1,000-lb useful load? +BRS?

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Antoine
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Re: 1,000-lb useful load? +BRS?

Post by Antoine » Thu May 16, 2019 4:58 pm

Rich wrote:
Wed May 15, 2019 10:25 pm

Your complaint about cabin design argues for a new airframe. While I don't disagree the US is a country of fatsos this has never presented a limitation in any flight I've made.

Your airframe empty weight complaint needs more validation. How much extra weight? 500 lb, 300? The Panthera, BTW, has a shorter wing, higher wing loading and much higher stall speed. A smaller wing is a good way to save weight. Much of the weight problem in DA40s is not related to airframe structure but all the various options that have come along, giving rise to limited useful load and/or problematic CG situations. (A characteristic that our early ones do not share.) In this we agree that it should be corrected, mostly by lightening up the structure. I just think it's a bigger job than you do. (JSYK, I used to work for managers who didn't appreciate the magnitude of actually implementing their "big picture" ideas.)

Dismissing a "trainer first" characteristic is disingenuous. It seems to denigrate docile handling, robust gear, etc. as suited for toys. Many of us fly these planes all over the country. You seem to say it should be more like a Cirrus, as though that's the gold standard.
You misunderstood me Rich.

I am not bashing the DA40 and its trainer roots, just saying what I believe would help make it more successful as a touring aircraft and eat the SR22's lunch.
I would never dismiss the "trainer first" characteristics - for a first aircraft. Quite the contrary, I have loved them.
Also, I am not "complaining" about the size of the cabin (I would not have owned two if I were not happy) - it is just a fact that quite a few potential buyers in the US buyers would need it roomier.
As I have proposed in detail many times,
(see here for one example: viewtopic.php?f=8&t=5843&hilit=redesign)

I believe Diamond should derive a more capable cross country aircraft from this excellent starting point. If anything, it would appeal to all of us who outgrew their DA40... and it would be a great competitor to the SR22.
And yes, absolutely I think the airframe is overweight. The steel landing gear is also very heavy and the nose gear is unnecessarily bulky causing huge drag issues and the infamous "corked nose wheel" problem.
It is very sad that as Diamond added "stuff" to the DA40, they spent so little energy on CG planning.
The 2007 DA40 XL was crippled in this respect. There was a retrofit (!) kit available with nose ballast and a field mod to relocate some avionics which had been placed unnecessarily far aft.
And no I am not a "big picture" manager. I did spend a LOT of design time, energy and money on these matters.
Redesigned the nose landing gear fork. See the thread on this.
Made design and cost calculations with a specialist company to produce a carbon fiber tailplane. Try doing the CG math on this and you'll see it removes the need for 17 lbs of ballast in the nose. This alone would give 22 to 25 lbs of weight savings.
Not only that - the lighter tailplane and nose would reduce the rotational moment during spin exit which may well be significant with regards to the certification issue that caused the restriction to aft CG in the 50 USG planes.
I absolutely believe that the DA40 is a wonderful aircraft and that it really deserves to be further developed. This is and has always been my view and I admit that I am sometimes aggressive in my stance, but it is out of frustrated love...
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Re: 1,000-lb useful load? +BRS?

Post by Rich » Thu May 16, 2019 8:55 pm

Here's an interesting wrinkle to consider. The lighter an airframe is the more impact on CG a given load will have. So I did a what-if analysis. Specifically, what if my plane had an empty weight of 1500 lb, rather than its current 1705, with the same forward CG it has now.

Here's a load graph with that condition, given the exact loading in my previous example:
N40XE Lightweight W&B.jpg
Bingo, now I'm out of rearward CG. And if I want to take advantage of the theoretically increased useful load by putting 4 200-pounders in the seats:
N40XE Lightweight W&B-2.jpg
So in order to make use of this increased margin between empty weight and MTOW you would also have to find a way to expand the rear CG limits, as well. Remember, my plane has a well-forward CG and is pretty light to begin with. The effect shows up for my plane even at 1600 lb, in case you're wondering. I did find that it'd work OK of there was not much in the baggage compartment. Experimenting with a couple of the more typical 40-gallon DA40s with existing higher empty weight and more aft CG's shows similar effects. I tried some boundary-condition 50-gallon versions. A 200-lb weight reduction leaves them with more severe CG issues than they already have.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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Re: 1,000-lb useful load? +BRS?

