BAK Questions Thread

[mnewbery]

https://en.m.wikipedia.org/wiki/Vapor_lock

 

Vapor lock has been the cause of forced landings in aircraft. That is why aviation fuel is manufactured to far lower vapor pressure than automotive gasoline (petrol). In addition aircraft are far more susceptible because of their ability to change altitude and associated ambient pressure rapidly. Liquids boil at lower temperatures when in lower pressure environments.

[Pearo]

So why can't I run car gas on a low wing plane without bigger lines and a stronger electric pump?

You can. bigger lines and stronger pumps have nothing to do with anything.

 

Answer 2Car gas has some polarity (like salty water) so it can carry a current as a liquid. Aviation petrol and kerosine is non-polar as a side effect of its other qualities. This also makes it less likely to mix with water, like cooking oil.

 

Car gas has a higher vapour pressure but petrol needs about 14 grams of air per gram of petrol to be explosive. In a car gas tank, the vapour is much more petrol than what is required to be explosive and any static charge will generally not result in an explosion because the right place for the spark to cause an explosion isn't near the filler neck. More likely the spark will be "wet" which means the static charge will leak into the vapour like on a cold humid day. Occasionally people use plastic jerry cans and fill them without putting them on the ground. Empty plastic container not earthed the same as the pump plus hot dry day equals boom. But it's uncommon. Tell that to the people who have lost cars from dragging their nylon covered arses off their microfibre seat covers before a bit of self serve action.

 

Aviation petrol/kerosine WILL carry their own static charge. This means bigger sparks more often without grounding and in the case of a Piper Lance room for 170 litres of air mixed with petrol fumes per side. Cruelly, the right mixture for an explosion is often very near the filler cap for the average bug smasher.

 

Should I earth the petrol station filler on the car first? Touching the pump nozzle and a metal part of the car at the same time before opening the tank or dispensing petrol is enough. Even the plastic bits of the filler nozzle are weakly conductive. Plastic petrol containers need to go on the ground preferably away from the vehicle so if anything bad happens, you can drive a safe distance away.

 

Same with planes. Brakes off for refuelling if you can push/pull it. If the plane does catch fire it needs to get moved away from the rest of the petrol.

Vapour pressure aside, most of what you say is nonsense.

 

Aviation petrol is controlled for various reasons.

 

Octane: to prevent detonation. Standard service station petrol around the world uses different tests for octane rating, aviation petrol is the same.

 

Volatility: Petrol on the ground deals with fairly constant temperatures compared with aviation, so depending on where you live will depend on the blend. Aviation fuel on the other hand, it needs to go from local temps to say 14000ft temps, maybe higher, so additives are very different. MoGas may have a higher vaopour pressure, but it might not also.

 

Polarity: You are talking nonsense.

 

Static Charge: irrelevant for avgas v mogas. Aviation Kerosene is a different story.

 

The key here is that AVgas in Australia is the same as the US, or the UK, or anywhere else in the world. MoGas blends are local and also vary with each distributor. I.e. Local BP and Caltex service stations might source their fuel from the one distiller, but the distiller will blend the fuel based on the customer.

 

 

[mnewbery]

http://www.autofuelstc.com/piper_airplanes.phtml

 

Challenge accepted

 

• Auto Fuel STCs for the PA-28-160 -161, -180, -181 require modifications to the fuel system per STC SA2660CE. If you have a PA-28-140 that has been converted to a 160hp engine, then it will require the fuel system modification
   
   
  

[mnewbery]

And for the record fuel differences WAS covered in my BAK theory

 

 

[mnewbery]

Can I have a reference to a motor gasoline that has a lower vapour pressure than 40-50 kPa at room temperature?

 

If Aviation petrol isn't prone to holding a static charge THEN WHY is the additive described by ASTM D7547 used?

 

Maybe I am a total tosser who can't read a fuel specification or motor gas STC for a low wing plane.

 

 

[DGL Fox]

So why can't I run car gas on a low wing plane without bigger lines and a stronger electric pump?

I don't know if my SportStar has bigger lines and bigger pump but I know in the POH that the electric pump does not need to be on while in cruise so the engine is just running off the standard mechanical Rotax pump and I use Mogas....so I don't see what a low wing has to do with anything?

 

David

 

 

[mnewbery]

You answered your own question. The airframe and engine combination was designed from the start as a mogas aircraft. I'm calling thread drift on this one. Do you know where the electric pump is and why it needs to be on during climb?

 

 

[mnewbery]

...or what the gascolator is there for?

 

 

[DGL Fox]

...or what the gascolator is there for?

As far as I know it is there to catch impurities in the fuel, basically its a fuel filter and as far as the electric fuel pump is concerned these are optional on SportStars so there is no real need for them but my aircraft happens to have one and in the POH it says that it is only to be used at start up and not used continually as seen below, so it does not need to be on during climb out, so some aircraft probably do but not mine.

 

"Electric fuel pump operates during engine starting

 

period only. It is not intended for long continuous

 

operation for long time."

 

 

[mnewbery]

Erecrik fool pump also suck out right tank and blow in left tank. Please for you being to suck from left tank only, when fool.

 

Also something in the manual about weels.

