Any Turbo-prop experts out there?

I am an owner/operator of a C90B King Air considering an upgrade to the newer C90GT. The only difference between these two aircraft are upgraded engines that are flat-rated to exactly the same 550 SHP as the original engines. Everything else is the same, including the props. Now here is the question. Raytheon made the change because the new engines have larger compressor sections and therefore perform better at high density altitudes. My question is why does this aircraft perform better at low altitudes. My 90 climbes at 1500 fpm and 160 knots. The new one at over 2000 fpm also at 160 knots. To me, horsepower should be directly related to thrust. However, clearly there is more going on. There is one important note, the newer design runs the props slower (1900 vs 2200) rpm during the climb. So, the newer design is taking a bigger bite of air. That said, if my assumption that shp is directly related to thrust is right (and it may not be), then what has changed.


Though I am no turbine expert, I have read a lot of the talk about the new turbo diesels and have learned that horsepower is NOT directly related to thrust. It sounds to me like the newer engine has more torque.

The torque is up to 50% greater with the new engines.

If you’re happy with your current C90B in all other regards, have you considered one of the upgrade programs like that offered by Blackhawk?

Yes, the torque is greater; however, the prop speed is less and the horsepower is the same (550 SHP). So, saying the torque is greater is not really a complete answer. It is my understanding that HP is torque times prop speed. So, the question still stands.

There are a few things that can explain the greater thrust here. It’s key to get over HP and think about thrust. HP correlates to speed, but it is not the only factor. Given all the things that are the same here, the newer engine is obviously creating more thrust.

One thing is that props lose efficiency at greater tip speed (higher rpm). As the tips approach Mach 1, the blade pushes less air because it is sending out shock waves that are not making thrust and loses more energy to making noise. Piston engines on certified planes are designed such that the greater rpm equals greater thrust, but if you had a poor prop design or could get even higher rpms you would see that at some point the greater rpm (and HP) would not produce more thrust.

What is likely key here is the angle of the blade. Because the HP is the same, and the RPM is lower you must have greater torque, and thus greater force. I don’t know the formula, but thrust is equal to some multiple of HP and torque. The bottomline for airplanes is thrust, not HP. Just like cars where the higher HP will not always win a drag race because torque, weight, and other factors like the suspension come into play.

Perhaps this analogy fits: If you were riding a ten speed bike next to a rider whose bike had slightly taller tires or taller gears you would fall behind if you pedaled at the same rpm. If he can match your best rpm, he is stronger than you and actually pedalling harder.

Also, you might find that they are not pushing the same prop. Change the prop, and the thrust at a given rpm will change. Prop diameter, pitch, and blade design all matter a LOT. Several models of planes are getting 5 knot increases with some of the newer props out. Think about a house fan and a windmill going the same rpm. Not the same thrust at all.

Hope all this helps. Hopefully someone with more knowledge and teaching skill will get in on this one.

When you used to have 2200 on your props your torque would be, lets say, 3000 lbs. When the props are pulled back to 1900 the torque will drop, mabye a couple hundred of pounds. Now you increase the torque back to the origonal 3000 lbs and wham you have more power. When I flew a turboprop for the commuters i would set the props back on the last leg to get some more speed.

Power = Force x Velocity
In this case, the velocity is the relative speed of the propwash to the plane. So If you can get a lower velocity (from a slower prop speed), you get more force from a given power.

Rate of Climb = Velocity x (Thrust - Drag) / Weight
In this case, the velocity is the relative speed of the plane to the air it’s flying through. More thrust yields a better climb rate.

torque is what matters. Moving the air is what is important and if you move more air because your prop takes a bigger bite, you are converting more of the available hp and torque into usable power.

Think of your car - you need torque to accelerate and any time you are climbing, you are ALWAYS accelerating against the gravity well of the Earth. Physics my boy . …

The lower RPM not only conserves the engine and gear [less wear] the prop thus needs a much higher range stop to stop in order to use the HP and available torque. A lower rpm saves wear and tear, and is much quieter since the prop tips are that much further from the speed of sound.

