MNModels

Thank you Anton,

Now i think this is what Anton and Likai will find when they test it.......Imagine the fuselage balanced on the cg on a ball so it can yaw easy. Now use small thrust turbine in centre and observe the yaw when 1 engine flame out. Now move the same 2 turbines to the tail and observe the yaw when 1 engine flame out. Now increase the turbine from the 12kg max as stated in manual to a 14 kg and then to a 18 kg turbine and observe the yaw when 1 turbine flame out. This is not rocket science - just common sence. When you have a flame out on a over powered twin turbine f18 with the turbines in the tail - you should reduce the power imediately and land. This is not so critical when you have under powered turbines and 1 flame out.

Also to note - that the angle designed by the factory was for max 12kg turbine. When you draw a centre line through the fuselage and a line through the cg - where these two lines cross is the start of the trust line. Draw a line throught this to the centre of tailpipe exit - this is the angle you need to install the turbine for compensate the yaw on 12KG THRUST turbine. For bigger thrust - only one option - reduce power to 12kg thrust and land.

Long time ago Dave Matthews from Ireland installed a single turbine off centre on Yellow Aircraft F18. He flew lake that for years - but those days it was 12kg with a single tail pipe...... So this shows that the F18 can fly with a single off set engine if you follow the laws of phisics....

Regards

gunradd

My Feedback: (9)

I will have to respectfully disagree with your statement. You seem dead set on my F18 crashing is all user error and I am 100% at fault. This is very short sighted and rather then slamming the end user I would recommend finding out what the problem is then fixing it.

The first part of your argument is I used larger engines then recommended. While yes this is true I have this thing on the left side of my transmitter called a throttle stick that I use to regulate thrust. When my plane lost its engine due to the RPM sensors being to close together I reduced throttle and was only holding enough throttle to maintain flight. I DID NOT have the engine wide open. Also if you would have attended Florida jets you would have seen when I wanted to land on the first lap their was traffic on the runway and I could not land.

please also take into account their is nothing in the instruction manual about setting the thrust lines. The stock mounts are not the correct thrust line. Also take into account the factory installed engine rails are not even set at the same angles. The left side has more angle then the right side. I have talked to others and they have said the same thing.

gunradd

My Feedback: (9)

All this being said I am very happy to see skymaster test the plane now and fix the issues. This is a great job in following up and making things right.

Anton gave my son a skymaster hat at FLjets and made his day. He is flying his skymaster Cougar and loves it. I am sure skymaster will make things right and Anton and Likia are stand up guys and a pleasure to do deal with.

Chris Nicastro

My Feedback: (3)

From the video I notice the jet yaw left immediately at lift off. Was this the moment you knew you had a flame out?
Was it the left or right engine that flamed out?

In the left turns the jet was nose high and slipping thru the turn. Was this right rudder compensation?

I ask only because the behavior of the jet in flight looked so unnatural that I'm having a hard time understanding this was caused by a flame out. It reminds me of smaller planes/heli's I've flown with gyros and how they behave especially when the gyro is overbearing.

With the engines so close close together the asymmetric thrust should be pretty well managed by the airframe with two huge vertical fins/rudders. Full scale Bug pilots will attest to the authority of the tail on this plane.

I dont put so much stock in the engine placement being the issue. The thrust exits the same place regardless. The airframe is balanced to fly regardless. The influence the thrust has on the jet comes out of the same point regardless. The jet was airworthy prior to this event and others are flying successfully to date.

It sucks to see you lose such a jet this way and it's been on my mind here and there since it happend I'm just trying to wrap my head around what I see in the video. Plus I have a twin project on my bench so this hits home a little too. I'll be measuring and checking everything twice with two sets of eyes at least!

gunradd

My Feedback: (9)

The airplane is being pushed from where the engine is mounted not the exhaust coming out the back. Once you get your head wrapped around that you can then understand why it reacted that way.

I flew it earlier that day and once the day before and all was good. Also pics confirm all the flight controls where doing what they where supposed to.

With full left rudder the engine asymmetric thrust could very easy over come it. During the last turn I had Ali standing next to me helping me and he was saying left rudder then looked down and saw I already had it full stick.

Engine placement has a huge effect and these where sitting in the tailcone. Could not have them any further from the cg. In theory if you get the angles perfectly aimed at the cg then it should be manageable. But with the engines so far from the cg the margin for error is huge. If its off even a little then it will have an amplified effect due to their location.

With both engines running it flies like a dream. I hope to build another one. I will just use mid mounted engines.

sideshow

My Feedback: (11)

I don't understand this. How do Harriers (or any aircraft) work with thrust vectoring....they wouldn't be able hover.

mr_matt

My Feedback: (10)

Thrust is a reaction force equal and opposite to the accelerated gas (mass) flow. If it were not, as you said Bob, vectored thrust would not work.

