FAA "dry" weight limit - incl removable battery?

Hey guys, new to the site here. I'm researching electric & hybrid aircraft, and have found answers to almost all of my questions, except for one (so far!).

Would removable/swappable batteries (specifically 18650 lithium cell packs) be considered "wet" weight, like the fuel? The aircraft I have in mind will be designed to fly without the supplemental batteries, at the expense of performance & duration, to stay within 254lb. I'm hoping adding more battery capacity/packs would be equivalent to increasing fuel tank size from 1g to 5g (24lb difference in wet weight).

Or maybe to word it differently. Lets say one of you guys want to add a 10lb lithium pack to your 254lb ultralight aircraft. Would this disqualify you from FAA Ultralight designation, or be considered "wet" weight?

Thanks so much for your help!

If the battery in discharged condition weighed less maybe, but it doesn't .Empty Wt is with all useable fuel removed. How would this apply to batteries? Nev

Hi. I have no idea, but I have found the FAA very easy to deal with so why not ask them?

Hey Nev, good question, as the mass of batteries experiences a negligible difference between charged &discharged. The mass of electrons is so small. This is why I ask, do batteries get the same exemption as heavy fuel (6lb/gallon for gasoline)?

Hi. I have no idea, but I have found the FAA very easy to deal with so why not ask them?

I've just had a look at FAR 103 and see what you mean. Your aircraft has to have an empty weight of 254 lbs but you can then add 5 galls of fuel plus pilot, but no mention of batteries or electric aircraft. If they classify the batteries as fuel you could have an empty weight of 284 lbs. If you are a member of the EAA, they may also have some answers.

See also the link below.

Does the weight of the battery count as "fuel" in a battery powered ultralight?

I would have thought that logic dictates it is a fuel source- as it powers the engine. So should be allowable to the same weight of the allowable gasoline limit.

Some will say- but it is a battery and is not consumed- bulldust the energy is consumed and must be refilled after flight just as gasoline does. The only difference is the battery will weigh only very slightly less after discharge.

It meets the spirit and intent of the part 103 rules- however the FAA seem to yet to decide.IF you had a parachute then the allowable weight limit goes up as well, assuming the chute is less than the extra allowable limit then that could also be counted for extra battery.

The same applies for floats- that adds a lot more allowable weight, make some very light floats and take up the rest of the allowance with batteries stored in the floats- to assist with C of G and stability.

And you can always have some luggage that just happens to have batteries in it.

Chutes and floats are an anomaly. Basic weight is defined and has a place in the calculations that one makes all the time. You add LOAD.. Fuel (Useable) Pilot etc to get an All Up Weight figure.

Your plane presents with a basic weight and an index that defines the state of balance in the

basic condition. That's your start point for all other possibilities and limits. It's specific to YOUR particular plane

Your max AUW may be structural ( performance limits may alter it to a lower figure on the day)

With an electric powered plane, you don't have useable fuel. Your battery is a fixed mass. Essentially part of the plane. You don't chuck it overboard at any stage. The design load limit has to apply or you lower the structural safety margins, category designations etc Nev

I would have thought that logic dictates it is a fuel source- as it powers the engine. So should be allowable to the same weight of the allowable gasoline limit.Some will say- but it is a battery and is not consumed- bulldust the energy is consumed and must be refilled after flight just as gasoline does. The only difference is the battery will weigh only very slightly less after discharge.

It meets the spirit and intent of the part 103 rules.

 

The same applies for floats- that adds a lot more allowable weight, make some very light floats and take up the rest of the allowance with batteries stored in the floats- to assist with C of G and stability.

 

And you can always have some luggage that just happens to have batteries in it.

This is also the reason for separating Pilot, Passenger(s) and their masses.

And also for separating baggage for its location ad mass.

To talk about batteries/electricity being a "fuel" might be OK for Philosophy class, but a battery as some has said, with a few exceptions, is something you can't tak off without, and will always be in its same place in relatiuon to the COG envelope, so isn't a "removable" item. Nor is the very clever "batteries in the luggage tip".

