I'd still like to know if the tailpipe velocities of turbines can be controlled.

Or do all turbines, including turboshaft engines put out a ~600mph eflux?

If so, then why can't a diffuser tailpipe instead of a nozzle be employed to slow down the eflux to something on the order of ~250mph?

It seems to me that the whole speed limiter issue is treating the symptom of the problem rather than the cause.

If the cause is a tailpipe velocity that is too high, then the obvious solution is to slow down the tailpipe velocity.

If I'm wrong, someone, please correct me.

Dan

Well it sure is not me!! It's like I am in a time warp or something, back to 1999![sm=confused.gif]

I am only dealing with the reality at the AMA as I understand it to be. In truth the thing that causes high speed potential is the turbine's high efflux velocity, maybe 450-500+ miles per hour. A little P-60 in a Stingray would probably go over 200.

The worse thing they could do is limit thrust, and limit speed without a speed limiter. Then we would not have enough power to fly the big draggy planes. I watched a friend fly a Eurofighter with a P-120, it flies good, but it needs every ounce that motor puts out. I doubt it would be very fun at 22 pounds of thrust.....


As for the speed sensor on the JetCat, yes it is integrated. For you new guys, there was even talk when the speed limiters were first discussed in 1999, that they would HAVE to be integrated, but someone backed off of that.

The speed sensor really has 3 technical elements, each that have to be handled correctly (most of the following meant for lurkers/newbies, not the veterans).

1.) The Dynamic/Static ports. The system has to fundamentally measure the difference of the dynamic pressure of the aircraft moving through the air mass, and the local ambient pressure. In order to do this, one port is configured to be pointing into the relative wind. THe other port is much harder to locate. I have seen other systems that vented the static port (that measures ambient pressure) into the fuselage or something....not always a good choice if pressure or negative pressure is generated in the fuse while the plane is flying. Another option is to mount the static port on the pitot tube itself, in a double wall arrangement, with the static ports located about 5X diameters of the dynamic port back from the pitot face...that is how we do it on the JetCat unit. As Mike and Tony have said, the installation of this port is most critical for the overall system accuracy. I have used the port in the nose (best results) to the vertical fin (also very good) to the little molded standoffs on the fuse (not as good)

2.) The Transducer. This is the little device that converts the pressure difference from the dynamic /static ports into a voltage proportional to the speed. We use a temperature compensated monolithic sensor from Motorola. It's full range is about 2 PSI (I think), and it has pretty good performance characteristics. IMHO, it does not add a significant error to the system.

3.) The Controller. This is the part that actually changes the power setting on the turbine to try to control the speed of the aircraft based on the inputs from 1.) and 2.), above. My guess is that this is where the other third party system is not behaving well. You really need to follow some design rules to have a decent response of this control algorithm. We run a full Proportional, Integral, Derivative (PID) control loop. This is a pretty standard control technique, we use similar loops to control the turbine in normal thrust mode. We also limit the amount of range the controller has on the low end, so you do not get that sudden pull back of the throttle to idle that a "bang-bang" controller might have. We can also tune some settings for better performance with different weight and drag coefficient airframes. I do not see why this same technology could not be put into a third party controller.

What were the findings?

Matt,

To quote you from a previous post you claim that the "speed limiters work". Then, in your last post you said that the controller is probably to blame for the "where the other third party system is not behaving well."

I'm not trying to start anything---Just trying to help clear the air so that others can accurately see where you are coming from.

Correct me if I'm wrong as I don't want to put words into your mouth----So, it is safe to say that you believe that the JetCat speed limiter works.....Right??? And, it is safe to say that in your opinion the third party version works but can cause problems....Right???

The reason I'm trying to nail this down is that I don't want an EC member to read what you posted about speed limiters working when in reality it is only the JetCat speed limiter that is working to anyone's satisfaction.

Kevin

I have used the MiniHobby unit and was very unimpressed. I would worry about my aircraft constantly if forced to use one.

Yes I agree with this statement. As I understand it, there is only one third party limiter, and it is not acceptable to people whose judgement I trust. I have never used it, but I have seen and heard enough about its detailed design that I know it has some real problems.

I have also said in the past that if the EC were to mandate speed limiters they would have to give industry some time to make new ones that work, or give the ECU manufacturers time to put them in.

