Twin Overweight
 

If I know that my light piston twin will climb at 500fpm following an EFATO at MAUW, is it possible to calculate the performance at 5, 10 and 15% overweight?

Please can we treat this as an academic exercise and take all the "don't do it" and "it's illegal" stuff as read? :}

Yes, fairly straightforward - you just need to go into the basic flight mechanics equations and play around with them a bit.

The further you get away from known test data, the less confidence you can have in the result, but up to 15% overload you should be pretty close.

A good book to get the equations out of would be John T Lowry's "Performance of Light Aircraft". A bit hard going, but very thorough.

G

....hmmm, that sounds a bit heavy going for my pea-brain, is there anyone who can get me started?

Why not attack the problem from the other end?

Plot the performance from the figures for 70, 80, 90 & 100% MAUW (if available) and then extend the resultant line / curve through 105 / 110 / 115% MAUW.

Simple, but pretty close I suspect.

Since reading this thread I have obtained "John T Lowry's "Performance of Light Aircraft" and tried to find the relevant bits. I can't see how this helps at all since you need to know variables which you don't have. Can you please explain which forumula and how it is applied?

Can I also be the first of I hope many, to point out that what Timothy is suggesting is dangerous and illegal and you wouldn't catch me doing it.

Elsie,

PPRuNe is always reliable in producing lots of, um, constructive criticism, but the FAA does produce permits for going overweight for long ferry flights and I would like to be cautious and know what the performance is likely to be.

I broached this in the test-pilot forum because I know that this is where the "envelope-pushers" are going to reside, by definition.

I do hope that you are not the first of many and that we can have one of those rare PPRuNe discussions that does not turn into a slanging match.

Timothy, I have a "friend"who has flown an Arrow across from Canada and he had such a permit. I have been trying to find out such information myself and obtained this book as soon as it was mentioned - but it doesn't help much. Perhaps some kind soul would help both of us?

So why do you want many people to queue up to tell me that it's dangerous and illegal?

Feeling a bit jetlagged Timothy?

Anyway you don't seem to be having much luck with an answer to this one. I intend contacting one or two ferry tank companies in the states who provide FAA certificates and ask them if they have such data. If I find any I will let you know.

It depends on type. Some Cessnas, up to 30% over. Most types around 10-15%. Don´t worry too much about exceeding those on the day, though. Goose is veeeery long.You´ll need to sit a short exam and have a bit of a chat with the CAA man, probably in Moncton, before you make the first one, after which you´ll be put on their approved list for life. Get yourself a good dry suit, too.

Clyde, I think you are mssing the point of the original question. He is asking about the actual performance on a single engine when overweight or how it is calculated.

Ah, sorry. Should have read the whole thing. Depends on the airplane of course, but you´re basically flying a big single when you over load most light twins like that. the Ferry permits are kind of based on the premise that it´s going to be a one off and the chances of having something go wrong for the relatively short time you´re overweight is slim. I´ve flown twins overweight, though, and you´ll find the performance will deteriorate very rapidly indeed when you get over gross.

Timothy,

I have been granted overweight approvals for single engine aircraft, and have operated twins overweight under approvals from the manufacturers of the aircraft. I agree with the "calculate it" means of producing a "comfort zone" of data. Once done, I suggest that you request the issuance of a temporary flight permit for the purpose of testing it, and take some sandbags for a ride to validate the data. Load by extrapolating the C of G limit lines in the flight manual, and do your testing right in the middle of the now salled C of G envelope. Include slow flight and stalls at a safe altitude, in your testing. Make it a long duration flight so you burn enough fuel to get yourself back under normal gross before landing.

With a flight permit, it's legal, and with caution, it's safe. A "G" meter and restriction on the "G" of manuevers while overweight should satisfy structural concerns. Brake and tire capacity are a concern in the case of an aborted takeoff.

If you would like to PM me, I can send you something which might help support your request of the authorities for flight autority.

Cheers, Pilot DAR

Gengis' mention of Lowry reminded me of the discussions he had with Denker on Usenet many years ago and his 'bootstrap' approach which apparently lets you calculate these sorts of things from an hour flight testing. Google found this link http://www.allstar.fiu.edu/aero/AirPer-BA.htm I don't have a clue how it works, just mentioning it in case Timothy might find it useful and can work it out. Google 'google groups' for john lowry and you will find many of his posts.

I've read that bootstrapping approach as well. I think the principle of it is that many performance data are interrelated and can be shuffled around in the various formula. Obtain enough of the right sort of data points and it becomes possible to derive much additional data.

Not sure but maybe also be analagous to solving simultaneous equations?

John Lowry's "Bootstrap method" is basically a form of parametric performance analysis.

What he does is create a really complex model of light aeroplane performance, then reduce all of the equations down to some complex-ish dimensionless parametric variables . He then shows how to do some (reasonably) simple flight tests to determine the values of those parametric variables, from which it's possible to then derive performance values for the aeroplane in virtually any circumstances.

The basic principle is absolutely brilliant - but the maths behind it is fairly horrible, probably beyond anybody without at-least degree level maths, science or engineering (and having read his book, and having the appropriate degrees, I do think that he goes a little over the top on occasion).

http://www.allstar.fiu.edu/aero/BA-Background.htm is the best start point

G