Right through the aircrtaft models, from rag and tube to Cessna 210s:

• if the flight is long enough, you will have to cut back on number of passengers and/or baggage.
   
   
  
• if there's a passenger in every seat, you are likely to have to flight plan on an upper limit of fuel, and possibly plan shorter trip legs, and there can be a minium time before you've burned off enough fuel to be able to meet the landing capabity of the undercarriage.
   
   
  
• if you're doing a long leg with medium sized passengers and need full fuel, you may have to cut back on baggage
   
   
  
• And with any of these combinations, you may have to cut something back, or relocated it even further to meet the balance requirement.
   
   
  

The way we calculate aircraft we use cantilever calculations, where the input mass unloads one side of the pivot; that mass just doesn't go up into the air but comes down on the other side, meaking its mass more than 1g (so if something weighs 10 kg, and lifts 30 kg from one arm from the COG it will input 130 kg to the other arm.

In a 60 degree bank (2G), that goes to -60 kg on one arm and 260 kg on the other. (which is why we had the concerns expressed a few days ago about cable ties around a ballast weight on the tail)

I recall someone on here reporting that he'd lost control of his aircraft (Morgan) after putting a 15 kg tool box on board, and just managed to get it down again, so even small mass changes can make a major difference to handling and safety.

I think the point has been lost a bit here, even tho the posts have been of quality. The MTOW the op is talking about would the the FAR103 REGULATORY weight, so to be far it does warrant a philosophical discussion as to whether the fuel source weight (batteries) should be considered part of the empty weight.

If we are to promote electric aircraft should those guys get penalised for having to carry batteries?

I am with Ben,

I assumed that the OP was talking about a part 103 aircraft and that he would naturally account for the weight and balance implications and the overall design loads of the aircraft.

A part 103 has a very restricted minimum weight hence the inquiry.

I disagree that fuel and battery should be different neither of them if removed would allow flight- remove the battery- no electric engine will work . Same for a fueled engine- remove the petrol it will not work. So are they are the energy source for the engine they should be treated the same. The idea that the petrol is consumed and the electric battery is not is incorrect- the energy available is most definitely consumed and a tiny amount of mass is lost. If it were the case that a part 103 could not safely land with its tank of petrol still full- then this argument may hold- but that is obviously not true.

Any aircraft will have a MTOW and this also includes a part 103. I did not imply that should ever be exceeded at any point, nor that any normal rules of airmanship or design loads should be exceeded.

The design for placement of the battery powersource is in fact more simple as it does not significantly change during flight. This is unlike petrol which gets its mass consumed during flight and thus effecting weight and balance.

IF we are ever going to see the intergration of electric flight systems into our sport, then these are the issues we need to consider. Just like the change from metal structures to composites, it is most successful when we stop thinking in the metal box but with the ability of composite in mind. The same applies for electric.

Tried to edit my post but was too late..

Wanted to change my wording as 103 isn't a MTOW limitation as I understand but a MZFW or a BEW or similar limitation hence the issue.

It's not about helping electric or not helping it. The mass of the plane is finite if the safety factors in the structure are to not be compromised. The exemptions in the case of floats and chutes are an anomaly as they DO compromise the airframe stress parameters, but apparently that's ok for some reason.

If I've missed something please point it out but don't start talking about being out of date or something. I'm all for promoting electric by the way. Incidently to talk of weight change during discharge I doubt you could measure it. Fuel is load. So is a battery and a pilot or tie downs. I doubt part 103 aircraft are restricted as to landing weight, which usually relates to undercarriage and sometimes wings and fuel load in them. The AUW (structural) considerations should be the PRACTICAL worst case and all loads apply In that instance. If this class of aircraft is restricted by application of the Empty (basic )weight limit there IS a case for modification of the rules.. Nev

All he is asking the FAA for is to replace the weight that is allowed for fuel tank and 5 galls of petrol with battery weight. No overall weight increase at all. Sounds like a fair request to me. Hopefully they will see it that way and it will advance the development of electric aircraft.