The C90GT is a great airplane. Have you considered a Merlin? We operate a Merlin IIB and several Merlin IIIB’s. While older aircraft, the trade off for performance, range, payload, volume, operational costs are untouchable.

The Merlin IIB-6 features are larger cabin than KA200, 250kts, 1300mi range, 3600#usefull, 8 seats + potty, 70gal/hr block, 65/hr engine reserve, 80/hr parts/mx reserve, easy to fly and work on $300-375K.

The Merlin III feature are 290-300kts, larger cabin than KA200, very large external storage, 2200mi range, 4500# useful, 8+potty, 87gal/hr block, we can fly at 12.5K# up to FL280 and burn 70/hr. Engines are $80/hr, last 5400hrs, mx/parts are 80/hr. purchase price 500K-1M.

Regardless of what people say we have owned Merlin’s for over 25yrs and are very low on maintenance as long as you have people who are familiar with the aircraft.

Good luck,

Hey Westavia, you should scroll down a few lines to the “Merlin” forum and give that Canadamooney guy a hard time for talking smack about your airplane!!



You wanna tell me what was “smackish” about my posts? Certainly, no “smack” was intended.

What you fail to realize Trafly is that to brokers it doesn’t matter all that much what the truth is about a model’s reputation. If the market believes its got problems, you can’t sell it. Whether it never breaks or not.

I am glad that Westavia enjoys such a bargain with his, but if the planes really were better than the King Airs, I don’t think they would be such a bargain. I will be glad to yield to his experience on the real situation with the plane in spite of hearing otherwise from other owners, pilots and mechanics; however, I still would have to recommend against buying one.

If you think you found an o-hella-deal in aviation, you will likely learn otherwise soon.


I’ll be the first to admit that Merlins are not for everybody, pilots need to be very proficient in high performance aircraft, the workload is pretty high for a plane of its size. Yes there are some bad apple airplanes out there that would be a nightmare to repair or maintain, probably from lack of flying. And thirdly, mechanics either lovem or hatem, finding a qualified shop familiar with Fairchilds is highly recommended.

Maybe Canadamooney knew of some of those mentioned above that had a bad experience with the airplane. In my experience I have not talked to an owner that disliked the airplane, primarily its performance, relatively low maintenance and low cost of operation.

Curious if you have sold any Merlin’s Canadamooney? I’m not in the sales industry but most brokers I have talked to tell me late model aircraft are stable and have actually picked up because Pratt & Whitney prices have gone up and they consume more fuel. We are looking at an aircraft now that is over 1M. I would agree with you that the older III’s may have a hard time on the market.

I haven’t sold a Merlin, but I did work with a buyer who was contemplating a purchase. A merlin would be on the top end of what I handle. He ended up going with an older Citation. Our findings are the source for my posts. The market looks exactly like the market for some other airplanes, and seems that way for the same reasons.

Yes, most of the bargain planes need a lot of work, and have likely gotten to be hangar queens from lack of flight. Do you know why so many of them seem to have been parked for too long? We definitely got the impression that there had been planes sitting for lack of parts. Also, you recommended that people find a mechanic with familiarity. That leads to two questions: How common are these guys, and what peculiar traits make you recommend that? Is there anything special about the pressurization? I seem to remember that being a common complaint.

I also know that the engines get more bad press than they deserve because they are different from the more common P&W’s. Often in aviation, different gets a bad name. I think this is because so many in the industry are of a personality type that favors routine and uniformity over innovation.

Sorry, Canadamooney, I just felt the need to stir up some trouble.
I’m well aware of the moodiness of the “market”, and how easily reputations, deserved or not, can sway the value of an airplane. Almost 20 years of flying has taught me that every airplane has its fans and detractors, and never the twain shall meet.

Westavia makes a great point about ANY complex aircraft, be it a Merlin, MU-2, Aerostar, or Twin Cessna. They all have good and bad features, and they all require seriously good maintainance. Each model has it’s own “gotchas” that must be addressed. Also, Pilot training and competience are key to operating them safely.