 

 

[Romeo Juliet Whiskey]

Just reading about the centre of gravity/centre of pressure and why the nose drops when thrust is reduce. This makes sense but it got me thinking about what would happen in the case of a sudden increase in headwind (i.e. windshear) and no change in thrust. Does the nose attitude momentarily pitch up, down or stay the same? With the centre of pressure (lift) behind the center of gravity id guess that you get a nose down attitude but i presume the tail plane has a say in this.

 

Rich

 

 

[facthunter]

Whether the nose drops with power reduction or not depends on the thrustline. It can be quite complex, as the propeller(s) may affect the tail and the airflow over the mainplanes.(wings). Nev

 

 

[mnewbery]

Drifters definitely go nose up on thrust loss. During a gliding descent as the wing encounters an increased headwind, the Drifter wing produces more lift and pitches up.

 

There is an axiom in flying that says "in the absence of other inputs an aircraft will return to its trimmed speed" (then overshoot through it and enter a fugoid oscillation). The pilot in command makes things better or worse by adding or removing pitch with the "houses" lever as discussed.

 

This means that in the case of an increasing wind gradient closer to the ground, the aircraft will initially record an increase in IAS then begin to pitch up. Rotational inertia (flywheel effect) will cause the pitch to increase beyond that required to regain the trimmed speed. Then it will record a slower IAS and pitch down. This is the source of the fugoid oscillation in pitch.

 

The behaviour is a design feature of all fixed wing GA aircraft and it is quite deliberate. An aircraft can be designed to do other things (like stall the tail plane first thus losing pitch control while still generating main wing lift or have anhedral wings causing deliberate instability) . But the insurance companies don't like it and charge accordingly.

 

The size and frequency of the fugoid can be easily determined (discussed perhaps in another thread) and it is specific to each example of aircraft because of manufacturing differences and centre of gravity plus mass at the time of flight.

 

Your instructor should be able to demonstrate the effect of trimmed airspeed and fugoid oscillation. I found that lesson quite fun.

 

Fun fact. The Cessna Citation and Lear aircraft have engines above the wings. Losing thrust on takeoff with this configuration requires considerable down pitch input to maintain control. This requires specific training to safely handle. Cessna 172s, Piper Warriors and the like, these naturally pitch down on thrust loss. Insurance...

 

 

[facthunter]

The main factor requiring pitch trim adjustment is changing airspeed. To get the same lift from a wing that is reducing airspeed you need more angle of attack OR select flap, if it is of a lift increasing design (which most are) Selecting flap may require more back stick (and retrim) because of centre of lift change. Nev

 

 

[mnewbery]

More lift and much more drag when applying flap causes the aircraft to slow and pitch which way? This is definitely a BAK and a CASA PPL cyber exam question by the way.

 

 

[facthunter]

It's not consistent. Some behave differently to others. It's also complicated by the speed change after the flap extended.. You are usually reducing to take the next stage of flap. or a threshold speed target.Nev

 

 

[mnewbery]

Bonus point: Name an aircraft that pitches down when flaps are applied.

 

When flaps are applied (in BAK land) the chord line of the main wing is lengthened and pitched up. It does this in relation to the relative wind, the horizontal tail and the airframe. This has the same effect as moving the horizontal tail plane.

 

Right now, a pen and a bit of paper might help show the result. Just extend the chord lines for the wing and the horizontal tail with and without flap until the lines meet.

 

I got this question wrong twice.

 

 

[facthunter]

Why would it be important? I know I've flown some that did but I can't be sure which one(s). When you first fly it you may not react instantly but you will quickly anticipate it once you have a bit of time on the plane. The biggest problem is at go round from the landing configuration Cessna's Hi wing and jets with underwing engines. Large forward stick force until trimmed out.. Nev

 

 

[mnewbery]

It's in the exam

 

 

[facthunter]

Luckily I've already passed. Apart from passing the exam, why would it be important? Nev

 

 

[mnewbery]

Did you read the title of the thread?

 

 

[facthunter]

Yes. trick or unsuitable questions are worthy of discussion. Being an ex chalkie it's one of my biggest concerns. Nev

 

 

[Romeo Juliet Whiskey]

More lift and much more drag when applying flap causes the aircraft to slow and pitch which way? This is definitely a BAK and a CASA PPL cyber exam question by the way.

In the Jabiru it pitches up. Flaps down, nose down I'm taught to maintain altitude.

 

 

[facthunter]

Yes and if you don't do that your speed will wash of pretty fast as well. In some aircraft like a technam the speed will change very rapidly if you don't have a fair bit of power on or descend when you extend flap. Having a low Vle exaggerates that effect. Nev

 

 

[mnewbery]

Edit: The OP and facthunter correctly pointed out the pitch up or down on flap application is dependent on the change in the centre of pressure plus the change in drag.

 

I earlier noted for a simple flap the effective wing chord is lengthened when maximum flap is applied. This is incorrect for the purpose of understanding the BAK question (even if the question is irrelevant and obsolete). It may actually get shorter, effecting the centre of pressure moving forward.

 

The BAK question assumes the simple flap full application will always move the centre of pressure of the main wing forward thereby causing a pitch up.

 

Aircraft that pitch down with full flap application include those with leading edge lift devices (think Boeing 737) and aircraft that use split flaps or a combination of split flaps with fowler action. I think some examples include the Cessna 340 and Beech Baron. Now I will need to go have a look.