If I've missed something please point it out but don't start talking about being out of date or something.. Nev

Just had a Google. It is EW

Regulation of ultralight aircraft in the United States is covered by the Code of Federal Regulations Title 14 (Federal Aviation Regulations) Part 103 or 14 CFR Part 103, which defines an "ultralight" as a vehicle that:

• has only one seat[1][2]
   
   
  
• Is used only for recreational or sport flying[1][2]
   
   
  
• Does not have a U.S. or foreign airworthiness certificate[1]
   
   
  
• If unpowered, weighs less than 155 pounds[1]
   
   
  
• If powered:
  
  1. 
      
     Weighs less than 254 pounds (115 kg) empty weight, excluding floats and safety devices
     

  [1]

   

   

  [*]Has a maximum fuel capacity of 5 U.S. gallons (19 L);[2]

   

   

  [*]Has a top speed of 55 knots (102 km/h; 63 mph) calibrated airspeed at full power in level flight[1]

   

   

  [*]Has a power-off stall speed of 24 knots (45 km/h; 28 mph) calibrated airspeed or less[1]

   

   

Read my last line. #13...Nev

This was the subject of extensive discussion on homebuiltairplanes.com, and while almost everyone felt that the only 'fair' thing was to consider the battery pack in an all-electric or series hybrid aircraft to be equivalent to fuel weight in an IC engine powered aircraft, and hence, for a FAR23 Pt103 'ultralight' the weight of the battery packs should not be included in the 254lb empty weight. However, that was nothing more than the 'wishes and dreams' of the discussion participants so the matter was referred to the FAA for a determination and they responded categorically that 'at this time' (it was about 2-3yrs ago) the battery weight would not be excluded from the 254lb empty weight limitation, not even to the extent of the allowable fuel weight for IC engine powered ultralights i.e. 5 US gallons.

Several manufacturers, including the one referred to in the link above, indicated their intention to pursue the matter but there have not been any changes to the ruling to date, it would appear that the best/only way around it at present is by individual representation for exemption somewhat in the fashion that the ICON A5 has approached special exemptions to the weight limitations for USA LSAs.

It's a real shame because it certainly puts electric Pt103 aircraft at a disadvantage compared to IC powered because the IC aircraft is weighed with an empty fuel tank and can then add the weight of 5US gallons whereas the electric aircraft has to be weighed with batteries on board and they weigh the same 'empty' or 'full'.

I think the FAA fears originated from a lack of understanding, they may have thought that it would require hundredweights of (lead-acid) batteries to achieve similar electric flight times that 5 US gallons would provide in IC aircraft. They'd probably not heard of LiFePO4 cells, where 5US gallons' worth of weight in cells might provide an efficient aircraft with 20 mins or so of flight time.

The logical thing would have been for FAA to find that the electric Pt103 aircraft could have an empty weight increase equivalent to the weight of 5US gallons of gas/petrol ... but no-one ever accused aviation authorities of suffering from an over-abundance of common-sense.

It's safe if the Max weight stays the same and the empty weight allows for the weight of an empty fuel tank to be superimposed on the battery powered empty weight (excluding battery). Maybe it's an old rule they wish would go away. Nev

All he is asking the FAA for is to replace the weight that is allowed for fuel tank and 5 galls of petrol with battery weight. No overall weight increase at all. Sounds like a fair request to me. Hopefully they will see it that way and it will advance the development of electric aircraft.

You increase the empty weight to include batteries

You decrease the empty weight by the equivalent weight of Internal combustion engine and its accessories such as exhaust system, fuel injection system, starter motor, alternator, battery, fuel tank etc.

Maximum Zero Fuel Weight (before useable fuel) and Basic Empty Weights are dry arguments if you're going down that path; what the wing understands is MTOW, and electric flying will open up some new horizons.

If that's what the argument is, then there's an equal argument that:You increase the empty weight to include batteries

You decrease the empty weight by the equivalent weight of Internal combustion engine and its accessories such as exhaust system, fuel injection system, starter motor, alternator, battery, fuel tank etc.

