# Lost Climb performance on twin

#### Flight81

##### OO BRO FO
Can someone please further clarify what all this means. And How can I better apply it and explain it.

The explanation says when you lose an engine on a twin you lose 50% of available power and 80% of climb performance. Obviously drag and all that is part of the reason but I also read that it is also that drag varies as the square of the airspeed and the power required to maintain that airspeed varies as the cube of the airspeed.

So what is that actually saying? Using and airspeed of 150 kias what do the numbers actually mean?

#### tgrayson

##### New Member
The explanation says when you lose an engine on a twin you lose 50% of available power and 80% of climb performance.
If you lost half of your income, how much would your entertainment budget decline? By a lot more than half, I bet. That's because your *entire* entertainment budget depends on your having excess cash over your expenses.

Same thing in an airplane. Pretty much all of the power of one engine is used just to maintain level flight. The other engine contains all the extra power that can be used to climb the airplane. So, when you lose half your engines, you've lost 100% (almost) of your climb ability, which is what they mean when they say performance.

#### Flight81

##### OO BRO FO
If you lost half of your income, how much would your entertainment budget decline? By a lot more than half, I bet. That's because your *entire* entertainment budget depends on your having excess cash over your expenses.

Same thing in an airplane. Pretty much all of the power of one engine is used just to maintain level flight. The other engine contains all the extra power that can be used to climb the airplane. So, when you lose half your engines, you've lost 100% (almost) of your climb ability, which is what they mean when they say performance.
Roger that..... However I more interested in understanding the meaning of the numbers associated with the statement drag varies as the square and power required is the cube of the airspeed.

When you work those numbers, they are large, so what unit of measurement are they refering to?

#### Polar742

##### All the responsibility none of the authority
I guess you're talking about a Pt 23 airplane, which means the left over motor doesn't have to do much.

SFAR41 (If I remember correctly) and part 25 airplanes have different performance requirements.

FAR Part 23 airplane may, or may not climb after takeoff (there are different Part 23 rules for "Commuter Category aircraft):

FAR Part 23 said:
§ 23.66 Takeoff climb: One-engine inoperative.

For normal, utility, and acrobatic category reciprocating engine-powered airplanes of more than 6,000 pounds maximum weight, and turbine engine-powered airplanes in the normal, utility, and acrobatic category, the steady gradient of climb or descent must be determined at each weight, altitude, and ambient temperature within the operational limits established by the applicant with—
(a) The critical engine inoperative and its propeller in the position it rapidly and automatically assumes;
(b) The remaining engine(s) at takeoff power;
(c) The landing gear extended, except that if the landing gear can be retracted in not more than seven seconds, the test may be conducted with the gear retracted;
(d) The wing flaps in the takeoff position(s):
(e) The wings level; and
(f) A climb speed equal to that achieved at 50 feet in the demonstration of §23.53.
Notice the descent part.

#### Flight81

##### OO BRO FO
Ok.... Let me better explain what I'm looking for. In the multi engine ASA oral prep book, a question asks "Overall climb performance is reduced by what value in an engine out emergency? Why?"

The explaination talks about "drag increases as the square of the airspeed while the power required to maintain that speed increases as the cube of the airspeed."

Now what does that mean? SO say we use 150 kias as the reference speed and we square it, as the answer says, we get 22500 as the numerical value for drag. What value or measurment does that represent? Furthermore, if we cube the airspeed to find the power required to maintain that speed we get 37500...What does that number represent?

I'm trying to better understand what these values are, I mean I can regurgitate the answer but I really have no clue what it means.

#### tgrayson

##### New Member
Furthermore, if we cube the airspeed to find the power required to maintain that speed we get 37500...What does that number represent?
It's not "airspeed cubed", it's *proportional* to airspeed cubed. There are other factors in the equation such as Drag Coefficient, air density, and wetted area of the aircraft and you need some scaling factors to actually make the number correspond to some recognizable measurement of power, such as horsepower or watts.

If you don't use the scaling factor, it's still a measurement of power, it's just not a unit of measurement that anyone uses.

Note that this subject has nothing to do with the idea that you lose 80% of your performance when losing 50% of your power.

