200lb Useful Load Increase?

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greg
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Re: 200lb Useful Load Increase?

Post by greg »

Antoine wrote: Sun Jun 02, 2019 9:20 am I CANNOT imagine that DAI re-certify the aircraft WITHOUT fixing this "50 Gal aft CG" mistake.
As Jeff Owen once wrote, a check valve between the outboard auxiliary tanks and the main tank would do the trick...
When the outboard tanks are empty, the aircraft's CG envelope is back to normal. and when you are fully loaded with passengers, you probably don't have more than 40 USG on board...
Is it as simple as adding a check valve? I thought about this sort of change some time ago and came to the conclusion that if you've got the 50 gal tanks, but only have 40 gal on board, it won't all be in the inner two tanks. Given the camber of the wings, I assume that some of the 40 gal will actually be in the bottom of the outboard tanks, while the middle ones will be somewhat less than full. If that's true, a check valve won't turn a 50 gal bird into a 40 one. You'd need to go to the two fillers on each wing and a manual valve or transfer pump model to make it work. Not that that would be too hard - the cut-out for the inner filler is already present on the wings.
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Rich
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Re: 200lb Useful Load Increase?

Post by Rich »

Antoine, I did exactly that. Used 25 ft. as the moment arm (probably a little generous but close enough). It does move the CG forward. 10 pounds is worth about 1.1 inch. This is a modification that would make a difference and is the only airframe-weight thing I could think of that would be reasonably retrofit. Some comments:

- Presuming that this is accomplished by using the construction technique of the DA62, the VOR/LOC/GS antennae needs to be externally mounted.
- In a retrofit, removing the nose ballast in concert with this mod as suggested in another thread undoes about .55 inch of this improvement.
- I worked these into the calculations for one of the representative post-2006 DA40s (1827/98.2) on the Wiki. Four 170-pound people and no baggage still leaves one out of that restrictive 100.4 in. limit even when tanks run dry.

For those with the 50-gallon limit that find the 100.4 in. acceptable today, consider that the additional 200 lb. has to be put somewhere. Except for the front seats, all those somewheres move the CG rearward from where it is today.

The 50-gal rear CG limit is the real kicker. It needs to be corrected, but the flapper valve is an over-simplistic suggestion. Take a 50-gallon plane, put 20 gallons on each side. Given the 5 degree dihedral a goodly portion of that 40 total gallons resides in the outer tanks. Section 7.10 of the POH will give you a sense of this. Consider also that those outer cells themselves, which includes fuel sensors, and connecting hardware and the moving of the filler cap and vents further outboard all contribute to the increased rotational inertia.
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Re: 200lb Useful Load Increase?

Post by MarkA »

An Excel spread sheet is attached that does the weight and balance calculation for four example DA40s (more could be added).

• An example DA40 with standard tanks
• A 2007 XL with 50 gal tanks, MT prop, and all the standard XL options
• A 2010 XLS with 50 gal tanks, Hartzel prop, and all the standard XLS options
• The same 2010 XLS with a 200 lb gross weight increase

Select a plane from the list in cell B1 and enter the other weight and balance load values in the “yellow” cells. The CG is plotted as the fuel goes from the initial value to empty.

The big assumption here is that the empty weight, moment, and max landing weight didn’t change for the XLS with the 200 lb gross weight increase. This is likely incorrect. It also assumes the 17 lb nose weight currently in the XLS is also not removed.

It’s interesting to note that if they're able to fix the rear CG limit due to the extended range tanks and kept everything else more or less the same, a 200 lb gross weight increase could in fact allow four 180 lb people and full tanks to be loaded into an existing XLS.
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Re: 200lb Useful Load Increase?

Post by Rich »

MarkA wrote: Sun Jun 02, 2019 6:39 pm It’s interesting to note that if they're able to fix the rear CG limit due to the extended range tanks and kept everything else more or less the same, a 200 lb gross weight increase could in fact allow four 180 lb people and full tanks to be loaded into an existing XLS.
I concur. The 102 in. is sufficient in this case. The baggage area remains problematic, but lightening the horizontal stabilizer helps there.

My SS allows taking an existing plane and adding/subtracting arbitrary weights at arbitrary stations. Changing props, e.g.
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Re: 200lb Useful Load Increase?

