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  #51  
Old 02-05-2009, 11:44 PM
whifof100ll whifof100ll is offline
 
Join Date: May 2005
Location: Mandeville, Louisiana
Posts: 179
Default props, IFR, and pitch stability

Quote:
Originally Posted by N941WR View Post
Maybe this should be on a different thread but I have to ask, why does a CS prop make IFR flight more stable?
Good question. Two reasons.

The first is that up/down drafts cause fluctuations in RPM. Since power is a function of MP and RPM, you will get fluctuations in power with FP. If you get a disturbance that causes in increase in RPM, the plane will produce more power and tend to speed up or climb. A CS prop will regulate the disturbance out and maintain a more constant power.

Reason 2 has more to do with weight than CS vs. FP, but FP props are generally lighter, especially comparing a wood or composite FP prop to a metal CS. A more forward CG is more stable in pitch.
I just completed my IFR recurrency in my RV with a light prop, and it was not too bad, but hand flying it takes more attention than my buddies CS RV-7 that I also fly. Way more challenging than my Cardinal. IFR flight in my RV makes me love Tru Trak. Plus/Minus 100 feet in the soup takes attention without the autopilot.

Landing
Unless loaded far aft, my plane is easy to land and can land really slow. An RV loaded near the aft CG limit can be a challenge to land well, so a light prop and a lot of bags or heavy back seater can cause some interesting landings. During AFT CG tests, I pogoed my first few big time! I learned to cope with this from my buddy who has an RV-4 with a wood prop. He just carries about five exrta knots when loaded near the aft limit and wheel lands, flying it onto the runway. This works fine at aft CG.

Agility
The pitch sensitivity due to CG change is also a factor in handling. Some prefer a more forward CG. Some prefer the lighter less stable but more aglile feel a light prop offers. A Hartzell equipped RV still handles great and is a real hoot to fly.

Again, I strongly encourage those with a choice to make to try both before you buy if at all possible. Many will choose CS, many will not, but all will be correct in their choice because they will have bought what they want rather than what someone told them they needed.
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  #52  
Old 02-06-2009, 08:00 AM
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N941WR N941WR is offline
 
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Location: SC
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Dale,

I have to disagree with your reason 1, up/down drafts may cause fluctuations in RPM but not power. Just like when coasting downhill in your car, you may accelerate even with your foot off the gas pedal doesn't mean you are putting out more power. Back when I was taking instrument training in a Archer, we just set the power and didn't worry about it.

As for reason 2, I don't understand. The airplane may be a bit more stable but that also has to do with passenger and baggage compartment loading. As long as you are w/in CG and W&B this should be the same between similar airplanes.

Agility has to do with mass, simple as that. If you have less mass then it is easier to accelerate that mass in a different direction. It is also best if that mass is in the center of the airplane. Therefore, a heavy prop will adversely impact roll, yaw, and pitch. It is kind of like a race car with heavy wheels and tires. They accelerate slower, turn in slower, and stop slower. All bad stuff. ?Agility? is a subject measure and means different things to different people. A very light plane may seem twitchy to some while a heavy version of the same plane may seem stable.
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  #53  
Old 02-06-2009, 09:42 AM
Sid Lambert Sid Lambert is offline
 
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Location: North Atlanta
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I agree that a CS prop makes your speed much more stable in rough air. When we fly in tight (tight for our standards) the CS guys are always complaining about us fixed pitch guys changing speeds even when we don't move the throttle. Also, minor alt changes (200') effect speed much more on a fixed pitch prop than CS.

I have no idea how that correlates with IFR as I'm a clear and a million, winds at 3 knots kinda guy.
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  #54  
Old 02-06-2009, 09:46 AM
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Bryan Wood Bryan Wood is offline
 
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Default Trust Your Own Gut

Quote:
Originally Posted by Wardo View Post
how much speed do you loose. surely the take off distance and climb is not as good with a fixed prop, as if the rv family has a problem with that. i guess what i am looking for is input from you guys that have been through this already.
You should find somebody in your area with a constant speed prop on an 8 and go for a ride. You will know right away what is right for you after a couple times around the pattern. Have the 1st take off be full throttle, 2,700 rpm, and then on downwind set up the prop for 2,300 rpm and just leave it that way during the landing and taxi back. With this setting for the next takeoff you will get a full throttle and 2,300 rpm departure similar to what the fixed pitch prop will give you. Feeling, seeing, hearing, and just overall experiencing the difference will give you the ability to make an infomed decision to the difference. The point of this is not to push the constant speed, but to give you the ability to see the difference in the same airframe and then you can choose what is right for you based on what you experience. If it is a budget thing there is nothing to apologize for flying with a fixed pitch propeller. As you can see from the previous responses they are the choice of people who can have anything they choose.

But on the other hand, to argue that having the ability to alter an airfoil to fit the current flight regime isn't an advantage, well frankly they are doing you a dis-service. You simply need to actually see if this advantage is worth the cost, and extra weight.

Good luck, see you in the nose wheel, tail wheel debate.
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  #55  
Old 02-06-2009, 09:54 AM
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Barry Barry is offline
 
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Default Stability In IFR Flight

RVs are airplanes that quickly accelerate when the nose is pointing down.
In the case of a decent in IMC through clouds the airspeed can quickly build up, this usually follows by tending to over control with a "chase the needles" syndrome quickly building up. Following good advice I once received from Alex De Dominicis it helps when one has a RV fitted with a CS prop to pull the power back to say 18 inches of MP. With the engine MP set to 18 inches MP it is much easier to fly a stabilize 500 ft minute decent whithout ending up chasing the airspeed. With a fixed pitch prop you would not have this advantage.
I am not a professional pilot however I am sure those that fly F16s and Jumbo Jets for a living will have no difficulty flying a stabilized 500 ft per minute decent with the pedal to the metal whether or not they have a CS prop!!

