DeRated Take Offs

[daberges]

I am in the middle of the 40 hours of my RV9A. I am still doing lots of Take Off and Landing (Full stop /Taxi back).

At the airport I am based at (KPMP) the patten is at 800ft. The winds are 10 - 13 Kts straight down the runway. Often straight off the sea. The ceilings here recently have been fast moving at 1500 ft. Very choppy at times.

The problem - I take off full power (Sensinich Prop/TMX320). I get to patten alt too fast at Vy / Vx. I am still over the runway. If I climb at 100 kts and get better visability - I am still at patten alt before the first turn. I am getting climbs over 1500 fpm with the wind.

Anything near and over 100 Kts - the ride can get too uncomfortable with the turbulance. The aircraft came in at 1060 with me at 245 and half fuel.

I am finding that I have to REDUCE power before my first turn or I am too fast in the patten or I bust the patten alt. But I am finding that I have to reduce this power to near 1300 RPM. (Take off is usually around 2300) I always make sure I have at least 650 ft below before I pull back.

I have heard of RVs taking off at as little 75 % power. Uses longer runway but climbs slower.

Just like a light 747 would do a derated T/O and Climb. (This gives better fuel efficiency and less engine wear)

What are the thoughts out there ? Should we always use all power available then hit the brakes or is this akin to hot dogging? I even have to extend my X/winds if I get a 152 takes off in front of me.

[jcoloccia]

Using full power in climbs dumps extra fuel into the cylinders and keeps the engine cool. I have the same problem where I fly. The 180HP 7KCAB combined with a super long runway means I'm at pattern altitude WAY before the end of the runway. So I just do what I do in any other plane. I level off at pattern altitude and reduce power whenever I get there, and I make my turns whenever I get to them. Why mess with it? I don't see a problem here at all.

A benefit is it buys you altitude as soon as possible. When you're near the ground, altitude = time = added safety if something goes wrong. In other words, it gets you out of the dangerous regime of low and slow as quickly as possible. Take advantage of the fact that you built an incredible plane that can do that.

[pierre smith]

John's on the money........you just have an incredible performer on your hands and you'll find that you have to ratchet up your finesse a couple of notches to meet the airplane's capability. If you have 400-500 feet, make your crosswind turn. There's no good reason to wait until the far end of the runway passes under you. If you happen to reach pattern altitutde, level off...in fact, as fast as these airplanes go, you probably need to start levelling off early since you're still going over a hump in the level off phase.

Don't be surprised to use only 1400 or so RPM on downwind, we all do. In fact, my 6A does 140 MPH at 2000 RPM. Adjust to the airplane's speed and power requirements and un-learn the Spam Can power settings you used to use. It's a new ballgame.....a fast one!

Regards,

[Kahuna]

Disreguarding the safety aspects, there is no problem with derated takeoffs.
We do them in formation all the time.
With my 6 banger, and leading a 150hp rv8 off my wing for a formation takeoff, Im running a sadly low percentage of horsepower for takeoff.
If you have a good reason, then by all means.
Best,

[Russ McCutcheon]

My -4 has 200+HP and CS prop, My normal TO is 80% power = 2650 rpm & 25? this still gives a climb rate of 2000 fpm solo and 1500 fpm with passenger. I do the occasional full performance evaluation witch is fun. I don?t think extra fuel is necessary, but rather the right amount will do just fine, also Turning before the end of the runway might be frown on in many places, I would not get away with that where I?m based so be careful. If I had a 160 HP and fixed prop and got 1500 fpm like your getting I would use it all to pattern altitude every time, you will get used to it.

[Low Pass]

It's totally up to you, but get used to making pattern altitude before the end of the runway. Especially with longer runways. Takeoff, climb, level at TPA, reduce power (if you want), fly the pattern.

Might be carefull turning crosswind early unless you're sure no one's going to enter the pattern out there somewhere you're not expecting. I mean you're trying to get 40 hours on the plane aren't you?

I'd split the diff somewhere with reduced power to say 80% and keep plenty of fwd speed for cooling.

[John Clark]

There are times, such as a formation takeoff, where there is no other choice. But it isn't a good idea to do it all the time. A full power takeoff is the engine's chance to purge the carbon and lead deposits from the combustion chambers.

And yes, you will get used to the performance.

John Clark
RV8 N18U
KSBA

[az_gila]

Originally Posted by John Clark

There are times, such as a formation takeoff, where there is no other choice. But it isn't a good idea to do it all the time. A full power takeoff is the engine's chance to purge the carbon and lead deposits from the combustion chambers.

And yes, you will get used to the performance.

John Clark
RV8 N18U
KSBA

Ahh... but a "full power" take-off isn't full power with a fixed pitch prop...

You'll get more "purge the carbon and lead deposits from the combustion chambers" with a high speed cruise at altitude and higher RPMs...

gil in Tucson ... with a fixed pitch Tiger that never does full power takeoffs...

[John Clark]

Perhaps I should have said "full throttle" or "max BMEP" takeoff. I understand the the RPM available is a function of the prop configuration. My point was that a habit of reduced power (partial throttle) wasn't going to do the engine any favors.

John Clark
RV8 N18U
KSBA

[fodrv7]

The reason Jets are derated for take-off is NOT because of excess performance, but because all the wear in a turbine occurs in the last 25?C before the TGT limit (Turbine Gas temperature). So the derate is to bring the TGTs down from redline.

As others have said, with a piston engine the reverse is true, for there is excess fuel fed at take-off power to keep the EGTs down. (see pic below) Reducing below about 80% removes the excess fuel. So you want to be either at 100% power or back around 70%.

From what you described, it sounds to me like you are doing it correctly. As I have a CSU I can't give you the RPMs for a fixed pitch to use downwind; but you can establish it yourself.

If you find you are arriving at downwind in excess of 100KT (ie . too far above the flap extension speed) then begin the reduction to your 1300RPM a little earlier on crosswind- say around 900'
If the 1300RPM is correct for downwind, then the speed should decay to below the flap limit before you turn base.
Additionally, if the 1300rpm is correct, then when you lower the flaps, the increased drag wil cause the speed will continue to decay on base without any power adjustment- which is a really smooth way to fly the circuit.

It is very satisfying to set the power turning downwind and not have to make any significant adjsutments until on final. A 'professional' circuit should be flown with no abrupt changes in power or speed, but rather gradual arrivals at the desired speed as if you were there all the time.

The only exception is arriving on downwind where you need to make a big pull on the throttle to arrive on downwind at the desired speed, again, as if you had been there all the time. With practice you will be able to make this big throttle pull without looking at the tacho, by simply listening to the engine.

Going through the circuit procedure on the ground and then practice for an hour will see you flying like a pro.

Good luck,
Pete.