Choosing an aircraft is an extremely personal decision. It’s all a matter of careful analysis of your missions, and finding the right traits in an aircraft that you can operate and maintain for the price you are willing to pay. There are very few “bad” designs out there. But there are lots of pilots who have had an airplane get the upper hand on them when they weren’t paying attention or on top of their game.

And of course, we all know how much pilots love to bitch.

As far as parts, we have never encountered any problems finding what we need and for a reasonable price. But we do have the resources and experience to know which vendors to use. Considering the Fairchild/Swearengin’s were built for 33yrs I’d hardly say there is a lack of parts available. With the Metroliner as the backbone of the commuter and cargo industries during the 80’s and 90’s, combined with over 1,000 Fairchild/Swearengins built, there are plenty of mechanics familiar with the airframes and systems.

The reason a familiar mechanic is desired is because the airplane is different than the more common Cessna and Beechcraft turboprops, the airframe is built like an airliner, I know some Metroliners with over 60,000hrs on them and still flying everyday. For the same reason you wouldn’t want a Cessna 310 mechanic working on the 737 you fly in.

The pressurization is a very simplistic design. Some unique features of the Merlin are a lightweight door frame that gets pulled tight to the door once locked, as opposed to the very heavy door frames on King Air’s. The outflow valve is located in the tail cone so what happens in the back of the plane, flows out the back of the plane, as opposed to King Airs where the outflow valve is upfront, so once the potty is used the air flows from back to front and lingers in the cockpit. Since the Merlin’s are built like airliners they are able to withstand higher pressures, offering a much higher pressure differential than King Airs all while having windows twice as big.

Ohhhh Westavia, and you were doing sooo good too!

Until you screwed up about the KingAir.

The Kingair pressurization system is not “reversed” as you mention. The outflow valves are located in the rear pressure bulkhead, as they are on most aircraft. However, the Beechjet I used to fly has outflow valves in the cockpit, with pressurized air entering at the back of the pressure vessel (makes a lot of sense if you think about it), and whenever the guy in the back farted, you sure knew about it!!!

Not on Citations, or Lears, or the PC12, or the P180 (i think).

Thanks for the correction Trafly, I was misinformed about the King Air’s. I do know that there are many aircraft with the outflow valves upfront, jets mainly since engines and bleed air are in the rear.

Holy S*#t !! I used to fly Citations and Lears and nobody told me the outflows were in front!!! I meant to state "many aircraft, not “most aircraft”, but my hands were faster than my brain, again. Thanks for not beating me up too much and making me cry

this post has been dead for awhile, but I could not resist the chance to add a QUICK comment.

We have owned and operated three different Merlin’s in our past. The Merlin IIb was an excellent plane. The range and decent speed of this craft made it a great first plane. The fact that we could slam it full of gear and luggage and fill EVERY seat (potty included), was amazing. This plane did have some pressurization issues initially, but a stuck bleed valve was easily repaired and no issues from then on.

Our next plane was the Merlin IIIC with -10’s. Now this plane was the best. Those 10’s were fast and powerful. We flew nonstop from Louisiana to Las Angeles easy and fully loaded. Unfortunately a minor issue in the engine was expounded by poor weather conditions and bad luck and this plane had a minor incident. We choose not to repair the plane, because we had our eyes set on plane #3.

Plane #3 is our current aircraft. The Merlin IV is in one sort word, a BEAST! This plane carries 11 comfortably, but can carry as many as 13 pax. Got a ton of luggage? Forget about it - the cargo door and bay make loading a hippo a possible feat. nonstop to NY or LA - easy! Performance is unreal. A totally loaded plane left half of KGKT’s 5506ft untouched on takeoff. I wish I had the numbers to throw at you.

King Air or Merlin??? I’ll take a Merlin any day as long as I have a good pilot in the front. This isn’t your weekend casual flyer. This plane requires the pilot to work for his pay. That is why I think that this plane has become a little unpopular. Let’s face it, when it comes to advice on aircraft, owners turn to pilots. And when pilots can sit and easily FLY a King-Air 200, or save his boss a few bucks and DRIVE a Merlin, they are gonna go for the easier job. It just seems that maintenance is the scape goat pilots use.

I’m sure I will be corrected if I’m wrong, so thought???