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Rough guess about drag of Tip Tanks in a small GA plane??? (Read 529 times)
May 10th, 2010 at 2:57pm

snippyfsxer   Offline
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I'm using several real life "hands on" reviews of an airplane to fine tune tune its counterpart in FSX.  One discrepancy that arises is that the model in the sim has Tip Tanks and the planes in my documentation do not.  I'm looking for a rough guestimate for a small GA plane of how many knots you will lose with the extra drag of tip tanks (empty) at a say a 150 knot cruise.  5 knots? 10?
 
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Reply #1 - May 10th, 2010 at 7:13pm

Tomcat5   Offline
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What is the head-on area of these tip tanks?  Are they streamlined, or what?  Can you give me a screenshot?
 

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Reply #2 - May 10th, 2010 at 8:55pm

snippyfsxer   Offline
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Sorry, this is the PA-30B, Twin Comanche.  I estimate that each one has a frontal area of .075 Meters^2.

Here is some background.  I am reading two different sources, that say, respectively:

1)  "With the gear down and throttles at 15-16" MAP, you should be coming down right at 140 mph [120 knots] and approximately 500 fpm" ....(a plane with no tip tanks and flaps retracted)

2) "With the gear down and half flap (10 degrees), 15 inches of manifold pressure gives us 110 mph and a nice steady 500 feet per minute descent"....(a plane with tip tanks and two notches of flap)

While the plane flies almost exactly on the numbers during every other phase of flight, with the gear down, it requires a signifigant more amount of thrust than indicated by the accounts above. After making some assumptions about gross weights in the above examples, I think the difference is substantial enough for a modification. So I found some scalar specific to the landing gear drag in the .air file and made it almost half.  Now my approach numbers are TOO close (+/- 2 mph!)...so much so that I think my tweak might cancel the effect of tip tank drag when the gear is down.  In other words, if the guy without tip tanks uses 16 inches, I might need a little bit more, and approximately how much is the question, hence a rough drag estimate.  If the adjustment to the landing gear drag scalar is small enough to account for this in the first example, its affect on the second example (at a lower approach speed, WITH tip tanks installed) should be negligible, because of the exponential effect of speed....right?

Any mod I make to an .air file is strictly for personal use only, so if anyone is inclined to give me grief for that, please don't Smiley.  Since the development cycle of this plane has long since ended, I may or may not bring this discovery to the developer's attention.  I doubt they would bother posting an official hotfix for such a minor issue.
« Last Edit: May 11th, 2010 at 12:44am by snippyfsxer »  
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Reply #3 - May 11th, 2010 at 4:28pm

Tomcat5   Offline
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I don't think it would matter much, maybe you would lose about 5 kts.  I have never been an aviator, so I wouldn't know.  Maybe someone else would know more than me.  Again, maybe about 5 knots, certainly not more than 10.
 

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Reply #4 - May 11th, 2010 at 11:06pm

snippyfsxer   Offline
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Nevermind about this.  Surprisingly, some back of the envelope calcs and some additional web searches indicates that for these sizes of tip tanks at these speeds, the drag difference might only chop your cruising speed at most a knot or two;  In other words, not worth accounting for within the fidelity of our sim or simulated guages.  This is surprising to me, but I'm pretty sure that is the correct answer.
 
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