I'm currently doing my MEI in the DA42 and according to the POH, the plane have a minimum flight mass of 3329 lbs. It does not say anything about the reason for having a minimum flight mass, but my guess would be that it's becuase of Vmca. The lighter the airplane, the higher the Vmca.
Are anyone in here familiar with the plane, and know anything about this?
Thanks!

Hi Marius.

I'm not specifically familiar with the DA-42 (although I know what it looks like :) ), but a lot of both single and twin engined airplanes do have a minimum flight weight or mass restriction. I don't think it's always related to Vmca (a single is not so bothered), but rather to a certification design requirement.

In some cases (e.g., the Columbia 400 single), MFW varies with CG such that at 105 in. forward it's 2600 lbs but varies linearly to 2900 lbs at 112 in. aft.

Pretty sure this all as to do with flight load factors, representing the ratio of the aerodynamic force component (acting normal to the assumed longitudinal axis of the airplane) to the weight of the airplane. The manufacturer needs to prove the airplane meets ALL certification requirements between the MFW and the MGTOW.

minimum flight mass of 3329 lbs

Sorry, but I can't resist this: You might have written "minimum flight WEIGHT of 3329 lbs", or "minimum flight MASS of 1513 kg", but not mix them. The Diamond/Austrian/Canadian/European use of "mass" is more "correct" than the typical American usage of "weight" (an airplane's weight changes with altitude, but it's mass remains constant - fuel burn not considered) :) Please take this as tongue-in-cheek, not an anal criticism on my part. :O

Someone else may "weigh" in here on the Vmca implications.

Cheers,

Tom

Tom, thanks for your reply. You are probably right, it seems likely that it is because the load factor. I do remeber reading that some light twins have a minimum weight because of Vmca requirements, but I can't seem to find that page again.

Sorry, but I can't resist this: You might have written "minimum flight WEIGHT of 3329 lbs", or "minimum flight MASS of 1513 kg", but not mix them. The Diamond/Austrian/Canadian/European use of "mass" is more "correct" than the typical American usage of "weight" (an airplane's weight changes with altitude, but it's mass remains constant - fuel burn not considered) Please take this as tongue-in-cheek, not an anal criticism on my part.

Seems like I mixed up weight and mass there. I'm not from the US, but I've done my flight training there. I'm more used to the european definitions of weight and mass.

Many multis, including heavies, have a minimum weight (mass, if you wish) included in the CG envelope and this, generally, would be to limit Vmca.

Looking at the Columbia example cited (an aeroplane with which I have no familiarity .. I presume you are considering the following TCDS (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/2c9a807e8a15cec9862573d20051899d/$FILE/A00003SE.pdf)) I suspect the limit is more likely due to tail structure strength or, possibly, pitch authority for the landing flare .. rather than load factor, per se. I am, as usual, open to dissent ...

In general, minimum flight weight is not a practical limitation as the empty weight will put the loaded weight above it although a minimally equipped aeroplane might well have to consider the limit operationally.

MFS, djpil, CliveL, and similar folk might be expected to wade into the discussion ..

Hi JT,

I thought V2 min would take care of VMCA - no matter what the mass, just as min Vref takes care of VMCL.
I also thought min flight mass is quoted simply because - that's what was demonstrated during the certification process and establishment of the C of G envelope.

Standing by for incoming....

I thought V2 min would take care of VMCA

Indeed .. but that presumes Vmca is fixed. Vmc, however, increases with decreasing gross weight, hence one needs to lock in a minimum weight (either implicit or explicit) to fix the declared Vmca. Much the same as aft CG limit, max engine thrust, etc.

Don't fly light twins anymore, but interested in the theoretical matter of the subject.

The relevance weight has to Vmca is concerning the 5 degs bank into the operating engine. A heavy aircraft will generate more lift to counter weight. Hence, when the lift vector is tilted 5 degs, the horizontal component will be greater and hence, be more effective in countering the asymmetric thrust.

Online DA42 manual (http://www.diamond-air.at/fileadmin/uploads/files/after_sales_support/DA42_Twin_Star/Airplane_Flight_Manual/Basic_Manual/70105e-Rev6-complete.pdf), list the minimum flight weight as 2756 lb and Vmca as 68 knots.

What is the Vmca of the model you are flying with a minimum flight weight of 3329 lb? Should be lower than 68 knots if your theory should hold up.

If not, applying the principle of parsimony (the most simple explanation is probably the right one), they probably just didn't fly it at a lower weight during test flights at the time it was certified, but later on did.

I'm flying the NG with Austro engines installed, and it has a Vmca speed of 76 kts. But the Austro engines at TO-pwr produces 165 hp, while the Thielert engines produces 135 hp. That could be the reason for the higher Vmca speed of the NG.

Seems like I mixed up weight and mass there.