Post by Don »

I special ordered my XLS from the factory with 40 gallon tanks. Here is my W&B for a day trip we made from L.A. to Vegas with four adults on board.
I am fairly happy with the existing W&B on my plane. That said, I would not mind another 50 to 75 pounds of gross weight increase if possible. As you can see below, there is no way we could have done this with 50 gallon tanks and be legal.
W&B.jpg
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Antoine
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Re: 200lb Useful Load Increase?

Post by Antoine »

Folks while I fully appreciate the legality of the 40 vs 50 USG debate, I think that the 50 USG aft CG limit is pure and simple BS.
Think about it. Some obscure certification rule states how quickly an aircraft must exit a spin.
The 50 USG DA40 can't do it fast enough. Some obscure ***** decides he's gonna fix it by restricting the way we, their customer, load our $ 450'000 planes.
This is a golden sample of very poor product management and disrespect for the customer.
Somebody should have been fired on this one.

And here's a practical question : how do you get a DA40-180 to spin in the first place?
In my DA40-180 W&B sheet I had not one but two aft limits.

Back to the fixes
- Point about the check valve being too simplistic well noted. There has to be a solution... and I am sure DAI will find it.
- Glad to see that the carbon tailplane would work.
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Re: 200lb Useful Load Increase?

Post by Colin »

The regulations in aviation are "written in blood." If you are flying your plane outside the limits in the POH you are, according to DAI, a test pilot. I got into this venture with an eye toward reducing my risk and *knowing* my risk ahead of time. Saying that someone arbitrarily decided when the plane had to exit the spin is incorrect. They moved carefully on a LOT of these regulations and certification rules.

For an example of when they didn't please read up on the 737MAX issues. "Come on, guys, the pilots know how to fly a plane, they don't need to know about this system in detail..."
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Re: 200lb Useful Load Increase?

Post by Rich »

I do agree with Antoine that the significant limit on the aft CG is an undesirable way to reach conformance with the regulation - especially as the incorporation of the large tanks coincided (contributed to?) a marked increase in both empty weight and rearward movement of the empty CG. Here is a graphic of the data from the Wiki showing the evolution into 2010 of the empty weight/CG. It includes some expression of known options. Every plane in this graph from 2006 onward has 50 gallon tanks. The 2006 model has the Hartzell AL prop, the others mostly MT.
wiki data.jpg
Doh! I apologize for the typo in the "year of manufacture" axis.
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Re: 200lb Useful Load Increase?

Post by Don »

Colin wrote: Mon Jun 03, 2019 4:02 pm The regulations in aviation are "written in blood." If you are flying your plane outside the limits in the POH you are, according to DAI, a test pilot. I got into this venture with an eye toward reducing my risk and *knowing* my risk ahead of time. Saying that someone arbitrarily decided when the plane had to exit the spin is incorrect. They moved carefully on a LOT of these regulations and certification rules.
^^^This^^^.
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Re: 200lb Useful Load Increase?

Post by Antoine »

Colin wrote: Mon Jun 03, 2019 4:02 pmSaying that someone arbitrarily decided when the plane had to exit the spin is incorrect. They moved carefully on a LOT of these regulations and certification rules."
Well Colin allow me to challenge this with a LOT of respect for your wisdom and attitude towards safety first which I commend.
In this case it is a paperwork issue, not a material safety issue. And here's my point of view:

This spin exit regulation does not (to the best of my knowledge) take into account the behavior of the plane prior to the spin.
It is virtually impossible to spin a DA40-180 unintentionally.
The regulation SHOULD take into account the intrinsic safety of an airframe while it is still flying, and not just impose a certain arbitrary spin exit behavior across the board.
An example of smart "adaptive" regulation is the 61 knots stalling speed. Some aircraft (TBM) have a higher stall speed BUT they make it up with superior passenger protection and are exempted from the restrictive rule.
A similar allowance should be applied to spin exit requirements. As they stand it's imposing that the airplane gracefully exits from a situation it cannot get in in the first place by design. And the cost of compliance is immense.

Does that excuse the outrageously sloppy product management and "AVGAS is dead" company strategy ? No.
Every time we discuss the DA40-180 I get all wound up. I love this bird and I am very very happy that the one person who decided it was no good is no longer there. I firmly believe that the DA40 is fundamentally the best 4 seater SEP in its category .
Give it 200 lbs (preferably via weight reduction), more power and watch sales...
If DAI do not offer a more powerful engine, Rod and his supercharger STC will make a killing!

Nice research Rich! thanks for posting
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