I was not trying to sidestep that issue!

Matt,

I knew that you weren't doing any sidestepping!!!

I'm hoping that the EC has heard enough from educated individuals so that the speed limiter issue will be canned. I'm anxious to learn of Steve Ellzey's findings. I'm also waiting to see how the AMA handles this entire issue, especially towards the planes that don't have a chance in hell of breaking 200 MPH. There are a lot of AMA regulations based upon the flier's honesty and maturity. I would like to see the 200 MPH rule fall under this area too.

Kevin

Agreed!

I just spoke to a member of the TRC, and he said they (TRC) were all feeling pretty good about the upcoming meeting.

I agree with you and the TRC, that only an increased level of pilot proficiency, and if necessary, more stringent certification, can adequately address this issue. BV's three tier system is looking better and better :-)

Time to write another note to my VP.

Matt is very accurate in his assessment of the other third party speedlimiter, it is not a smooth transition like the integrated jetcat unit is. It is a bang bang type unit, when I used it years ago while I was still flying RAM engines I almost crapped myself on a couple of flights when I thought my engine had flamed out. The throttle was not pulled back gradually but it was cut to idle.and then bang back on when the speed got down below the preset value. My experience with the Jetcat unit has been very positive and I think it could work very well but the probelm is that the Jetcat unit is integrated to the ECU and even if it could be developed as a third party device it could not possibly be tested on all radio/engine/ecu combinations in a short period of time.

Without starting a debate on other branches of the hobby why is it that jets are deemed as so much more dangerous when a Giant scale racer weighing in at almost half again the weight of most sport jets and is capable of the same speed as most of the really fast sport jets are? Yet there is no talk of limiting their top speed, or engine size. It just seems like the jets are the whipping post.


Matt,

I think the tiered system is the ONLY way to insure that the really high performance aircraft are in the hands of the people that are #1 willing to jump through the most hoops, and #2 are ultimately the caliber of pilot that can handle them.

That's inaccurate. The BMT engines have speed limiters included as standard equipment.

http://www.bairdtech.com/bmt/BMT120.htm

My bad...I did not know about the BMT's. I have seen very few of them at events.

And knowing Andre's work I bet the limiter is top shelf.

Ok Matt... you are a tease

What are those jetcats? 70, 120, 200??????????????

David,
What were the high end speeds recorded as an average? And what was the average speed in general? My guess is 180 average on the high end and about 140 for the cumulative average.

From what I've observed at the Ralleys I attend each year the 200mph mark is broken from time to time but not as a general rule. Furthermore with very few exceptions pilots are flying in a consistently safe manor. Overall the quality of flight operations, number of minor incidents at the Ralleys I attend has consistently gotten better and fewer respectively --- year after year.

As an example this year at Superman, considering the number of flight operations very few crashes and or incidents occurred. This year the total number of flights was up over the last two years, mainly because every day was a great day for flying. Recall the rained out Friday in 2001 and the high wind day of 02. Jerry increased the user fee's this year, and for whatever reason the total number of pilots was down a little from 2002. Another significant difference this year at Superman was the flight line was controlled almost totally by the user's ---- no official flight line boss and the flight line ran very smoothly overall.

My belief is a 200mph speed limit is basically in effect now ----- there maybe a few exceptions but overall speed limiters installed or not 200mph or lower seems to be the reality of what is happening. I my view the total disappearance of competition speed traps which promoted the highest speed obtainable was the beginning of the end of speed as an "issue" in Jet Modeling.

It's been a few years since I raced large scale aircraft at Madeira type events, but several years back in the unlimited classes speeds were well over 200mph. And by the way the the real world safety record in large scale racing is far worse than in the Jet World. Knock on wood.......