Nah, you didn't. Until the first US aircraft flies on a planet where the acceleration of free fall is significantly different to 9.8 m/s^2, mass in aviation will be measured in pounds (http://en.wikipedia.org/wiki/Pound_(mass)). :)

C'mon, when is someone going to spout off about slugs and hyls?

Since Diamond publishes an empty weight of 3153 lbs (which will be the best case and virtually all aircraft will be heavier) the 3329 lb min weight is from a practical point of view a meaningless number as your total pilot and fuel load would have to be no more then 176 pounds (or less if the empty weight is higher) for this to be limiting.

John T.,

Many multis, including heavies, have a minimum weight (mass, if you wish) included in the CG envelope and this, generally, would be to limit Vmca.

Looking at the Columbia example cited (an aeroplane with which I have no familiarity .. I presume you are considering the following TCDS (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/2c9a807e8a15cec9862573d20051899d/$FILE/A00003SE.pdf)) I suspect the limit is more likely due to tail structure strength or, possibly, pitch authority for the landing flare .. rather than load factor, per se. I am, as usual, open to dissent ...



You'll get little dissent from me since you've identified some of the certification requirements in addition to Vmca for the multis. And yes, the TCDS you linked does indeed cover the specific model to which I referred. I'll let others comment on the details further. :)

Cheers,

Tom

bookworm,

Nah, you didn't. Until the first US aircraft flies on a planet where the acceleration of free fall is significantly different to 9.8 m/s^2, mass in aviation will be measured in pounds (http://en.wikipedia.org/wiki/Pound_(mass)).

Please remember, I said "tongue-in-cheek". :) But if we're really getting technical, and depending on which gravitational decay with altitude formula that you accept, a fully loaded Boeing 747SP at 670,000 "pounds" MGTOW will weigh less at 41,000 feet than it does at sea level - by an astonishing amount equal to at least a fully loaded Cessna 182.

Not trying to start a "weight" vs. "mass" war here, but the latter is technically more correct than the former. :)

(Disregard fuel burn of course).

Bests,

Tom

Marius, see what you've started? And such a simple question, really. :):)

Tom

PS: Where are you from? PM if you don't want to say here.

Cheers.

Not trying to start a "pounds" vs. "mass" war here,

You are mixing apples and bananas.
Kg and Lb are both units...

(not trying to stant a weight vs. mass war here ;) )

Marius, see what you've started? And such a simple question, really.

Tom

PS: Where are you from? PM if you don't want to say here.

Cheers.

I'm from Trondheim, Norway, but I live in Daytona Beach, FL for the moment to work as an instructor here.

Cosmo, you are of course quite correct. My mistake, corrected above. :OTom

It does not say anything about the reason for having a minimum flight mass, but my guess would be that it's becuase of Vmca. The lighter the airplane, the higher the Vmca.Since Vmca can be scheduled as a function of weight, IMHO a more likely reason is the difficulty of getting the test airplane to lower weights for certain required certification tests, considering flight test crew, instrumentation, provisions for data recording, telemetry, ballast, etc.

My bad... that reply was to another thread!

The Diamond/Austrian/Canadian/European use of "mass" is more "correct" than the typical American usage of "weight" (an airplane's weight changes with altitude, but it's mass remains constant - fuel burn not considered)

Wouldn't the use of "weight" actually be more correct? Full disclosure: I'm Canadian! haha

As altitude increases, weight decreases and therefore the required lift would also decrease! If you were to be looking for a way to accurately calculate performance figures, weight would be the most correct I believe.

But minimum flight mass is not a performance figure.

It's a fixed value, where the mass of the aircraft has to be a certain amount of lb/kg. Even if taking off from an airfield with an elevation of 5000 feet, where the weight theoretically would be slightly lower for a specified mass.

The mistake is that Diamond mentions weight for lb and mass for kg. Both would be mass, regardless of unit used.

Taken with the usual grain of salt, wikipedia explains this nicely:
"The pound or pound-mass (abbreviations: lb, lbm, lbm, ℔[1] ) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound which is legally defined as exactly 0.45359237 kilograms."

"Usage of the unqualified term pound reflects the historical conflation of mass and weight resulting from the near uniformity of gravity on Earth"

Cosmo...

If you actually calculated out F=ma you will see that kg is mass and lb is weight. The conversion factor you mentioned (0.45....) is compensated for gravity on the surface of the earth at 9.80665 m/s^2. Anytime something is measured in kg, that is the actual mass of the object. If it's measured in lbs, that is the actual weight of the object on the surface of the earth.

Example:

F = m a, the units being: Newtons = kg m/s^2

F = 1500 kg 9.80665 m/s^2
F = 14,709.975 kg m/s^2
F = 14,709.975 newtons = 3306.93 lbf (pound force)

If you convert 1500 kg to lbs you will see that it equals 3306.93 lbs.

Edit: You will see weight listed as either lbs or kg and you'll see mass as either lbs or kg. It's technically not correct. They're just assuming that you're on the surface of the earth.