readyturn

My Feedback: (9)

Just curious as if anyone knows what the loss of thrust is with the FOD screen installed? 5%, 10%, 12% or15%.. loss could answer some people's question about a lack of perfomance.

I know from full scale testing that the number is significant and it is why you don't see screens on planes like the 737.

Rick

Eddie P

My Feedback: (4)

I suspect the thrust loss is not really measurable on standard sea level flying days but when the temperature rises significantly or the airfield altitude is higher, then the thrust loss will occur once the maximum engine power is limited by max temperature as seen by the ECU/FADEC rather than RPM. This depends greatly on the engine type and the condition of the engine.

I had a nicely running P-60 that I initially was not running a FOD guard in. As soon as my FOD screen showed up I installed it and tested. I noticed the thrust was the same but the temperature was about 20 degrees C hotter in all aspects. On a typical day my thrust was limited by Max RPM. But on some days when I was flying in the mountains at density altitudes reaching 8500 feet the maximum engine thrust would become temperature limited vs. RPM limited. This is specifically where the FOD guard could be limiting thrust slightly compared to a fully FOD exposed engine. Most people don't operate in these conditions. But plenty of people have higher time engines that could benefit from an overhaul at the service center to some level and those engines will be more likely to show signs of limited thrust from max temperature first and the FOD guard will make that more apparent.

To be honest there is not a reason I can think of to not use a FOD guard.

SinCityJets

The loss we have seen with different brands on a test stand is in the neighborhood of 5-10% depending on screen and turbine.

It's pretty simple, less air, less thrust. The screen does inhibit (even if slightly) the amount of air the turbine can ingest. Still, everyone should use a FOD screen.

Chad

Eddie P

My Feedback: (4)

It's actually a lot simpler than that, this is not a ducted fan moving air so much as burning air and fuel for thrust. The engine will get as much air as it needs to burn the fuel it needs, and as long as it has "enough" air it can do this as designed. Thrust generated is a result of the fuel burned and as long as enough cooling air flow can be crammed into or sucked into the compressor the engine will not temp out too soon for RPM reached. An example of this is when you throttle up slightly before shutdown and the temp actually goes down when it stabilizes, and the goes down more on the deceleration before flameout - it's an example of what more air into the combustion chamber does with fuel flow returning to idle levels. A FOD screen will not always limit thrust. This isn't a simple EDF, ehrre air movement is the motivational propulsion force. This is an air breathing burning engine where as long as the engine can get enough air to not be temperature limited the chemical reaction of the fuel burning will provide the thrust out the back as a big part of the equation.

Xairflyer

My Feedback: (1)

This is not a ducted fan ic or electric, and much different.

A turbine is basically a thrust generator. The thrust produced will be the same, so long as it is still able to reach its maximum rpm.
It may have to work a bit harder (burn more fuel, higher temps) to produce the same thrust, because it will basically needs to suck harder, but the thrust will not suffer, more so the efficiency.

The restriction would need to be a lot before you will see any difference, a turbine dos'nt actually need a ducted setup, having one just helps with the efficiency and keeps things a bit cooler.

Actually it could be possible that the thrust will increase as a higher egt will produce more thrust.

Eddie P

My Feedback: (4)

Full scale doesn't use tea strainer type design FOD guards like we use simply because it is not a structurally stable system under the vastly dynamic conditions airplanes generally fly in. In icing, for example, the tea strainer type FOD guard in front of a jet engine would be a nightmare to keep clean/ice free and would likely cause many more problems than it could ever solve. And structurally these type of FOD guards would not at this time with our structures available stop a huge Canadian Goose from whacking an engine, or it's FOD guard, to death. So it would just be a hinderence.

But there are plenty of aircraft that use systems that "act like" the RC engine tea strainer FOD guards, to keep ice from building up on the compressor inlets (ice is called called soft FOD in the industry). Turboprops with "ice vanes" for example, like King Air's. These don't look like tea strainers at all but they effectively do something very similar. When the ice vanes are extended, they effectively block FOD or ice from entering the engine inlet aerodynamically and by using the FOD's own momentum to divert it from the compressor. And, it places a pretty severe restriction on the intake air, and the ITT's (internal turbine temps) go up quite markedly. but there is no change in power available what so ever so long as the engine doesn't reach it's ITT (temperature) limit. When you reach higher altitudes, like above FL180, that's when the Ice Vane (FOD Guard) operation starts to give the engine it's first ITT power restriction. You'll reach an ITT limit before you'll reach a gearbox or RPM power limit. Unless of course the engines are high time dogs, then they may get restricted a lot lower down. For takeoff performance, I believe there is a restriction placed when the Ice Vanes are in use but that's not a "measured thrust is lower" restriction - it's a performance calculation change that occurs where the math constants used to calculate the performance change to give more pad to "average pilot, average plane" environments. It's not a reflection of the delivered thrust available using the Ice Vanes. So anyhow, that's just one real world example of how full scale airplanes use something similar to FOD guards and the effective performance considerations that occur when they are in use and not in use.

highhorse

My Feedback: (2)

Nope, sorry. Higher thrust is usually accompanied by higher EGT, but higher EGT does is not necessarilly accompanied by more thrust.