Post by TimS » Thu May 16, 2019 10:01 pm

Basically you are stating the CG range is too small, or the load pattern is not close enough to the center of thrust making it have a disproportionate effect on CG.

Tim
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Re: 1,000-lb useful load? +BRS?

Post by Rich » Fri May 17, 2019 12:05 am

TimS wrote:
Thu May 16, 2019 10:01 pm
Basically you are stating the CG range is too small, or the load pattern is not close enough to the center of thrust making it have a disproportionate effect on CG.

Tim
For purposes of this thread, I'm pointing out that the nominal CG range adequacy is reduced if the plane is made lighter. For me, the range is fine as is because it's in balance with my plane's empty weight and CG. But if you fool with either of those too much, the CG range becomes a problem. What I'm pointing out is a case no one's brought up. Making the plane lighter has the effect of increasing the influence on the CG of everything you put in it. The CG envelop becomes less adequate by dint of the empty plane's influence being reduced.

Here's a thought experiment, using some magical boundary conditions, assuming the often-wished-for 1,000 lb useful load:

Using today's DA40 50-gallon variant CG limits, assume the planes weigh 10,000 lb, CG of 97 in. Load it up with 50 gallons of fuel, 600 lb of people and 100 lb of baggage. Resultant CG is now 98.45 in., well within the 100.4 in. limit for a 50 gallon plane. The stuff loaded in the plane moved the CG less than 1.5 inches.

Now make the plane weigh a mere 1,000 lb, same CG of 97 in. Now with 50 gal of fuel, 500 lb of people (300/200 front/rear), no baggage whatsoever, we have CG of 100.46, surpassing the 50-gal plane's aft limit. In this case considerably less stuff loaded in the plane moved the CG by almost 3.5 inches. We're also almost 155 lb below MTOW, thereby making the 1,000 lb "useful load" not entirely useful.

The efficacy of the of the CG envelope is clearly affected simply by magically making the plane heavier or lighter while keeping the empty CG fixed at a pretty common value for DA40's.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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Re: 1,000-lb useful load? +BRS?

Post by Antoine » Fri May 17, 2019 9:45 am

I hear your point Rich. Good find.
The problem is real and can be addressed by proper redesign.
I can't accept the concept that only a heavy airframe can have enough loading flexibility.
As I wrote above, removing weight where it has a lot of moment arm (tailplane) is a good way of reducing polar inertia which should allow relaxing the aft CG limitation. We don't have all the cards in our hand here.
Maybe look at the Pipisteral Panthera to see how they overcame the issue in a much lighter airframe?
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Re: 1,000-lb useful load? +BRS?

Post by TimS » Fri May 17, 2019 11:47 am

My impression of Panthera and Cirrus is both have a wider CG envelope than Diamond.
As for the engineering reasons why they have a larger envelope, no idea (e.g. no way to know if wing selection, where variable loads sit, fulcrum effects, size of tail feathers...).

I know for Cirrus, most of them have a forward CG issue when you put more than about 400lbs in the front. At which point you start needing to add weight in back. I recently did a training flight with 470lbs in front, and full tanks for a three hour flight. I needed about 5lbs in the baggage area for take off, and just under 20 lbs by the time we landed to keep it in the CG envelope.

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Re: 1,000-lb useful load? +BRS?

Post by Rich » Fri May 17, 2019 2:31 pm

I have actually been seeking W&B info for the Panthera and have been unsuccessful. There's no way to know about its loading capabilities until one has that in hand. Even then, in the absence of actual, delivered, empty weights/CGs the proverbial grain of salt must be taken.

I'm being misquoted here. I'm not saying that only a heavier plane can have a larger loading flexibility, it's just that it can be achieved with a narrower CG envelope than a lighter one. Likely a larger empennage would probably take care of it, but this might be a bit oversimplified.