 

Maximum Zero Fuel Weight (before useable fuel) and Basic Empty Weights are dry arguments if you're going down that path; what the wing understands is MTOW, and electric flying will open up some new horizons.

Far 103 basically has one main restriction. The empty weight must be less than 254 lbs. You can then add up to 5 gall of fuel plus the pilot. Of course the MTOW must be within aircraft limits. What the poster is saying is that as he is not adding petrol can he add the equivalent weight of the 5 gall (30 lbs) in batteries. The MTOW does not change one iota.

ELECTRIC

Replacing the internal combustion engine and the items I mentioned above (including useable fuel), and replacing that with a battery powered motor is going to reduce the empty weight by a lot more than 30 lbs, so the question is irrelevant.

HYBRID

I haven't seen any hybrid designs for RA aircraft so far, but the hybrid specifications we have seen, each developed at a cost of several million dollars, contain an electric drive motor (s), drive batteries, and a much smaller petrol motor than usual to drive the power line (prop) at maximum power demand, and an alternator while the machine operates at cruise power demand. The 30 lbs for 5 gals would be overly simplistic in this design.

Just as a matter of interest, and I accept there might be a more accurate comparison, but this was a quick check.

The Toyota Prius C is 13% heavier than the Corolla Sedan

The Prius at its GVM only carries 400 kg, so four big guys would make it overweight.

The Corolla carries 22.5% more.

So I wouldn't be getting too excited about a hybrid aircraft, particularly since an aircraft cruises at constant rpm, i.e. without brake regeneration.

Just as a matter of interest, and I accept there might be a more accurate comparison, but this was a quick check.The Toyota Prius C is 13% heavier than the Corolla Sedan

 

The Prius at its GVM only carries 400 kg, so four big guys would make it overweight.

 

The Corolla carries 22.5% more.

 

So I wouldn't be getting too excited about a hybrid aircraft, particularly since an aircraft cruises at constant rpm, i.e. without brake regeneration.

Potentially there's an absolutely immense benefit to be gained from series hybrid application in aircraft. The big thing about planes is that we're forced to cart around a big engine the whole time when it's only actually needed for a tiny percentage of the time i.e. for the take-off and initial climb. The rest of the time it's an inefficient heavy-weight gas-guzzler operating well below its best torque.

The beauty of electric motors is that they can be run to whatever power setting you like (and rpm, within reason), provided you can get rid of the heat they generate. With modern liquid cooled brushless out-runners you can over-power the motors massively for a few minutes (overpowered to 3-5 times their continuous duty power rating) so you can have a nominally 30hp motor weighing very little but which can put out up to 150hp for the short period of the climb to cruise altitude. After which you reduce power to let the range extender recharge the batteries and the motor can then cool to continuous duty levels.

Additionally - and here are the even bigger rubs - electric motors put out max torque at any rpm (even if stalled and therefore not turning) so they're the absolute best power unit for an aircraft. And ... they put out the same power at sea-level or 50,000ft so don't require any form of turbocharging for density altitude compensation, and ... they don't need constant speed or variable pitch props because by using a carefully designed scimitar style blade, under high power settings the sweep will result in sufficient pitch change to provide optimal pitch for climb and v.v. for cruise.

Further - batteries actually increase their efficiency when heated to about 70-80C, so when you need the battery to have a higher power density i.e. deliver more power more quickly, you can heat the batteries using the waste heat from cooling the electric motor, which of course is producing more waste heat at that time so it's a win-win situation, which is rare among the usual compromises we have to endure in aviation.

Agree, straight electric will be the way to go, you only have to look at the transformation of RC to see the advantages.

In which case we will be looking at a far lighter empty weight anyway.

There is no answer to me other than logic.

I start the day in my Tesla 80% charged, 420 kms later it weighs exactly the same.

If a 747 takes off and the Captain has a problem such as has happened to a member of this forum, he then flew for a bit while dumping all the fuel until the plane reached landing weight and landed.

If the 747 was electric, that weight wouldn't change, it's not rocket science.