Jetstream 61
Topic Author
Posts: 111
Joined: Wed Jan 05, 2000 5:10 am

A Few Technical Questions

Fri Mar 09, 2001 12:17 pm

To any person reading this message that holds a private or commercial pilot license: Please answer the following questions for me. The questions are "interview" questions for my 10th grade research project on the mechanics of flight. In your response, please include your Name, title (captain, first officer, etc), employer, and a way to contact you so that I can get full credit for the assignment.

1. What are some of the direct causes and consequences of a stall?

2. When installing powerplants on aircraft, what are some of the conditions that are taken into consideration when choosing the powerplant?

3. With fuel tanks in the wings, how is fuel kept from freezing at high altitudes?

4. How are aircraft able to compensate for "thin" air at high altitudes?

5. What are some dangers of relying on exposed surfaces for flight control?

6. How could aircraft manufactures make aircraft in such a manner that stalls can be eliminated or made less common?

Thank you very much for your time and concern, I greatly appreciate it!

Yours truly,
Jack Wheaton
Posts: 2295
Joined: Wed Nov 03, 1999 3:54 pm

RE: A Few Technical Questions

Fri Mar 09, 2001 2:02 pm

I'll try to help with some of your questions.
1. The cause of an aerodynamic stall is the separation of airflow from the upper surface of the wing, causing turbulent flow. The consequence of the stall is that the aerofoil can no longer produce enough lift to balance the weight of the aircraft, therefore a descent will ensue.
2. When installing a powerplant on an airframe the manufacturer must consider the planned weight of the airframe, the projected performance and the weight penalty of the powerplant itself. Also included are the ongoing cost of maintaining the engine as opposed to the earning capability of the aircraft.
3. Most aircraft have an oil to fuel heat exchanger in the lines between the tank and engine to ensure no ice enters the engine. The fuel in the tank may be heated by element, however in aircraft such as the B747 the fuel doesn't freeze until approximately -50 degrees celsius. The friction of airflow over the wing generally keeps the fuel temperature above this point. If not the aircraft is accelerated to increase friction or descended to increase outside air temperature.
4. Can you rephrase the question, what about high altitude operation do you need to know?
5. Icing would be the major issue, particularly predeparture icing jamming the control surfaces.
6. Some manufaturers of canard equiped aircraft claim stall resistance, as the canard (a forward mounted lifting surface) stalls first, thus pitching the nose down and preventing main-plane stall. Conventional aircraft can be fitted with stall prevention, such as stick pushers, which lower the aircraft's angle of attack to prevent the stall.
Send me an email to the address in my profile and I'll forward all my details to you.
AJ  Smile/happy/getting dizzy
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: A Few Technical Questions

Fri Mar 09, 2001 2:15 pm

1. High angle of attack or too slow speed. Lift on an aircraft is created by the difference in pressure on the top surface of a wing compared to the bottom of the wing. As you increase the angle of attack or decrease speed - the pressure isn't sufficient. Compressor stall is basically same in principal - the compressor does not have enough airflow. Most modern aircraft has stall warning systems which react b4 a stall occurs. One such system that helps to prevent a stall automatically is called autoslats. When a approaching stall is sensed, th slats will automatically extend to provide extra lift.

2. ----

3. First of all - JetA fuel has an approx freezing point of -50 or 60. Something like that. And that's just the point where it even starts to frezze. VEry hard to freeze. You'd have to be flying over the north pole or something like that to even be concerned with that. In anyevent, there are Hydraulic heat exchangers in all Boeing aircraft fuel tanks. They help cool hydraulic fluid and to a lessor extent, help heat fuel in the tanks. Its effects are pretty minute.

4. Not sure what you mean. Jet aircraft are actually more effcient at higher altitudes because they don't have to work as hard to move the aircraft through the air.

5. Dont understand question.

Will try to answer # 6 later

Cdf MxTech
Aircraft MX Technician
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

RE: A Few Technical Questions

Sat Mar 10, 2001 4:45 am

I'll see if I can help out a little:

#1 - Pretty well covered.

#2 - Not quite sure, but I bet it has a lot to do with intended use. I once flew a Cessna 172 that had a 180 HP engine, instead of the normal 160. It made an average short field airplane into a great one. I think size of airplane comes into play... I mean it doesn't make to much sense to strap an IO-360 on a C152. I'm sure weight and balance comes into play. You don't want to put a powerplant on that will throw the airplane out of it's forward CG limit...

#3 - Well, not sure here either. I know that avgas doesn't freeze in the normal temperature range where flight occurs. I'm not sure about Jet A though. Someone else will have to answer that one.

#4 - A prop can only get you so high. Plus, normally aspirated (not turbocharged) piston engines lose hp rating as they climb, because the air is less dense, and the pistons can't
compress as effeciently as they would at a lower altitude. And like I said before, props lose effeciency with a higher altitude, once again because of the decrease in ambient air pressure. All jet engines are basically just big air compressors, and, well, they spin a lot faster than a prop does. A jet squeezes the air a lot harder than a piston or prop can, so producing thrust at high altitude isn't much of a problem.

#5 - Pretty much covered too - icing. But I'm not quite sure how else you would control other than exposed surfaces. Maybe I just don't quite understand the question.

#6 - Yes. As mentioned earlier, there things such as stick shakers and stall horns to warn you of an impending stall. Another technique of controlling a stall is what's called a spanwise airfoil variation. Basically, the airfoil is designed so that when a stall occurs, the section of wing near the root stalls first, so that the pilot still has effective use of the ailerons for control. There is no way that I know of to eliminate the possibility of a stall. It's a by-product of the way that lift is produced. When you use an airfoil to produce lift, there is always a possibility of the stall. It comes with the territory. You can minimize it with airfoil design, but you can't eliminate it.

Hope that helps...

RE: A Few Technical Questions

Sat Mar 10, 2001 8:39 am

3.Fuel is used to cool IDG oil (Integrated Drive Generators) and then returned to the tank. It is not the OAT that counts but TAT (Total Air Temp). The higher the speed the warmer the skin of your a/c gets (and the fuel)
Min. fuel temp for CFM 56-5B engine :
Jet A1/JP8 :-48°C
Jet A : -36°C
Jet B : -46°C
JP4 : -54°C
4. Speed.(altough high speed buffet and low speed buffet close in)
6.Spoiler named it all exept that you can not stall the Airbus 320 family. If speed gets too low it will autom. put TOGA (take of/go around) Thrust and eventually lower the nose.

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