Hummm, I wonder if anyone has entered a turbine powered aircraft in unlimited class racing? I'll bet they came up with a rule against that right-a-way. I know there was a hue and cry when the Lance Air airframes began to win in the unlimited classes cause it wasn't the pro-to typical WWII War Bird airframe..... By the way the year Madeira had the "paid-shooter" incident ---- I believe BV was present with a noon time demo of a turbine powered model aircraft, the first I'd ever seen ---- was it propane powered? I know it was so quite we couldn't hardly hear it --- but it was fairly fast as I recall.
Lee H. DeMary
AMA 36099

I have installed the speed limiter from bmt and breifly read about it but never used it. They do come as standard equipment. Looks like a quality piece of equipment.
I don't really understand why anyone make speed such as issue. I think the people who do would be suprised to see that most jets fly well below 200. Some jets are capable of reaching this infamous level, but dont do it in normal flying conditions. Most jets fly around 120-160. I think flying styles have changed and speed isnt really an issue like it used to be in past with ducted fans. Aerobatics have become more prominent, not high speed race track patterns. Thrust to weight isnt the answer either. A stingray with wren 44(7lb) will do 200, but I doubt a eurofighter with a P200 would do 200. Speed isnt the safety issue. How many accident are speed related? I would bet the highest amount of accident come from equipment failure. So why would we want to implement another potential equipment failure? I've never had an accident because I was going to fast. But I have because of an equipment failure. Weed out the peolpe who shouldnt be flying and make it challenging to those who are beginning who will prove their ability. I attained my waiver fairly easy, but I had much practice with many different models. I dont really think it has anything to do with how long you been flying or if youve flown ducted fans. I saw a kid start flying a few years back and far exceeded most of the skills I have seen of many of others modelers in just a years time. I dont really know what the answer is, but is there really a problem?
Scott

Lee,

I have not heard any official results yet. I do know that we did some unnofficial testing with a set of the JPO speed traps earlier this year at a local field. When pushed hard and with the speedlimiter set at 200 mph several airplanes were able to break past 200 mph some by a fair margin. Most of those passes were from fairly steep dives and hammering the throttle on the way down, in all cases you could hear the engine throttling back as the limiter kicked in. It was interesting to note though that several planes (my Phantom being one) that we thought were going faster were only around 165-185 when pushed hard. I personally believe that 200 mph might be exceeded for short durations but it is just not that much fun to fly 200+ mph for any extended period of time. Most guys find themselves very content to fly in the 150 mph range. I also agree with you that the overall safety record seems to be improving. I attended 9 jet events this year and numerous local flyins. Several jet events had nothing more than a few bent landing gears and scuffed up wingtips, I can't say the same thing for the local flyins. As a matter of fact the ONLY time I got the skit scared out of me this year was when a .40 sized profile plane flew between my tent and my trailer at my local field, it was BELOW the height of the tent, amazingly it did not hit anything.

I think another problem is the fact that limiting speed to 200 mph does not automatically make a safe pilot. There are several people I know who are unsafe (IMO) at 150mph. The people on the sticks have to be safety minded from the moment they open the kit box, all the way to the shutdown of the turbine after a flight.

What I think we need is:

1) Factual data (not perception, guestimates or heresay) on performance characteristics of our aircraft.

2) Tiered waver system. Don't penalize those of us who are safe and can handle the high speed/high complexity aircraft due to people who cannot. We also need people to stop signing off their friends just because they are their friends. The waiver structure needs to have clear guidelines, and people willing to stick to them.

3) Enforced rules (notice I didn't say enforcable). We need CD's who are willing to be tough on people who break the rules. We need people who aren't afraid to be the 'bad guy' and ground someone who is out of control, continually breaking rules, or putting people at risk REGARDLESS of who that pilot is, well known or not. How many of you have seen a Bandit or Hotspot with a P-120 at full thrust at a jet meet you've attended? These airplanes will do over 230mph with 19 pounds of thrust, let alone 28 pounds. Eddie Weeks made a good point as well. When was the last time you saw anyone check the T/W of an aircraft? We don't have people enforcing the rules we have NOW.. What makes you think anyone will enforce any future rules?

Lee,
Your guess is pretty much dead on. Of about 14 models we got a reading on only two could break 200, and they had to work for it. Had we not been taking data no one would have broken 200 because no one pushed that hard at any other time.

Steven

Doug,

The Mississippi Gang tried this a few years ago and it was so inconsistent it was ridiculous. You could take the same airplane and use two different methods and get two different values. You could check the airplane on two different days and get slightly different readings as well. The rules have to be enforcable for them to have any teeth. Over the course of the last year I know of several pilots that have been approached by other pilots about "unsafe" flying and in all of the cases the pilot toned down their flying. What may seem unsafe to some may not to others. The community as a whole seems to be policing itself very well.

