W&B and CoG Jabiru 170D/230D

[RFguy]

Flying tailplanes aside, and that is has downforce due to being inverted, is the aerofoil shape of the tailplane of a similar ilk to the main wing? (similar curves) or quite different ? I guess the airflow at the rear of the aircraft is rather complex and highly attitude dependent.

[Bruce Tuncks]

The tailplane usually works at different Reynold's numbers and so is a different section, well in an advanced design it is.

It acts downwards most of the time. There was an LS3 ( glider) where the tailplane broke with a loud bang during a high=speed start run. Being FRP, it stayed together enough to land safely.

The loads on the broken tailplane were clearly downwards. It sported a lot of anhedral when it landed.

[facthunter]

It generally has a different section as it has different job to perform. Often it's just flat with a round metal section being it's structure. It has to provide a force in both directions at times. Most of the time it will be providing a down force. While this causes more total drag it's the most pitch stable arrangement. In some STOL types it's designed to provide more down force than usual with a curved convex undersurface. Nev

[Old Koreelah]

It should be realised that all this knowledge about CoG limits has been learned the hard way; I suspect many crashes in the early days could have been avoided if their design had been more pitch-stable.

A well-known designer showed me how he greatly improved the flight stability of a vintage replica by moving the CoG forward about 100mm forward of where it was in the 1920s.

[facthunter]

They knew a lot about pitch instability 90 years ago. You really can't make a "chuck" glider work without it being right. Nev

[Old Koreelah]

They knew a lot about pitch instability 90 years ago. You really can't make a "chuck" glider work without it being right. Nev

I guess that's true Nev, but I'm just relying on what I was told; the replica was reported to be much less tiring to fly (and presumably more stable) than the original.

Edited August 20, 2020 by Old Koreelah

[RFguy]

Back to CoG and W&B and competency.

 

Given that LSAs etc have MTOWs often 2x their dry weight, it would seem that there are a few gaps to be filled in the RAaus PC and GA RPL, IMO. The POH handbook certainly spells it out but there seems to be a disconnect with what is actually being done in the field. I was reading through various silly-bus , and there isn't much discussion on the potential of lost of control aspects of CoG errors.

 

Overweight but CoG right on the money is one thing, CoG errors leading to instability and LOC is another. Which brings me to another question: If permissible CoG range reduces to an infinitely small range at MTOW , what happens beyond MTOW ? Example, say permissible CoG range 100mm at 500kg, permissible CoG range is 1 mm at 600kg (MTOW), what happens BEYOND MTOW? given that you cannot have negative permissible CoG range. no such concept. But something must happen. does the aircraft begin to need a bigger tailplane to compensate for the fore weight ? and so becomes a bit nose down in behaviour (lowering AoA, lift etc). Given that available lift is coming into this then height (pressure and temperatures) aspects would also become relevant. (but they always were).

 

a good question for you doyans me thinks.

 

Glen

[turboplanner]

Back to CoG and W&B and competency.

 

Given that LSAs etc have MTOWs often 2x their dry weight, it would seem that there are a few gaps to be filled in the RAaus PC and GA RPL, IMO. The POH handbook certainly spells it out but there seems to be a disconnect with what is actually being done in the field. I was reading through various silly-bus , and there isn't much discussion on the potential of lost of control aspects of CoG errors.

 

Overweight but CoG right on the money is one thing, CoG errors leading to instability and LOC is another. Which brings me to another question: If permissible CoG range reduces to an infinitely small range at MTOW , what happens beyond MTOW ? Example, say permissible CoG range 100mm at 500kg, permissible CoG range is 1 mm at 600kg (MTOW), what happens BEYOND MTOW? given that you cannot have negative permissible CoG range. no such concept. But something must happen. does the aircraft begin to need a bigger tailplane to compensate for the fore weight ? and so becomes a bit nose down in behaviour (lowering AoA, lift etc). Given that available lift is coming into this then height (pressure and temperatures) aspects would also become relevant. (but they always were).

 

a good question for you doyans me thinks.

 

Glen

The problem we have in RA is just getting pilots to understand their obligations for the base aircraft in normal operations without the complexities.

It will be interesting to hear your reactions after the touch and feel control modualtions that will fill out the picture for you happen at Cowra.

[facthunter]

It's not just an RAA problem. Perhaps it's as much or more in other areas of aviation. The most common place I recall for issues is PNG before independence probably because of the pressure to carry just about anything as there were no roads.