Just my 2 cents worth.

Barry RV6A
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  #56  
Old 02-06-2009, 11:58 AM
David-aviator David-aviator is offline
 
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Location: Chesterfield, Missouri
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I thought this thread would end a long time ago but it seems to have a life of its own with a new perspective every day.

I just came off 4+ years behind a CS prop and it is the cat's meow - as long as it works. Take off performance is a rush some days when there's a bit of head wind and the temps are moderate, no denying that. But I came near putting one in a field one day when the thing failed in the fine pitch mode. It doesn't matter what engine you have, if it fails in the fine pitch mode, you don't have much thrust while trying to stay under the engine/prop rpm read line.

When I switched engines, I decided to accept the down side of FP in favor of getting rid of a failure mode. In fact I got rid of several failure modes going to the old technology - like a PSRU, an ECU, liquid cooling system, a manditory alternator, and the CS prop.

That's my perspective for the day. I like the Catto for its simplicity and no sweat operation, to each his own. I been forced down one to many times and getting very conservative or something.
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  #57  
Old 02-06-2009, 12:12 PM
JHines JHines is offline
 
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Location: Rock Hill, SC
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Quote:
Originally Posted by David-aviator View Post
I thought this thread would end a long time ago but it seems to have a life of its own with a new perspective every day.

I just came off 4+ years behind a CS prop and it is the cat's meow - as long as it works. Take off performance is a rush some days when there's a bit of head wind and the temps are moderate, no denying that. But I came near putting one in a field one day when the thing failed in the fine pitch mode. It doesn't matter what engine you have, if it fails in the fine pitch mode, you don't have much thrust while trying to stay under the engine/prop rpm read line.

<SNIP>
I'll second that. The first time I ever flew behind a C/S prop, in an old Arrow, I had a runaway prop (it had just come out of an overhaul, BTW). Fortunately it happened near the pattern, but to stay under the redline there was no way I had enough power available for a go-around. Oh and it goverened just fine on the ground during the runup and after landing.
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  #58  
Old 02-06-2009, 02:03 PM
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burgundyja burgundyja is offline
 
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Location: maple grove,mn
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Default 8a catto or hartzel c/s

my 2 main concerns are weight and acro. should i go with the catto for the weight and money or a used c/s hartzel? i dont have the budget for new c/s. is there a large gain for acro in the c/s prop?
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  #59  
Old 02-06-2009, 02:17 PM
Finley Atherton Finley Atherton is offline
 
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Location: AUSTRALIA
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Quote:
Originally Posted by David-aviator View Post
But I came near putting one in a field one day when the thing failed in the fine pitch mode. It doesn't matter what engine you have, if it fails in the fine pitch mode, you don't have much thrust while trying to stay under the engine/prop rpm read line.
I can see the point you are making. However I would have thought that if the low pitch stops are set correctly for max static rpm on the ground, then you would still be able to use considerable throttle without exceeding red line rpm in flight especially at slower airspeeds even if it fails in full fine.

Fin
9A

Edit: The low pitch stops on my new Hartzell as delivered from Vans were set a long way too fine. I can believe that had I not adjusted them, then little throttle would have been available if I had lost oil pressure to the prop.

Last edited by Finley Atherton : 02-06-2009 at 02:30 PM.
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  #60  
Old 02-06-2009, 02:51 PM
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Rick6a Rick6a is offline
 
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Today, the weather finally cooperated so I seized the moment and took my Sensenich FP equipped RV up for a bit of airwork. This thread has morphed into a sort of FP versus CS dialog and some of the bull it has generated was still fresh in my mind. Upon takeoff on runway 27, I made a right hand departure and headed north. A few miles north of the airport I did a 180 and headed back in. Crossing the field WOT at pattern altitude the run generated a GPS ground speed of 178 MPH. The number represents a typical RV cruise speed. That's important. After crossing directly over the runway at pattern altitude, I reduced the power and made a left downwind for 27. At no time while in the traffic pattern did I range farther than 1.5 miles from the center of the airport . This is a much tighter pattern than many of the spam can drivers around here use I can tell ya. Guess what? The RV slowed down enough to allow the first few degrees of flap deployment when the airspeed indicator flirted with 100 MPH at the corner of downwind and base. As anybody who flys RV's knows, the airspeed can drop off dramatically once the flaps are even slightly deployed. Soon thereafter, final approach proved as normal as normal can be and 30 feet or so above the numbers, I shoved the throttle forward and climbed out to repeat the exercise once again. The second run WOT at pattern altitude proved a bit faster and produced a 184 MPH groundspeed directly over the runway. This time I ended the exercise with a completely uneventful squeaker of a full stop landing.

As I taxied back to the hangar at the end of the two runs, I just had to shake my head at those naysayers who insist on spreading the false, misleading, inaccurate poppycock that when piloting a FP equipped RV, the pilot (as opposed to his CS brethern) must start slowing down some miles from the airport. Those guys probably had trouble with proseal and canopy cutting too.
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