Screens are not magic thrust makers. If they were, then they would be standard equipment in this competetive market.

FenderBean

If you fly on a active runway at a airport do you really need the fod screen? The runway is clean, from the looks of mine small sand/pebbles could still through the screen.

gruntled

This is where you need a cod screen.

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FenderBean

Yeah sorry just saw that and fixed it, sometimes I get in a hurry and this iPad makes it hard to type.

gruntled

Just funnin with you.

Xairflyer

My Feedback: (1)

Note the smiley after the sentence, that was not meant to be taken seriously.

Xairflyer

My Feedback: (1)

It is amazing the amount of fod can come from inside the model ! screws, washers, bits of fibreglass, cable ties.

bevar

My Feedback: (27)

Don,

Then why the FOD screens on the exhaust of Citation 1's?

Ba ha ha ha ha ha!!!!

Beave [sm=bananahead.gif]

highhorse

My Feedback: (2)

A bit off topic, but think...........birds.

highhorse

My Feedback: (2)

Xairflyer, sorry I didn't get it first time, it went right over my head. Oooooops !

Boomerang1

Yep, good thinking, no problems here, the bird did not go through the turbine!


http://www.rcuniverse.com/forum/m_10496234/tm.htm

I have seen flys go through model turbines though.

No hiccup, no fart, no puff of flame - nothing.

A bit of a disappointment actually! [&o] - John.

Desertlakesflying

My Feedback: (28)

Because Citation 1's are the only jets that get bird strikes from the rear.................LOL

RENO

My Feedback: (17)

An excellant mechanic and absolutely avid stick man, who once helped Bob Wilcox, told me many flying beetles
are the cause of expensive repairs to our small turbines! John Redman

olnico

I have been testing all the turbine with and without the FOD guard. No measurable difference.
As it was said earlier, the engine takes whatever air it needs. This is one of the nice aspects of the centrifugal compressor. It is not very sensitive to inlet drag.
It is a different story with an axial compressor where each blade is acting as a mini propeller ( although it is not propelling but compressing ). Also, it is rotating at fairly low RPM. This makes the longitudinal velocity an important component of the vectors triangle.
On the centrifugal compressor at 100 000 rpm, the longitudinal velocity is fairly small compared to the the rotational velocity. The shape of the blades ( long and close to each other ) also make them very un-sensitive to inlet drag or lower longitudinal velocity.

Nevertheless, the more restriction you put in front of the engine, the higher EGT you get.
Our engines are not really EGT limited in normal operations since they can usually go all the way to 900° whereas a typical max thrust EGT is 750°.
As a matter of fact it is the RPM limitation this you'd normally hit first...

rctech2k7

My Feedback: (1)

In full scale you're right airflow efficiency is very important because they fly in much higher altitude for a long time. Lower air density up there could cause higher operating temperature and higher fuel consumption therefore lower overall efficiency. Also the risk of mechanical failure plus maintenance problem and the overall benefits is not compensated...

FOD screen obstruct airflow on the intake side of the engine. Thrust loss is on the output side. Much loss happened on the exhaust pipe due to heat transfer and obstruction, the good thing it's not that big enough to affect the overall thrust performance.

Here's the loss that might develop. The ECU monitors exhaust temp and rpm. Computer maintains max rpm at full power below EGT max setpoint. During this process, you need the same pressure to propelled turbine wheel for cycle process from compression to combustion and meet the setpoint power level regardless of intake condition. However at the very far range when the temperature is too high the engine will go into alarm and ECU might shut it down. The only problem is the compression process since you have restricted airflow, the air volume is lesser. At higher altitude air density goes down therefore lesser air mass. At the combustion chamber since you have lesser air mass you need more added fuel to raise the temp in order to increase pressure. The result at the exhaust side is slightly higher temperature, slightly lower air mass flow, slightly higher exhaust velocity but same thrust.

Engine max EGT can affect output thrust if the setpoint has been reached due to less likely too much air flow restriction and lower air density or slight combustion problem. However you got loss on engine's overall performance because there are more energy that are being converted into heat than mechanical thrust.

rjbob

My Feedback: (8)

Your explanation is spot-on, Eddie. Thanks for saving me the trouble of writing all that stuff! The only time the ice vanes prevent max power output is when either of 2 parameters are maxed out...N1 or temp. Then we lose about 150 ft/lbs of torque.with the "boards" out. When N1 AND temp aren't maxed out, you merely push the levers closer to the instrument panel to come back up to max torque.

When are you gonna get back to Anchorage?

Eddie P

My Feedback: (4)

Hi Bob, I have not been doing much Anchorage stop overs lately. But I do have a few on the books and I'll have to give you a call when they come around. How are you liking that King Air? Take care my friend!

AndyAndrews

Yes.