Bootstrapping on takeoff with new Merlyn and intercooler

[Scott H Poms]

I thought the Merlyn was supposed to ease 'bootstrapping' it is very difficult to restrain the surge in MP with new Merlyn and intercooler.  The old engine with stock fixed wastegate and no intercooler was a breeze to set at take off power.  Any ideas? Vernier throttle cable?   

Thanks and sorry if you saw this before, seems it got deleted.

[Aerodon]

Check your Merlyn system for leaks - sounds like something was disturbed and any small leak will result in your symptoms. (been there, done that).

 

Don

[jlunseth]

Well, wait. What do you suppose a turbocharger does when it spins up? You should be getting a distinct surge in power when you push the throttle in for takeoff. This is a good thing, it means you have a turbo. If you are a former NA pilot used to a set and forget throttle you just need to learn new technique. The Merlyn does not stop the turbocharger from working the way a turbocharger works. Bootstrapping is when the turbo and engine start to chase each other, up and down, back and forth. 

On takeoff you put in about half or 60% throttle, you let the turbo kick in, then you progressively push the throttle in to make full power. You can’t shove the throttle around in a 231, with or without a Merlyn.

[N201MKTurbo]

On mine (not a 231, but mostly 231 parts), as I advance the throttle, there are three phases. The first is the prop hitting the governor and the RPM stabilizes at 2700, next the turbo comes up to power and stabilizes, the next is when I set TO manifold pressure. 30 in in my plane. A new pilot may not even notice these things, but after a while you do.

This all takes place in 2-3 seconds.

Edited July 21, 2021 by N201MKTurbo

[81X]

231 owner here with the Merlyn and Intercooler.  Can you describe your process for takeoff power setting and also what the MP is doing in greater detail?  
 

It could be a leak or DOA Merlyn; but it could just be just the need to learn a new technique since the engine certainly reacts differently with a variable wastegate vs the fixed wastegate. 
 

@jlunseth uses the same technique I do for my 231.  The process takes about 10-15 seconds for me as part of the power up process is brakes on and then I’m still adjusting (typically adding) power on the takeoff roll to get to 36” of MP.  

[PeteMc]

As the others have said, you will get a surge.  Just a nature of the turbo spinning up.  And remember, the whole point of the Merlyn is for the turbo to spin WAY DOWN if it is not needed.  So you're getting it to spin up as you add power.

I have a vernier throttle, so I just screw in my throttle vs. shoving it in.  It takes me only a few seconds to get the power in, but it allows me to see/hear the surge and watch the MP as I near TO power.  Also gets you into the prop gov. range as @N201MKTurbo mentioned.

But if you're already basically doing this and are still having issues.  Then it sounds like a leak or sticky plunger that is giving you issues.  So time for a trip back to the mechanic.

 

[NicoN]

Sorry to ask. I have no experience with a Merlyn wastegate.

Only with my stock 231 and a Rotax 914 turbo driven powered glider.

WIth the Rotax, normally everything is easy: push the throttle half-in and wait for a second, then push all-in . The MAP will slightlys overboost 42, then stabilize at 42. No change in take-off run, no changes in the air. The MAP stays nailed no matter how the airspeed changes

With the 231 you have to be careful in take-off-run. As you gain speed, the MAP will rise and I have to pull out the throttle to avoid overboost.

Same in the air: Setting the MAP to 25 with 150kts, I will end at ~20 as the airspeed drops.

I thought, a Merly wastegate completely removes these speed-induced changes in MAP

 

 

 

[jlunseth]

The Merlyn wastegate is not a truly automatic wastegate, although I believe it has been marketed that way. It is a differential pressure controller. A differential pressure controller senses the pressure change across the throttle and tries to maintain the same pressure difference. In other words, it senses the pressure when the induction air enters the induction system, and senses the pressure on the downstream side, after the air passes the throttle, and works to keep that pressure difference the same. The idea is to reduce bootstrapping, so imagine that you takeoff and the aircraft accelerates. As it accelerates, there is a ram air effect that boosts the power output. When the power output is boosted, that in turn produces more exhaust pressure, which drives the turbo faster and outputs more compressed air. This relationship - increase in engine output increases turbo output - is not necessarily linear, in other words, an increase in power output of, say, 5%, does not necessarily mean an identical 5% increase in output from the turbo, the turbo might put out more than that, which in turn would feed back to the engine in the form of more compressed air. This can be exaggerated on takeoff when the engine starts the takeoff roll at idle, there is turbo lag before the turbo spins up, and then suddenly the turbo spins up and power output spikes. All the Merlyn does, is to try to keep that pressure differential stable, so an increase in throttle does not lead to a relatively large increase in turbo output, which feeds back into the engine, which then feeds into the turbo, etc. 