Also I don't accept the criticality at high weights being specifically a C of G issue for all planes The empty Wt v/s all up Wt ratio is much the same right across aviation except for a few different ones (about 55%) WHERE the loads are carried is more important. If you get far from the balance point, the effect of any weight , or wt change will be more. Fuel located far away will change the Cof G in flight perhaps by a large amount.

There are some RAAus types that are near impossible to load outside the range. They have to be 2 place side by side or single place. Tandems always get complicated for obvious reasons. The operator /pilot must KNOW their plane .Nev

[turboplanner]

Also I don't accept the criticality at high weights being specifically a C of G issue for all planes The empty Wt v/s all up Wt ratio is much the same right across aviation except for a few different ones (about 55%)

.

The Weight part of weight an balance is whether the aircraft at the flight planned phase of flight is within the All up weight.

(a) There's MTOW

(b) The MTOW is reduced by a set of calculations for surface, grade and altitude of the airfield and outsided air temperature

© Quite a few airctraft have a lower undercarriage capacity than MTOW, so the flight plan has to include the time when you can land again. No point taking off at MTOW for a field a short distance away because you'll collapse the landing gear, so an emergency stop might mean flying around and burning off fuel until you are light enough to land.

 

The Balance part of W&B is the calculation for

(a) reserve/flight fuel mass

(b) baggage/freight mass

© people mass

(d) any other mass which is not part of the empty aircraft mass

 

The balance calculations are not guessed because the net result is a combination of how far they are from the datum and each individual mass.

 

WHERE the loads are carried is more important. If you get far from the balance point, the effect of any weight , or wt change will be more. Fuel located far away will change the Cof G in flight perhaps by a large amount.

This is one of the reasons you don't guess. In general terms aircraft with fuel tanks in the fuselage will perform differently during the flight because their "moment" arm is usually longer than a wing tank.

There are combinations of heavy passengers and full or empty tanks which the mind's eye deson't pick up but a ten second cal will.

 

 

re some RAAus types that are near impossible to load outside the range. They have to be 2 place side by side or single place.

If the fuel tank is on the datum there will be no change in flight, and if the person/people are on the datum you are not getting the cantilever multiplier, and the calculation here is more to decide whether you have to drop a passenger or some fuel before the flight.

 

In some cases the Ultralight designer has placed the seats so it doesn't matter if the pilot doesn't calculate, and so might specify a particular seat for the pilot if he's solo, but W&B calculations will still optimise your flight.

 

Tandems always get complicated for obvious reasons.

This is because of the person further away from the datum, so best to calculate.

 

The operator /pilot must KNOW their plane .Nev

That would have worked some years ago, but since Endorsements ceased, you will be pointed towards an aircraft you have never flown without any training, except in some cases being told to rear the POH. If you do your full P&O checks including W&B, you will know the airctraft is within its envelope before you start the engine, so this is not an issue as far as W&B is concerned.

[facthunter]

You just can't leave some off my stuff alone. If you read carefully what I post and understood it , you wouldn't write such irrelevant responses. You and I are in different worlds on a lot of this. Not everyone has your particular approach to these problems and clearly your experience is limited in some areas. Nev

[turboplanner]

You just can't leave some off my stuff alone. If you read carefully what I post and understood it , you wouldn't write such irrelevant responses. You and I are in different worlds on a lot of this. Not everyone has your particular approach to these problems and clearly your experience is limited in some areas. Nev

I'm going pretty much straight from the syllabus, that's why I took the trouble to post. People think they don't have to do the W&B calculations.

[facthunter]

Well, don't accuse me of stuff I've never advocated or said. .Nev

[Yenn]

Glen. I cannot think of any aircraft such as you describe,, where CofG limits are 100mm at 500kg and 1mm at 600kg.

Is there such an aircraft? I am wondering if this is a real situation that I have no knowledge of, or is it something someone has said without proof?

It is certainly necessary to ensure that the attitude of the plane is correct when weighing it. My Corby is well within C of g limits, but if I pick up the tail by hand and lift it to abour 1.6m height, the plane will tip forwards if I let go. This leads to the tongue in cheek question. If I am in a steep dive does my Cof G move forward?

[RFguy]

Hi Yenn,

 

The 100mm and 1mm example was about using limiting case , in math speak.

 

My example right at the beginning of this now four page topic- J170D v J230D (which is a lightly loaded J430)

reproduced below :

 

J170-D Aft Limit 272-mm (10.718”, 27.5%) aft of datum at all weights

Forward Limit :

180-mm (7.09”, 18.2%MAC) aft of datum up to & including 440 kg

255-mm (10.0”, 25.8%MAC) aft of datum at 600kg

and.... 272 - 180 = 92mm, (up to 440kg) then at MTOW 272-255 = 17mm

there are plenty of aircraft similar, this narrow range is not unusual.

