William Robison

My Feedback: (3)

Jared:

Got my multi rating in a Piper Apache. A first version Apache. The low powered Apache. Now that you know this, you also know that I have spent many an hour "Behind" the power curve. That was almost the normal condition for that plane.

Agreed, many factors have to be considered in almost any regime of flight, and even more in a twin than a single.

Your points on alpha are noted, and as it comes to zero the air entry into the propellor disc also comes to 90 degrees. At a 90* entry angle P-factor, by definition, does not exist.

Putting all this together, since torque reaction is a factor independent of alpha, the simplification (right, here we go again) that works for me is considering airspeed, rudder authority, and torque. It works for me. You use whatever works for you.

This is my last turn in this game.

Jared, you get one more turn, and everybody else gets one more too.

Then let's just drop it, OK?

Bill.

Rotaryphile

Just to get my two bits in, and maybe get off on a bit different subject, I too have had the misfortune to have flown an Apache, belonging to a friend. I played around with its engine-out handling, and was absolutely appalled at the horrible handling, and the amount of rudder input required. This thing, along with its brethren, including the Cessna 310, needs about twice as much vertical tail area. Stall/spin accidents seem to happen far too frequently with light twins, and I lay the blame at the door of insufficient vertical tail area. I doubt whether the designers of these airplanes ever made even the most elementary calculations of required vertical tail area, but just slapped on a tail that would "look right". Renowned WW2 era test pilot Frank Courtney said, "I have never flown an airplane with enough vertical tail." I get around squirrely handling at high angles of attack by simply using lots of vertical tail area, particularly subfins and subrudders.

William Robison

My Feedback: (3)

Rotaryphile:

The Cessna 310 A or B model, whichever it was, gave me a feeling of total security and freedom, after having flown nothing but that early PA-23.

An entirely different world.

Bill.

FLYBOY

My Feedback: (11)

Ok, my last turn, go to the factors related to designing a Vmc number for a plane. The one that says "critical engine inop and windmilling" is so that the one that is still running has the highest amount of P factor. On a counter rotating engine, most people believe that they don't have a critical engine. Actually, in most cases, both are critical because if one quits, you are going down. Planes like the Seminole have CR engines and if one quits, I don't care what the book says, you are going down. I have only flown my 320 one time on one engine, and it is a dog. That one however doesn't have CR engines. (boy do I miss my jet)

anyway, I don't remember Torque coming up ever in twin traing of any type, but P factor came up over and over and over.

If you are running on 1 engine, you are more than likely going to be nose up, pitching for blue line or what ever you are doing at the time, and the P factor will play a part.

Everyone seems to have a different theory though. Some were taught 10 factors to determine Vmc, some were taught 7, all I know is when I am flying the plane and one quits, you better know what you are going to do, or you will be a statistic.

Just my 2 cents.

mucksmear

Great thread folks, thanks agian Bill.

I think it's safe to say that P-factor has been defined rather clearly here.

How it deals with you and how you deal with it is apparently open for some extended discussion, but the clear definition is a must to begin with.

For the average semi-informed R/Cer (that's me), the well put definition should be of some value. With this clear definition of P-factor in hand, a clear thinking modeler SHOULD be able to examine the basic conditions surrounding his/her model's flight and then determine whether or not P-factor exists at that given instant, and, if it does, whether or not it's of sufficient force to cause concern. If it is, again, given this clear definition, a number of solutions should become aparent (eg. increase airspeed? reduce power? brace for impact?....).

Disclaimer:
Of course, I've also met modelers who insist their models are being pushed by the wind when flying down wind.

Cheers,
-E

ps2727

The definition of critical engine is the one that most adversely effects performance if you lose it. It doesn't mean that the airplane will be able to climb or even maintain altitude, it's just a worst case scenario. If you lose any other engine performance will be a bit better, that's all.
I would agree with Bill that we are probably making too much out of p factor thing because in the end it's simply a little bit of yaw, and the counter to unwanted yaw is a bit of rudder. I have seen many modelers fly airplanes with a pronounced yaw on climbout where a little right rudder would have straightened it out nicely but the fact that they didn't makes no difference. The models flew just fine and the only time it would matter is if they pulled into a power on stall. Since nobody does that regularly its all academic unless you want to have an aerodynamics discussion.
Paul

Jared

My Feedback: (1)

I'll toss in my last with a ditto on the idea that VMC has nothing to do with climb rates during an engine failure; and the 10 factors that you are thinking of are merely 10 standards that the FAA has mandated designers to use when designating a number to put a redline on the airspeed indicator. The speed at which directional control is not possible is not constant, it changes; that is why the feds made up the 10. The 10/7 discrepancy comes from the manufacturer's guidelines having 10, and the pilot training references having only 7. For instance, pilot training references omit that the force on the rudder pedal must be no more than 150 lbs... etc. To get back to the p-factor point, I'll tell you that the best way that I have seen to demonstrate p-factor is with the good old prop-on-a stick. You can find these in toy stores, and to a simple mind like my own, they are loads of fun. You spin the stick between your hands, and the whole thing flies away. If you tilt it forward for the launch, it will fly away, but as it moves along, it will begin to "pitch" up, changing the angle between the stick and the horizon. This is a really easy way to actually "see" p-factor for yourself. The explanation is that each blade of the prop has its own airspeed, due to rotation. Let's say that is 20 mph. When the stick starts moving forward at 5 kts, then the advancing blade is 25, and the retreating blade is 15 (or close to it). Thus, the advancing blade produces more lift, and since the lift occurs 90 degrees early, the resulting change is that the stick reverses its angle. Try it, but be prepared to spend all day playing with the little prop thing... at least that is what I do. You can have contests to try and make the stick land in a coffee cup, or if you get two, you can juggle them with another person. If you stand in an elevated place and spin it backwards, it will autorotate, and actually pick up rotational speed on the way down. Very fascinating, if you ask me. Thanks for all the fun discussion.