The Merlyn is very much an "all things remaining equal" type of system. If something happens to cause MP to rise, such as ram air, the Merlyn does not try to maintain the original MP, rather, it continues to try to keep the pressure difference across the throttle, the same. Thus, MP rises, it results in an increase in turbo output, just not a high spike in turbo output.

The Merlyn does some other things as well, but they are not related to the differential pressure regulation. One principal additional advantage over the fixed wastegate is that the Merlyn can fully close and fully open. A fixed wastegate has a fixed opening, it is always wasting some amount of exhaust overboard. A 231 with a fixed wastegate has a critical altitude of about 15,000 (dependent on day pressure and temp). At that point the turbo is working as hard as it possibly can, the fixed wastegate vents some exhaust overboard. The Merlyn, on the other hand, can fully close, so the turbo can output more compressed air at higher altitudes. The critical altitude in my aircraft, with a Merlyn, is about 22,500. Technically, CA is when the aircraft can no longer maintain a climb rate of 100 ft. per minute. My personal definition is simpler, and that is when the throttle stem is fully in and the engine can no longer make 100% HP if altitude continues to increase.

In true carusoam tradition, let me say I am just a pilot, not an engineer nor flight instructor. However, I have had some firsthand experience with a turbocharger company and their engineers, who are pilots as well. Hopefully I am translating their education correctly.

 

[81X]

3 hours ago, NicoN said:

Sorry to ask. I have no experience with a Merlyn wastegate.

Only with my stock 231 and a Rotax 914 turbo driven powered glider.

WIth the Rotax, normally everything is easy: push the throttle half-in and wait for a second, then push all-in . The MAP will slightlys overboost 42, then stabilize at 42. No change in take-off run, no changes in the air. The MAP stays nailed no matter how the airspeed changes

With the 231 you have to be careful in take-off-run. As you gain speed, the MAP will rise and I have to pull out the throttle to avoid overboost.

Same in the air: Setting the MAP to 25 with 150kts, I will end at ~20 as the airspeed drops.

I thought, a Merly wastegate completely removes these speed-induced changes in MAP

 

 

 

 

It does not remove those speed or altitude induced changes, it only helps with bootstrapping and increases the critical altitude.  
 

To over simplify this, the Merlyn is an automatic relative pressure controller, not an automatic absolute pressure controller. 
 

 

[whiskytango]

Agree with what @jlunseth and @81X have said.  My experience has been that very small throttle movements are needed to get from 30 to 36 inches MAP as the airspeed builds (ram effect) and the turbo spins up.  I plan to have a vernier throttle installed at my next annual to reduce the chances of MAP overshoot.   

[RJBrown]

On 7/22/2021 at 9:17 AM, jlunseth said:

The critical altitude in my aircraft, with a Merlyn, is about 22,500. Technically, CA is when the aircraft can no longer maintain a climb rate of 100 ft. per minute. My personal definition is simpler, and that is when the throttle stem is fully in and the engine can no longer make 100% HP if altitude continues to increase.

 

My understanding of the definition of the term “critical altitude” is that altitude above which a turbocharged engine can no longer make 100% power.

Has absolutely nothing to do with climb rate.

The service ceiling of a normally aspirated airplane is that altitude were the aircraft can no longer maintain a climb rate of 100 feet per minute.

The ceiling of a turbocharged airplane is part of the certification and is not related to climb rate.

Note, my experience was with a stock 1980 231/M20K and same plane as a Rocket.

The Rockets certified ceiling did not change with the conversion. 231s kept the 24,000’ ceiling while 252s kept the 28,000 ceiling.

Rocket Engineering considered raising the ceiling to 30,000’. All testing was done to that altitude but certification was then not sought. Since the certified ceiling is a regulatory thing and does not restrict part 91 flight. Highest I ever had a Rocket was 26,000’. Done 3 separate flights. Each flight was solo so not near gross. The climb rate remained at 1500 fpm the whole way.
If my understanding above is not correct, please post correction.

Randy.

Stock 231 critical altitude is 14,000’ Stock 252 critical altitude 24,000’ Rocket’s is 24000’ also.

 

Edited July 26, 2021 by RJBrown

[Pritch]

Is your engine driven fuel pump set to the proper settings that are required for the intercooler?  When my pump was starting to go out, I was always chasing the manifold pressure, (did not know it was going south).  When it finally went belly up and I put a rebuilt pump in and my A/P set the proper flows it was a whole new plane.  Many mechanics will set the pump to stock settings not knowing that they are different with intercooler and Merlyn.   

Pritch