[RFguy]

and as for your steep dive example, I would say the answer to that riddle you have provided it is about relative airflow. The relative airflow over the aircraft would be the same as in level horizontal flight, and the CoG conditions are based on some airflow benchmark (presumably cruise but I have little knowledge of the specifics of aircraft design) , . did I pass?

[Roscoe]

and as for your steep dive example, I would say the answer to that riddle you have provided it is about relative airflow. The relative airflow over the aircraft would be the same as in level horizontal flight, and the CoG conditions are based on some airflow benchmark (presumably cruise but I have little knowledge of the specifics of aircraft design) , . did I pass?

Glen, with respect, I think you are going into this W&B thing far too deeply.

When you start your flying read the POH for the Aircraft you are operating, and go through some loading scenarios with your Instructor.

Dont overcomplicate with in depth mathematical formulas or you will find your Instructor might get fed up fairly quickly.

By all means, read up to your hearts content, but i believe you are going way beyond whats necessary in understanding basic loading stuff.

Just enjoy your flying!!

[RFguy]

......I am deeply aware that I have the capacity to annoy or distract the instructor, and will refrain. grin. Remember... engineering detail is what I do for a living.. When I was 25 I would probably p1ss off the instructor without thinking, but I am a bit older than that now...

cheers

[facthunter]

Yenn your plane only tips forward as you have the wheel contact point as a balance point. In the air it misses the ground by a large margin.( For a while).Nev

[Kenlsa]

To those who may be numbers people ( I include myself here) don't over sweat this stuff.

My wife's cousin was an aeronautical engineer in the Uk for one of the big manufacturers and got so concerned about the numbers and how "little" the margins seemed matter he died without ever flying. Too much thinking!

Ken

[Bruce Tuncks]

Turbs, you said " legal" and "safe" as if they were the same thing. Is this what you really think?

[turboplanner]

Turbs, you said " legal" and "safe" as if they were the same thing. Is this what you really think?

I went back a few posts and couldn't find the "legal"; can you tell me which post # I might have used the wrong word.

[Blueadventures]

......I am deeply aware that I have the capacity to annoy or distract the instructor, and will refrain. grin. Remember... engineering detail is what I do for a living.. When I was 25 I would probably p1ss off the instructor without thinking, but I am a bit older than that now...

cheers

To agree and reinforce what Roscoe said (I have been considering what to comment) and also to provide support. At training hold your thoughts and respect the instructor. They are skilled at progressing students through flight training. Yes some have certain quirks and certain thresholds of safe flight. A type you don’t want is one that remains on the controls showing you their skills. Read up the flight manual and POH. Over loading should not be a concern as only training, 2 persons and fuel. No cargo or baggage. As you progress you will have a better view of the whole flying / pilot picture / pie. Treat the engine that it will stop and be ready to correctly take emergency actions, by flying the aircraft to the presented situation and discard the unnecessary bits. Enjoy the journey and most of all don’t play any mind games or ask challenging technical questions like some of the Engineering tribe do. I have Engineering quads and some Engineers I have known just ask too many questions, that although they want to know the answer It totally disrupts the flow of the work or task. If your ever in Mackay your welcome to a fly in my 19 Reg Skyranger Nynja.

[RFguy]

At training hold your thoughts and respect the instructor. They are skilled at progressing students through flight training.

BlueAd, that is good advice , They have done this before with many different students. I find the best teachers are ones that can adapt/vary a little to suit the student. I'm not at all concerned about the W&B for the training.... We are not going up there with a toolbox of housebricks..... BUT:

 

Actually the thing I wonder is : will I easily develop the intuition of 3 axis coordination, or will my brain / visual +hand eye acuity just not be up to scratch learning late in my life (49) rather than earlier. I guess I am dazzled by the learning rate of my 5 yo and 7.8 yo , and wonder if humans continued to be able to learn at that rate (at 5 years old) their whole life, what sort of place we'd live in.

 

Thanks for the Mackay offer ! I have a feeling the aircraft I buy where ever will be a long ferry home. Sort of reminds me of driving my first car I bought home some distance, barely held together learning to drive a manual gear shift for the first time doing it..-GLEN

[facthunter]

You can pull to the side of the road with a car. Flying is not that hard but it sure is different and very unforgiving of errors or mistakes. Nev