Instructor Rating Briefs - Help?

[oneday_soon]

Can anyone help me explain the power and drag graphs for the straight and level flight brief.
I need a simple way of explaining it, but keep getting my self stuck and don't think i sound very clear in my explaination. I guess there must be a ( KISS ) "keep it simple stupid" explaination.

Any help greatly appreciated.


ODS

[OZBUSDRIVER]

From sitting on the other end of the table...All I remember is "Jones, see this part of the graph (pointing to the tail end of the drag curve) Do not allow yourself to get caught here".. end of briefing!

Read an article linked from Peter Garrison explaining best cruise speed and how it was worked out Carson (http://www.melmoth2.com/texts/Carson.pdf)

My understanding of it is the guys who formulated the limit line , specific power proportional to speed(equivalent to a line drawn at 45deg rising to the right) came up with the proposition that as I would explain it better where the L/D curve forms a tangeant with a line parrallel to the the limit line. This tangeant represents the point of best economic cruise speed. This speed doesn't represent the most economic fuel efficient speed (that occurs at the best ratio between the L/D curve and available power) It represents the best value for speed vs time in the air vs fuel burn. The closest to the best ratio!

My Instructor just put a cross about so much along the L/D curve and said this was the best cruise speed. I think he was trying to explain the Gabriel/von Karman line

So, you could say, a bunch of engineers got together and come up with a line on the graph that represents the most perfect efficiency for any vehicle. The Limit Line. When we place our aeroplanes L/D graph near the limit line we can work out where a tangeant would occur on our L/D line compared to the limit line. The speed at this point represents the best cruise speed for this aeroplane. The point of getting the best return in airspeed for fuel consumed.

Hows that sound:}

[tio540]

Mechanics of Flight by AC Kermode is excellent, as are many stocked at your local airport bookshop.

Buy a couple of different books as they cover different subjects at different levels.

Understanding the principles is the basis for explaining them to a student. Good luck with the course, you will find it rewarding.

[MakeItHappenCaptain]

VERY basic guide
For S&L;
Induced Drag - High at low airspeeds/high AoA
As the path of the molecules over the upper surface is longer with higher AOA, there is a higher dynamic, therefore lower static pressure. This larger difference creates greater movement spanwise movement (as the pressures try to equalise) tending outward underneath and inward above the aerofoil. The blending of these spanwise flows creates the wingtip vorticies (well demonstrated by a photo of the tyre smoke from behind the big jets landing or dragging a spoon through a cup of coffee). Higher AOA = greater spanwise flows = bigger vorticies = more energy taken from the forward motion of the aircraft.
Parasite Drag - High at high airspeeds/low AoA and increases exponentially. (Double speed = ~Quadruple drag.) Break it into Form, Skin Friction, Cooling and Interference Drags. (Spats, fillets, flush riveting and retractable u/c ways of reducing these.)

Add the two together on a graph to achieve Total Drag.

Progressing to C&D, Total Drag = Thrust Req. (D=T to maintain constant speed.) Then you move onto Thrust available (for explaining AoC or Vx) and deriving with respect to time for Power Req. and Av. (RoC or Vy).:zzz:

Advanced Pilot's Manual by Kirschner is another good source or check with a few instructors at your school for another angle.:ok:

[VH-MLE]

Hello oneday_soon,

If you are a flight instructor, I can't believe (with all due respect) that this wasn't covered in your I/R training!!

I would also have thought that as a matter of course, you would have a theory of flight refererence eg Kermode etc, to assist you. Trevor Thom also does a good job using the KISS principle.

Regards.

VH-MLE

[Tinstaafl]

It doesn't sound like oneday_soon doesn't understand it - his question relates to how to share that knowledge in a clear & concise manner. Understanding something in your own mind is easier than being able to explain it in a manner that lets someone else's mind understand it!

I'm not sure where in your briefs this topic is intended to fit however something like the following might help:

1. Start with a *short* revision of forces in S&L flight: Weight therefore need Lift which produces Drag which in turn will require an equal amount of Thrust.

2. Expand Drag: Using a diagram show Parasite Drag vs Speed graph. On the same diagram overlay Induced Drag vs Speed. Also on the same diagram sum each point on both curves to give the Total Drag curve.

3. Remembering back to 1. above, explain that the Total Drag must be matched by Thrust so the two curves are the same. Cross out the Drag label & re-lable it Thrust Required

4. Highlight the way that the Thrust required changes with speed as Drag changes. Make sure you identify that there is an increasing amount of thrust needed at low speed/high AoA, reducing to a minimum at some speed then an increasing amount as speed increases.

5. Explain engine thrust output: Mention that there's a limit to what an engine can produce. Draw the Thrust available on the T.req graph. Use the overlayed curves to show how the Thrust avail limits the max & min speed due to the increasing T requirement.

6. Review forces in a climb: The key point is for the student to remember that Excess T. is why the a/c climbs. Greater the excess --> the greater the climb angle. An analogy for the student would be something like pushing a fridge up a loading ramp. The harder you can shove more than would be needed to slide the fridge level then the steeper a ramp you can use. Doesn't matter if it only slides up the ramp slowly, the important bit is how *steep* a ramp can be used.

7. Use the curves to find max. excess T and therefore Vx. Also min T for Vg

8. Explain that Power is related Energy released over time ie the faster you can get the same amount of work done (eg the fridge up an incline but more quickly this time) then the more power is needed. Mention that piston engines burn fuel to produce Power which is then converted into Thrust at the propellor and that fuel consumption is ~ proportional to power produced.

9. Use this concept to lead into a Power curve drawn on a graph below the Thrust curve. **Make sure the common speed points are aligned with the Thrust curve**.

10. Look for Max excess Power to derive Vy & min Power required to find Max Endurance.

11. To derive best range speed talk about how best range is all about getting the most number of miles/hr per gallon of fuel used/hr. Pick an artificially low speed/high power consumption point the curve to show how lots of fuel is being used but not many miles are being gained. Draw a line from the graph origin to this point. Choose another point a bit further along to show how speed per gallon is improving. Draw another Origin line to the new point. Keep going until your final point produces a line from the origin at tangent to the curve. Label this point Best Range Speed.

Depending on how much deeper you need to go into it you could also:

12. Show other Thrust/Power Required/Available curves that are the result of airframe configuration, altitude, temp, weight etc.

13. Include headwind & tailwinds and show how the origin is moved by them which shifts the tangency point.

14. Explain the region of reversed command ie 'Behind the Power/Thrust curve(s)



A point to note: If you do this on the whiteboard then you need to be able to draw reasonably accurate graphs of these curves that are appropriately aligned. I prefer this because I can pace what I draw with what I'm talking about. Otherwise you'll need to have OHP or presentations made with the changes being progressively included.

Good luck!


Added about finding best glide speed from the Thrust Req. curve. Forgot that bit last night.

[oneday_soon]

Thanks All for the info, greatly appreciated. Especially MakeItHappenCaptain and Tinstaafl for their longer information.

VH-MLE, Yes, I do get it, but i am doing a renewal after a very long time out of the game, and trying to remember a clear, concise and easy way to explain it to the student.

Cheers

ODS

[Valdiviano]

Hi, thinking of renewing my instructor rating from LONG, LONG time ago. Anybody wanting to sell either of the books mentioned on post #6, I am interested.
Regards