Drag Index?

[Flagrum]

What is a drag index and what does it tell me?

 

I know that it somehow describes the amount of drag that an object that moves through the air produces. I know that this is an important factor when planning your load out as the different payloads have different drag indexes as they produce different amounts of drag.

 

But what exactly tells me such a number - besides the relative drag one item produces? Like, bomb A has a drag index of, lets say 1.33 and bomb B of 1.12

 

I know now that I would be better off if I load type B. Is that all the number tells me? What other information can I derive from the drag index (of a single item or the total of plane + loadout)?

 

Or this example: for my loadout I have a total drag index of, hrm, 12.43 (totally made up ... no clue if this is even remotely realistc ... but just for the sake of the example ...). An other loadout gives me 25.03 ... about twice of the first! Now, what performance characteristics of my aircraft are now twice as bad when compared to the first loadout?

 

A bit noobish approach to these aspects of aviation, I know ... but if you could give me some pointers of how I could learn more about it? Thank you! :o)

[Flamin_Squirrel]

It's not noobish, in fact it's quite an involved subject.

 

You have the correct idea in general; drag indices are cumulative (IIRC they are summed exactly as you have done). How the index affects you depends on many factors, including phase of flight, and is not linear. There are charts in the A10 flight manual where the drag index is a variable that can be used to improve the accuracy of whatever it is the chart is being used for. For example, the cruise fuel burn chart will use drag index.

 

I've not explained that well, but should become clear if you refer to the manual. I don't have the link handy for you at the moment I'm afraid.

[GGTharos]

You're more or less on the right track.

 

To give you an idea, off the top of my head, for an F-15C the centerline tank has a DI of ~12, and the two wing tanks ~5.5 each (since we carry them in pairs, we say 11).

 

So I know if I take 2 wing tanks, I have similar performance to taking 1 wing tank, ie. I can hit M1.6 with my weapons onboard for a sprint-and-shoot.

 

If I take all 3 tanks, with a DI of 23, I'll be lucky to his M1.4 in a timely manner (note: I could go faster, but not in a way that is practical for combat).

 

To give you more insight, if you think about your A-10C, and you grab some performance charts such as say, a sustained-g turning chart, and you want to know how much g you can sustain at altitude x with gross weight y, you also need to toss in the DI to compare to the DI of that chart (Was it made with a clean a-10? Was it carrying some bombs etc?). If your DI is less, you will turn marginally better ... of more, you might turn considerably worse.

 

A high DI will also prevent you from reaching higher altitudes and higher speeds compared to a high DI payload - again, all of this is stuff you would look up in the charts, and depending on the chart, the DI will be one of the variables you look up.

 

Generally speaking, for flying, low DI good, high DI bad. :D

 

Or this example: for my loadout I have a total drag index of, hrm, 12.43 (totally made up ... no clue if this is even remotely realistc ... but just for the sake of the example ...). An other loadout gives me 25.03 ... about twice of the first! Now, what performance characteristics of my aircraft are now twice as bad when compared to the first loadout?

 

A bit noobish approach to these aspects of aviation, I know ... but if you could give me some pointers of how I could learn more about it? Thank you! :o)

[Svend_Dellepude]

It translates to the drag of the aircraft carrying the stores and relates to the max speed achievable or the powersetting to reach a given speed. Not sure what exactly.

 

Dont bring more than necessary though.

 

Here is a link from f-16 net:

 

http://www.f-16.net/f-16_forum_viewtopic-t-14939.html

[Flagrum]

I shall have a look into the A-10A dash one then ... but how well do these charts apply to our DCS A-10?

 

Perhaps well enough ... I am not keen to calculate my fuel consumption up to the last drip. But to get some good estimates about max climb rate, air speed, turn radius for different altitudes - that would be great.

[GGTharos]

They apply very well.

[PFunk1606688187]

I don't go much for in depth fuel planning, mostly I just check the AOA gauge markings and my Fuel Flow gauge, but if I ever were to there are people who have produced updated Drag Index charts for the available ordnance in DCS for the A-10C.

