grecoaj

One of my students asked me why the Short Field Takeoff and Landing tables are only made for max gross weight. We fly a 1980 Cessna 152.

The question arose when we were talking about practicing on some shorter grass strips in the area and I pointed out one turf airport that we would never go to (for PPT, anyway) that was 1,300', due to the information in the tables. The other question was "what if we weighed less than 1670?" The answer of course is a shorter T/O and landing, but by how much? Anyone have any ideas on how or where to get this information, short of playing "test pilot"?

The elevation of the airport is 978', assume a normal summer day in the northeast, 4:1 slope to clear trees.

tylervet

Quite simply, the performance tables are meant to be simple and easy to use. A 2 seat airplane should be simple, there is little need to publish data for such a small change in performance.

I have never seen additional performance charts for lower weights in the C-152. As I'm sure you know, extrapolating from the max gross T/O chart is certainly not wise.



400 hours in 152's.

Cubdriver

Nice questions. The obvious reason they provide performance numbers at max gross is because you can't load it any heavier, easy one. The short field idea is that as a pilots we are supposed to be able to do reasonably skillful takeoffs, otherwise it would be a guessing game as to how poor the pilot is.

I suggest making a conservative use of the chart by assuming the performance you get will be better than the max gross weight performance shown for your particular pressure and temperature combination. Easy. Anything you get over that will be a bonus, but you should not any worse unless pilot technique is poor or the engine is worn out.

Ok, now for the engineers.

1. It is certainly possible to figure out what the airplane will do using performance mathematics alone with no real test data. I used to have a job doing this very thing for a couple of the local manufacturers. This particular problem could be solved by assuming some value of minimum unstick velocity V,mu or VLO and some value for soft grass rolling resistance Mu,r , CL max with the flaps at 10 degrees, and a reasonably accurate thrust number. All of this is published somewhere and the rest can be accurately guessed. The ground roll equation is below and the air portion for a 50-foot obstacle is very simple as well. Of course this is not FAA-validated or certified, so use at your own risk. It should give you and ideas what is within the realm of possibility if you are interested, but I would not use it to get out of a radically short field any time soon- just use the published charts.

SkyHigh

To me a better question is why would a 30 year old Cessna 152 be even considering going into places where a few feet of differentiation on take off roll matters?

The big picture is to use gross weight numbers and then add 30% for a tired engine, rusty pilot and old out of rig airframe.

I own a 150 and am a part of a 150/152 club and I can testify that they all fly much differently. Mine happens to do better than the performance data. Most others however do much worse.

The performance data on a Cessna 152 is a guideline at best and not a scientific certainty.

Skyhigh

jsfBoat

A C-152 at 1500 lbs will not use any more distance on takeoff than a C-152 at 1670 lbs. Guess it's a fudge factor.

FlyerJosh

+1

See- Even SkyHigh and I can agree from time to time!

grecoaj

Thanks for the advice everyone!

Cubdriver: superb response. I graduated last May with a BS in Aerospace Engineering and that information really piques my interest. Not to mention that John Anderson is my favorite technical author.
I don't have the book you referenced, could you give me the names of the variables in the equation you posted? Also, do you have a guess as to what I could use for Vmu value?
I also want to mention that I'll simply be crunching these numbers to satisfy the AE in me. I wont actually expect the aircraft to perform as I will calculate. Not until I get a job as a engineer-test pilot, anyway.



wings up, wheels down.

PearlPilot

Interesting thread. I am in love with the 1978 150 I rent. On a side note I recently calculated fuel burn on a cross country flight and came up with 5.8. GPH. Much worse than the 4 point something the book said I would get...hmm..But still I was impressed...
Uh I don't know if anybody experiences this or it is just a quirk for this particular bird. I have found that once I level off at cruising altitude, it helps for me to add a bit of nose up trim and then ever so lightly hold it and it works perfect.

Cubdriver

Not trying to be unsporting but I spent an hour typing them and there are so many more than appear in the equation above due to supporting equations it is hardly worth attempting. It's a big pile of terms and I had defined over 25 technical terms on my sheet by the time I realized it's too much work and got a beer. Internet is a lousy chalkboard and does not lend itself to analog lettering. The only good way to do it is to type them into MS Equation Editor which is time consuming. I knew answering you was not a good idea and after an hour was reminded why most of these technical discussions get bogged down. Buy the book or one similar to it and toy with it on your desk using a pencil. That's what I do and I rarely attempt to put anything up here. Not trying to be mean.

Not sure how much good it can do you without the other 6 pages of stuff you need to go with it but Vmu is somewhat higher than Vstall and it is the slowest speed the airplane's weight is equaled or exceeded by lift and is also right at the speed of stall. Just use stall speed, or about 45 knots with flaps at 10. Vlo is 1.1 x Vstall. I "may" want to come back here on a rainy afternoon and post the full blown thing, so check back in a few weeks. Even though there is a lot of stuff to consider it only involves basic math.

grecoaj

No worries! I understand the hassle and thank you for the effort. I'm convinced to buy the book. I'll mess around with the numbers and try to post in a few weeks as well. Thanks again!