As a participant in the speed data gathering at Hearn this past weekend, I'd like to chime in on this subject with a few points of observation.

Of the dozen or so airplanes that participated in determining average and top flying speeds, only two were able to break 200 mph.

All top speeds recorded were achieved after a dive with up to full power followed by a gentle pull to level flight over the trap at approximately 40' AGL.

At least one of the airplanes that broke 200mph had less than a .9:1 thrust to dry weight ratio.

I know that at least one of the planes had a greater than .9:1 thrust to dry weight ratio, and it was not able to break 200mph after two very aggressive attempts.

Several flights each were made over the trap by some of the planes, and the recorded speeds seemed consistent.

Some that turned in lower than expected recordings in fact were easily overtaken in the pattern by those that did turn in higher recordings.

During the weekend there was a guy practicing with a glow powered pylon racer. Now that was scary! I'm not kidding. That thing was pointed at me at least once every lap and made me quite uncomfortable.


Conclusions?

The speed trap seemed accurate enough for the intended purpose.

You had to try very hard to break 200mph from a dive. Level flight max achievable speed would likely be much less than those recorded this past weekend, and probably none would have exceeded 200mph.

The thrust to weight ratio rule is meaningless when it comes to governing max achievable speed.

Speed limiters would seem unnecessary since most jets would not exceed 200mph on a regular basis.

I have a question for you all and the AMA rule writers. What is the difference between a crash at say 80mph and one at 250mph when it comes to risk management?


Comment: I can't keep up with the little jets that go 200mph anyway, so, if it makes the AMA feel better, I don't care if they set the limit at 200mph so long as the rule does not also force me to install an expensive speed limiter that is not needed in my case anyway. I also fail to follow the logic that says if I use a single turbine, it must not put out the same thrust allowed for a multi turbine powered model. I say let the model manufacturer publish the thrust and speed limitations for their products and leave it at that. If you want to exceed these thrust limitations by use of a speed limiter, then so be it. I just want to be able to pull vertical at takeoff and could care less about exceeding 200mph!



Cheers!


Reid

Tons of difference..... E=1/2mv^2. Lots more energy at 250 mph. I know you said "risk management", but you can see the picture!!

There are few sport jets out there that can do 200+ mph all day without having to dive. BUT, most jet pilots are respectful of such planes and manage their speeds well. Flying at 200 mph thru the 5-7 min flight is no fun and like I said, most pilots fly their jets at good speeds that are pleasing to the eye and are enjoyable to the pilots.

Uh, I understood there was to be no math

My point is that I believe the damage is done at the lower speeds just as effectively. If you are going to hit the windshield of my truck, I'd prefer you do it at 300mph so that it will at least be worth watching. I have not seen a rule proposal based upon equipment limitations yet that was going to prevent an accident.

To borrow from Greg's quote at Hearne this last weekend, The prudent man rule is more effective at preventing accidents than any aerodynamic theory you wish to apply.

Just go fly responsibly and have fun.


Cheers!


Reid

Actually, the AMA is VERY much enforcing and placing limitations on giant scale races. Pretty much the only way to be able to meet the distance requirements set forth by AMA for racing is to hold the venue at full scale airports and very few people are actually out on the flightline. Even then, it is difficult to get insurance otherwise. Some interesting documents.
http://www.modelaircraft.org/templat...-files/535.pdf
http://modelaircraft.org/templates/a...-files/573.pdf

Also, go try and hold a small type of club race where the engine is greater than .40 cu/in. You'll have to get a waiver from the AMA in order to hold the race, unless you want your pylons 450 feet from the pilots and the spectators another 225 feet back!

Jets are not the only one getting restrictions placed on them.

Michael

Unless the restrictions apply all the time on giant scale race planes, rather than just when an actual event is held, the above comparison is apples & oranges. Same with your example about racing > .40 engined models. Jets are significantly restricted on an every-day basis, not just when we decide to get together and have an event.

Well, have fun with the rest of this discussion ... I'm off to Kittyhawk for a week.

Later,
Gordon

I'm trying a different approach. Just ordered a Bob Fioerenze Airspeed Indicator. Downlinks the airspeed to a handheld receiver. Hope to have it in a few days. Will post more when I have something to tell.