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CDS Flap Setting


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A question came up around the squadron the other day. Let’s say we have five CDS bundles loaded, but we’re only going to drop two on this pass. When we choose the CDS flap setting, do we use the column for 5 bundles or 2?

The numbers aren’t important. The question really is…

Do we use the number of bundles loaded or to be dropped?

Most importantly, why?

Keep in mind, looking at the CDS Flap setting chart, more bundles require additional flaps. This means that more bundles require a lower deck angle.

I have my own opinions on these, but I’ll keep them to myself so I don’t bias the responses.

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I'm a Loadmaster but I'll attempt to answer anyway. Because you are dropping only two, you need to use the flap setting for two. Keep in mind that CDS is a gravity airdrop. There isn't as much weight pushing two bundles out as five. Therefore, the higher deck angle is required.

For the first drop, the other three bundles are still on the aircraft and are part of your operating weight at that point. They are essentially part of the airplane until they are dropped. Much like fuel not burnt.

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Thanks for your reply; a Loadmaster may be just the right person to solve this mystery.

Initially I provided myself with the same logic you have presented, however; I have to consider a few situations in which this explanation does not seem to hold true.

Situation 1.

I naturally assumed that more bundles would mean more weight. But I must consider that I could potentially have 4 bundles weighing 1000lbs each or I could have 6 bundles loaded each weighing 500 lbs.

(4 * 1000 = 4000lbs vs. 6 * 500 = 3000lbs)

Why would I drop a heavier load at a higher deck angle?

Again, the numbers here are just examples; there are many possibilities. I’m just showing that more bundles don’t necessarily mean more weight.

Situation 2.

Also, consider loading 16 bundles (8 per stick). I’ll have two separate rows of 8 bundles. If I drop one stick at a time, 8 bundles, I would use a flap setting for 8 bundles. However, if I drop both sticks simultaneously I would use a flap setting for 16 bundles. (Higher flap setting/lower deck angle required)

I’ve attached a .doc file containing the CDS Flap Setting Chart for reference.

As a result, I’ve determined that the weight of the bundles is not the determining factor…but I don’t know what is.

I thought, then, that positioning (Fuselage Station) of the bundles must be the key. However, situation 2 makes that hard to prove. So I’m at a loss.

Any thoughts or ideas are welcomed!

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Reason #422 why we should fly 50% flaps and pop up about 100-200ft in the last five seconds...AATTC used to teach this method, the Brits still use it, and there's no good reason not to (other than it requires a bit more "hands" from the stick monkeys up front, but that, like landings, can be trained).

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I think I know where you're getting jammed up-

The more bundles you have, the more the airplane weighs. Keep in mind that your Minimum Controllable Airspeed is higher at heavier weights. Therefore, you need more flaps to maintain the same amount of lift over the wings.

Sorry for my delayed response. I was without internet for a few days.

I agree that gross weight is a factor. However, I disagree that it solves this mystery. Consulting the CDS Flap Setting Chart I provided in an earlier post, we can see that the gross weight variable is accounted for. The flap setting is increased for an increase in the number of bundles at ANY given gross weight.

An example...

130k lb Gross Weight and at a drop speed of 130 knots - with 1-4 bundles we use 17% flaps, 5-10 bundles uses 25%, and 11-16 bundles used 31%.

So as we add bundles we add flaps (lower deck angle). 17% - 25% - 31%. This is all at the same gross weight of 130k lbs.

So, yes, gross weight is a factor... We can also see from the chart that as the gross weight increases (number of bundles remains the same) the amout of flaps required increases. This is what you explained, and I agree. But a 130k lb airplane uses different flap settings when it is loaded with 2 bundles or 12. As long as the aircraft operating weight, fuel and number of bundles * bundle weight = 130k, the flap setting is determined by the number of bundles. 2 or 12...as long as it all adds up to 130k we would use different flap setting for each case.

So I think the mystery continues. Ideas? Or am I mistaken?

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I believe the chart was probably developed using an average bundle weight. This is the missing weight piece that may be catching you up. Also, exit time can vary quite significantly.

This weight piece is precisely why CRL at ABQ and the way HRT does them varies. W/O getting too specific, HRT typically does them early in a mission with a full load and high overall gross weight, so even at 50% flaps, there is a significant deck angle. ABQ does them at the end of the night on a training sortie with no gas and an overall low gross weight, so 50% flaps has a relatively low deck angle. Speed is the same in both cases, but when you're 145K your deck angle will be higher at 130kts & 50% flaps than if you're 110K and same config/speed. ABQ was having issues with long exit times and whether or not the LM should assist the load or not (and I don't want to get into that and derail your thread here).

Deck angle will be based on speed, operating weight and center of lift/pressure on the wing. Nothing else really affects it. And while it's true that as your bundles exit, it changes the weight and you have to change another variable to keep the plane from changing the rest of the variables, the change is so small and so quick it is barely noticed through the drop.

At the end of the day, the flap setting chart is nothing more than over-simplified tab data. If we wanted exactly 6* deck angle, we'd make the FE run a series of spider charts for every single drop...but that's just dumb. This works quite well. I guess you could have another variable in the chart - "weight of bundles to be dropped", but again, what we have works quite well...

EDIT - after posting & re-reading your post above, another piece is bundle proximity. You don't want the bundles falling onto each other & collapsing chutes. As we add bundles using your example of 130K @ 130Kts, 1-4 bundles going out vs 11-16 bundles going out there's a much higher chance as the forward bundles build more speed on their way to the ramp that they will each get successively closer to each other and raise the risk of impacting each other on exit, or falling into each others' chutes.

I think the question you're raising ends up having to do as much with bundle exit speed & separation than simply weight and bundles.

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