feet above Sea Level - the P-51 is zipping along at ~ .67M. Even with 3500 Hp it has just about reached its absolute limits because a.) prop is well into supersonic drag conditions, b.) the Compressibility Factor for a stock Mustang at .67M is about 1.4, and c.) approaching Q load dynamic pressure limit. The wing and the prop are really the only variables left to play with to squeeze a few more knots as the Merlin has tapped out in context of squeezing HP further (IMO).