#### The Gardener

##### Terrafirma Phobic
Ok.... Let me better explain what I'm looking for. In the multi engine ASA oral prep book,

That right there is half of your problem. I can't stand the ASA prep books. They create robots who can regurgitate the proper answer without the proper understanding. I compliment you on wanting to find the correct answer and I would recommend you start with the FAA Aircraft flying handbook.

#### tgrayson

##### New Member
I can't stand the ASA prep books. They create robots who can regurgitate the proper answer without the proper understanding.
I agree. The author really isn't good on technical issues. I picked up a number of crippling ideas from his books that took me years to undo.

#### Flight81

##### OO BRO FO
Gotcha.... Thanks for the explaination.

#### woutlaw

##### Well-Known Member
Here's what finally made it stick for me.

Take your twin up with an MEI and reduce power until it's just maintaining level flight. You'll probably be somewhere around 40% power depending on weight. (Technically, I suppose you'd want to be somewhere around Vyse but something above Vmc+5 for sure.)

On a Duchess, for example, that means you're using, say, 40% of 360 horsepower, or 144 hp total, which means you have 216 excess hp available for climb. (ignoring losses due to prop inefficiencies, density altitude, etc...)

Now take one engine out of the equation and you now have only 36 additional horsepower available for climb. (180-144=36.)

That's a decrease of roughly 83% in available climb power.

Now factor in the loss of lift generated by the loss of accelerated slipstream on the wing with the dead engine, increased drag, etc... and it's amazing the thing will maintain level flight, much less climb. Of course, heavy, on a hot day it won't.

Hope that helps.

#### GuitarMan

##### New Member
Here's what finally made it stick for me.

Take your twin up with an MEI and reduce power until it's just maintaining level flight. You'll probably be somewhere around 40% power depending on weight. (Technically, I suppose you'd want to be somewhere around Vyse but something above Vmc+5 for sure.)

On a Duchess, for example, that means you're using, say, 40% of 360 horsepower, or 144 hp total, which means you have 216 excess hp available for climb. (ignoring losses due to prop inefficiencies, density altitude, etc...)

Now take one engine out of the equation and you now have only 36 additional horsepower available for climb. (180-144=36.)

That's a decrease of roughly 83% in available climb power.

Now factor in the loss of lift generated by the loss of accelerated slipstream on the wing with the dead engine, increased drag, etc... and it's amazing the thing will maintain level flight, much less climb. Of course, heavy, on a hot day it won't.

Hope that helps.
Agree and love this explanation.

#### CFmike

##### Well-Known Member
Ya I agree with the last explaination. Thats how I learned it. Power required to maintain level flight is somewhere around 150 hp (ballpark estimate). When you lose an engine you only got 180 hp and straight and level takes 150 hp of it.

I also learned how to draw a generic chart with power required and power available for the visual learners. I'll see if I can't post it up later!

#### CFmike

##### Well-Known Member
Not the greatest picture. Did it in paint but you get the idea.

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#### tgrayson

##### New Member
Not the greatest picture. Did it in paint but you get the idea.
Hmmmmm.....Power Required curve looks all wrong. Should be U-shaped, since we all know that as you get slower, there is a point of minimum power and getting slower than that means an increase in power required.

The power available curve looks more like a jet's, where power increases with airspeed. Power on a prop should be a lot flatter, with a peak towards the middle. And it puzzles me that the power available for single and two engines is the same at low airspeeds, but the difference widens with speed. That can't be right.

Here are mine:

#### CFmike

##### Well-Known Member
Oh wow those look much better. :yeahthat:

Thanks for clarifying that up. Its been nearly 2 years since I've done any flying in a multi.

I also like the 'descending' portion of those charts.

Hey tgrayson do you mind me asking what program you used to make those charts and can I save these to my computer for future reference/notes?

#### tgrayson

##### New Member
Hey tgrayson do you mind me asking what program you used to make those charts and can I save these to my computer for future reference/notes?
I use CorelDraw. If you have the bucks, Adobe Illustrator is probably the program of choice, but CorelDraw is almost as good and less expensive. Sure, you can save them, but if you publish a book with 'em, I'm coming after you.

#### JK7177

##### Well-Known Member
I think the entertainment budget analogy is excellent at least to grasp a rough idea of what happens