Rule of thumb for ROC based on weight.

[damiens]

Is there a rule of thumb calc for changing weight and rate of climb.

 

As an example. If at standard conditions, sea level, 1013 and 15 degrees and 550kg takeoff weight I achieve 650fpm climb at VY. Is there a calculation to say what rate of climb FPM I would expect at 600kg (extra 50kg).

 

So what would the extra 50kg decrease the rate of climb by, all things being equal?

 

 

[mnewbery]

I can do an energy calculation if you want. PM me if this is of interest

 

 

[Ryanm]

Lots of variables to have a generic rule of thumb.

 

The POH/AFM might already give you the figures for various weights, as well as penalties for atmospheric conditions. Check the Performance section.

 

 

[mnewbery]

I agree with Post #3. Won't help if the plane is an experimental or heavily modified

 

 

[mnewbery]

The rough number I came up with was 595fpm which only takes into account the relationship between the power available for increasing altitude and the change in mass.

 

Any PPL or CPL level aerodynamics book can give the details about changes in TODR, LDR, Vx and Vy as mass changes that I am neglecting here.

 

 

[poteroo]

The POH/AFM might already give you the figures for various weights, as well as penalties for atmospheric conditions. Check the Performance section.

Most of them have a barely believable ROC shown, and certainly not broken down into conditions vs weight. Also, whilst these 'numbers' are supposedly for standard atmospheric conditions, you need to be aware that in Australia, we are usually on the wrong side of these. The result is that we rarely achieve book numbers due to operating in higher DAs, but you also have to consider all the other variables such as engine and propellor performance due settings and wear/tear, and airframe rigging by individual aircraft. ROC gets 'lost' due to so many things that it becomes meaningless to try to fine tune it in the way suggested. You'll usuall find that the IAS needs to be increased above Vy for engine cooling purposes. In practice, with low HP types, if you can maintain 500fpm then plan on that and just take the extra speed. With my RV, I just set the vertical speed on the autopilot to 750 fpm and set, say 75-80% power, - and take whatever IAS results. It will always be faster than Vy, and the engine stays nice & cool. happy days,

 

 

[damiens]

The rough number I came up with was 595fpm which only takes into account the relationship between the power available for increasing altitude and the change in mass.Any PPL or CPL level aerodynamics book can give the details about changes in TODR, LDR, Vx and Vy as mass changes that I am neglecting here.

So is this based on an approx 9% weight increase (from 550kg to 600kg) and corresponding 9% performance decrease from 650ft per minute to 595ft per minute. Does this imply that the climb performance degradation is linear with the weight increase. That sounded way more complicated then i intended.

 

Understanding that things like density altitude and other factors are more significant, I thought it would be handy to have a rough guide given that the POH is next to useless on the issue. My thinking being that if I am very used to flying a particular aircraft at say 500 kg and I chuck in a passenger and some luggage in, I could expect X fpm in climb. Of course, the other way is to go for a fly and check it out :-). Thanks all for the input.

 

 

[facthunter]

I've never applied any rule of thumb for that , but invariably you check your take of performance charts carefully, particularly when you are high and hot where your density altitude is a real trap. If you are used to your plane (type and condition) you will be familiar with it's reluctance to climb on hot days with a couple of big people and full fuel onboard.

 

Where density altitude is the same ,you should climb at a best angle of attack which will require a faster actual climb speed than normal when at heavier weights.. This will keep your lift co efficient constant and probably produce a relationship between weight and thrust which is a straight line approximately. By flying a bit faster the thrust does more work( Force x Distance) but it's a small % and can be ignored, so you are trading energy from the engine for Potential energy (By virtue of position (height increase) which is Mass x gravity x height so a direct relationship with mass and height. Hope this "musings" fits in with your logic. because it's the same answer. In reality your "service ceiling" may be a quite low altitude at heavy weights so will enter into the picture if the plane is generally underpowered. Nev

 

 

[mnewbery]

One thing I was planning to avoid mentioning is that Vx is the indicated air speed that maximises vertical force and Vy is the indicated air speed that maximises the difference between propeller generated thrust and total drag. As mass increases to MTOW, the total drag is increased by the required increase in angle of attack to generate the lift. This doesn't means the aircraft nose pitches up, it means the path through the air is less steep and the nose pitch relative to the earth stays the same. Depending on the wing that could be anywhere between 9 and 14 degrees typically.

 

The description of Vy here and its relationship to power is in agreement with post #8. The POH only sees noticeable large differences between Vx and Vy speeds when there is a lot of thrust and big differences between the lowest and highest all-up weights (like the Boeing 757). One Cessna 172G I fly has no measurable difference between Vx and Vy with two people in it. Its rather old and slow.

 

From this, there are two things robbing the systemof rate-of-climb:

 

1. The power available from the propeller is constant and can't be increased, at 650 fpm its only about 17-18 Kw devoted to lift and the rest is drag

 

2. As mass increases, more drag is induced due to a higher angle of attack pointing the lift vector backwards

 

The coefficient of lift is not normally a straight line change but it can be

 

 

[facthunter]

To be efficient at higher weights you do not use a higher angle of attack. You increase speed (if you can) and retain the BEST angle of attack for THAT particular airfoil . As an example your best holding speed,( Least drag speed) will vary with your actual weight as does your stall speed . (another angle of attack phenomenon). The Lift/ Drag ratio is best at one angle, which is not pitch as you say but the angle to the relative airflow. Your line 3 in the first para conflicts with your #2 in the third para. on that matter.

 

People often when discussing these things often consider the drag can be divided into two types. Drag associated with the wing providing lift and other "parasitic" drag. Wires wheels struts fuselage etc Nev

 

 

[mnewbery]

Yes it does conflict. My bad. Such an easy mistake to make which is why I'd rather not have gone there

 

 

[facthunter]

I don't find it easy to explain clearly in simple terms, either. It's an area of some neglect and misunderstanding. Most people only think of one speed when stall is mentioned. . If things get too complex people glaze over. . I hate being picky but people are mucking this stuff up and paying for it, dearly. Nev