I've reached a certain enlightenment. In the case of my DA40 there's a certain harmony. I run out of carrying volume, weight capacity, and CG limits at just about the same point. The resultant envelope of these three items also encloses anything I might want to carry. It certainly does for anything I've had a desire to carry so far. Interestingly, there is a certain small dissonance that might be introduced if I go ahead with a couple of upgrades that have the side-effect of lightening the plane by about 8-9 lbs. Notably the replacement of the KCS 55A with a G5 suite might move the empty CG back by about .1 inch. It depends on the single factor of whether the KG 102A is beneath the rear passenger seat, as stated in the AMM or is located in the front instrument panel as implied by the POH.

The flight envelope is more usable in DA40s like mine precisely because of its bias toward nose-heavy. It's only a limitation with certain heavier loads than normal in the front seats with full fuel and nothing at all in the rear. The later ones with empty CG's at 97 in. or greater effectively have their practical forward limit shortened to something like 96 in. So while I can use the entire 7.5 in. (itself a bit tight) of envelope, Many Stars are in reality limited to as little as 4.5 in.

N40XE is essentially the original Diamond design, though I have tweaked it with 3 things pertinent to load-carrying: MTOW increase, extended baggage, and Powerflow. This is a net increase of about 5 lbs. These and other mods appear to have moved the CG forward a net of .4 in. and an overall net increase in weight of 4 lb. (There is some question about the original CG as provided by Diamond.) So it kind of reflects the early "vision", if you will. From there, lots of various factors have caused a casual corruption of the product. Some by Diamond, some by post-sale behaviors of the customer base, adding this and that with little consideration for the existing airframe limitations.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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Re: 1,000-lb useful load? +BRS?

Post by Steve » Fri May 17, 2019 3:38 pm

Rich wrote:
Fri May 17, 2019 2:31 pm
Notably the replacement of the KCS 55A with a G5 suite might move the empty CG back by about .1 inch. It depends on the single factor of whether the KG 102A is beneath the rear passenger seat, as stated in the AMM or is located in the front instrument panel as implied by the POH.
It is definitely under the rear seat (right side). I had to pull it out to have it overhauled last year. :scream:
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Re: 1,000-lb useful load? +BRS?

Post by Antoine » Fri May 17, 2019 4:02 pm

Rich wrote:
Fri May 17, 2019 2:31 pm
N40XE is essentially the original Diamond design, ... From there, lots of various factors have caused a casual corruption of the product. Some by Diamond, some by post-sale behaviors of the customer base, adding this and that with little consideration for the existing airframe limitations.
You said it all Rich. Allow me to re-iterate that the DA40-180 is in my opinion the most wonderfully balanced, most pleasant and most unfairly treated child in the family. I still remember the perplexed look in the eyes of my C182 pilot friend after a "cloud dancing" party and a sprint to 145 KTAS on 9 GPH in my little wonder.
Diamond hit gold with the airfoil, general aerodynamics and engine choice. In retrospect I believe the G1000 was overkill.
After reading other people's input here I believe Diamond have a market for both the DA40 in its "original" form as a trainer and basic cross country aircraft and would open up a new market segment with the DA41.
If they can keep the price below 500 K USD while delivering 1000 lbs useful, a wider cabin, 165 KTAS with optional BRS, AC and de-icing, and last but not least - sexy paint, I am confident they will sell several dozen per year.
Couple this with a trade-in strategy whereby existing DA40's are rebuilt as flight school workhorses and sold at a profit.
This way they will be able to get a bigger chunk of the primary trainer market which Piper seems to have recently taken over.
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Re: 1,000-lb useful load? +BRS?

Post by TimS » Fri May 17, 2019 6:51 pm

This whole discussion brings to mind a question. I have looked at the CG, W&B for multiple new Cessna models, and multiple generations of Cirrus.
They have all slowly gotten heavier; with more creature comforts. However, they have really done a great job of maintaining the UL and the ability to use the full CG envelope.
So here is the question: how much effort/cost has it taken the companies to do this? Or has it been just "luck" with the initial design?

Tim
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