 

The simple rule I have is never more than 2 CBUs, never more than 2-3 rocket pods, and I never carry 2 CBUs with 3 rocket pods, that'd be awful. Those two things have obscene drag for their relative value. If you fly around with a couple mavs and a few slicks then land, load even 2 rocket pods you'll feel the difference in how it handles.

 

For the most part performance hits based on drag index are something you can just feel out I think. You should know intuitively how every type of loadout affects your plane's handling characteristics. For in depth planning of fuel usage for a long mission thats when this stuff really matters, which I must admit I never bother with, but one day maybe.

[alfredo_laredo]

Drag Index refers to the Drag generated by the aircraft, each rack and weapons generate drag, but also Landing Gear, FLaps, Slats, Airbrakes.. etc

 

from page 407 in the A-10A Manual comes a list of the drag number for each stores and suspension equipment.

http://www.476vfightergroup.com/downloads.php?do=file&id=42

 

Drag Index is useful in the performance charts of the airplane.

 

For example. in the next chart is to calculate the Single Engine Rate of Climb after takeoff

 

The situation with the Red Line is as follows... (start from top left)

1.- Temperature 25°C

2.- Pressure altitude Sea Level

3.-Gross Weight of 46692 lbs

4.- Here, the line goes to the Guideline and starts to go down until you reach your Drag Index... In this example the sum of all drag index is 7.89 with a load of...

 

6 Maverick DI 0.35 each

6 GBU12 DI 0.51each

2 CBU105 0.38

ALQ131 0.75

AIM Sidewinder (dont know the DI)

Litening Pod

2 Lau 88 (for mavericks) 0.61

 

The Line then ends at -550 ft x min... this is a negative value.. this means that if you loose an engine during takeoff, you wont be able to climb!

 

As you can see, the Drag Index doesnt have to much of an impact... with a Drag Index of 0.0 you will archive around -450 ft *min

 

But if you loose some weight... (fuel) you can have a Gross weight of 37500 lbs you can see the green line will give you 0 ft * min and with the blue line in a Gross Weight of 35000lbs you can archive a climb of +200 ft * min

 

(this situation is taking into account that you have a takeoff configuration, FLaps 7° Landing Gear Down.. so note the little box at the top right)

 

DI is a factor in other several charts...

Edited August 14, 2013 by alfredo_laredo

[Bushmanni]

But what exactly tells me such a number - besides the relative drag one item produces? Like, bomb A has a drag index of, lets say 1.33 and bomb B of 1.12

 

The drag index is meant to be used in conjunction with the charts they are provided for. For example, you can have a negative drag index, which only means your drag is less than the baseline (by removing some stuff normally always carried). It doesn't relate to aircraft performance in any easily decipherable way other than less is better unless you use it with charts. You can also have different drag index for the same munition in a chart of a different aircraft. A-10A flight manual has fractional drag indexes while modern manuals tend to have much larger integer drag indexes that can be summed in head more easily.

[Flagrum]

Allright, thanks everyone. The information provided here give me now a better understanding of what it is all about.

 

The drag index is a relative figure that only relates to a specific aircraft, and is primarily used to read these charts.

 

Alfredo, thanks for your explanations as well - dang, those charts look complicated ... but if one knows how to read them, it gets a bit easier ... :D

 

Are these charts available in better quality by chance? The quality of that fotocopied scan of an old and used manual are a bit difficult to work with, especially if printed out ...

[alfredo_laredo]

no, as far as I know, but if you find of better quality make sure to let me know!

 

And yes.. they seem a bit complicated at first... but once you know how de basics, its really simple.

 

if by any chance you read spanish.. I made this post explaning most of the Takeoff performance charts....

 

http://www.boe117.com/foro/index.php?/topic/852-tablas-de-performancia-despegue/

 

I you dont... just follow the Red Arows =)

 

if you need help in particular, just let me know!