As I posted several pages ago (I think it was this thread, no time to check) centrally mounted turbines probably make it easier to line the center of thrust up (or harder to mess up as it were), with a rear mounted turbine you could be way off if not careful

gunradd

My Feedback: (9)

Matt you are correct however remember we are not flying full scale. With normal mounted turbines on our models the pipe is bigger then the turbine nozzle. The pipe does not accelerate the air its simply their so the exhaust does not burn the plane up. That is why the thrust is created from where the engine is mounted and not at the back of the plane. High pressure seeks a low pressure. On our models the high pressure is at the tail cone of the engine it self not the rear of the plane.

Now full scale the engine nozzle is at the back of the plane so its thrust is from the back of the plane. It has an enclosed exhaust so the pipe on a full scale is part of the engine. The nozzle accelerates the air using at the rear of the aircraft. Bob the harrier is the same. It has an enclosed tailpipe and I would imagine the nozzle is the smallest part of the exhaust. This means the exhaust gets squeezed and accelerated until it leaves the nozzles.

Now why does the full scale fly fine on one engine?The thrust lines are set up perfectly. Never worked on a hornet but any twin jet I have worked on had a rudder bias system to help feed the correct amount of rudder in. Also the full scale has computers helping out. I am pretty sure the CG on the full scale is well behind the CG on our models.

mr_matt

My Feedback: (10)

Then back to Bob's question, why do vector nozzles work on models as well as full scale?

gunradd

My Feedback: (9)

Same effect as an elevator or rudder on a prop plane while its hovering. Airflow from the prop makes them effective flight controls even though its not moving. The thrust is still bring developed at the prop.

mr_matt

My Feedback: (10)

The force on the motor mounts is the same no matter what . But the center of thrust can be somewhere else. That center of thrust vector is the sum of all of the forces, not just the one on the motor mounts

Imagine the CG. It can be in the middle of the air. Same as the center of thrust. It is not necessarily on the motor mounts.

lavi rider

My Feedback: (6)

I apologize but you are totally wrong .
The angle of the airflow at the end be it generated in the mid or the very end of the subject dominates its vector angle .

The sole difference is with the length of the thrust tube the longer the easier to determine the angle desired , with a short thrust tube be it the nozzle itself is more complicated to accurately aim the flow angle each millimeter offset can be conveyed to multiple degrees due to the short length while the longer the thrust tube the offset in milimeters greatly minimizes the offset in degrees .

All the the rest you've written about CG ... computers .. full size VS the model is mumbo jumbo...

the sole difference between sizes is the over all Reynolds number and it has nothing to do with this discussion .

This model if designed properly should have no problem flying safely on a single engine . Considering proper energy management .

If as as described the pilot reduced the throttle accordingly there should have not been problem bringing the model back to safe landing or even performing a go around without any issues ( turning into the live engine etc' as per emergencies executed on one engine status ).

mr_matt

My Feedback: (10)

Pretty much ^

Edit: second paragraph I meant

lavi rider

My Feedback: (6)

I will elaborate on "proper design".
A . Setting the thrust angle ( offset of each thrust line ... nozzle pointing outwards , front turbine end pointing inwards to better compensate assymetric thrust , this has to be determined by calculation .. Mig -29 has some 3 degree offset ... ).
B. Proper calculation of total tail volume (determines total tail moment).
C. Proper choice of airfoil selected due to and here the Reynolds number steps in ! To ensure desired aerodynamic behavior from our model because we are not flying the full size and yes the full size airfoils won't do us grace here . Still this has nothing to do regarding the thrust tube and thrust generating point ! ...

this is is what I mean proper design . For a model such as this it's really quite basic ... can be done in a single working day if one knows what he's doing ...

lavi rider

My Feedback: (6)

The C of G isn't a single point mostly ...

There are 3 C of G ..
lateral ,vertical , longitudinal .

In most cases they are in different areas ... just to explain , see a situation when the model is balanced on a certain point as far as nose vs tail but has one wing heavier than the other .. same as vertical C of G .. try and balance the model longitudinal while on layuing on its side ...

the desired total C of G of a model is of course should be on a single point but it's likely not so .
im 100% sure it's not on this models thrust line ! Wish it was however

mauryr

The CG is one, and it is a single point in 3 dimensional space. It may shift during flight as the weight distribution inside the plane changes (fuel sloshing, fuel consumption, moving parts).

lavi rider

My Feedback: (6)

Absolutely not correct my friend , pls read the explanation and you will understand .

lavi rider

My Feedback: (6)

The x ,y, z sum at the end can be where it might be even a point out side the physical dimensions of the subject .... for example if one wing is extremely heavier than the other ...

lavi rider

My Feedback: (6)

It's true it shifts due to various physical reasons changing their physical positions .
By the way , to properly analyze the effects on the aircraft or any other subject of the CG shift due to fuel mass shifting a centograma of the fuel tank in various amount fuel quantities is carried out .

lavi rider

My Feedback: (6)

I apologize ,
i meant that in most cases the overall CG isnt on the wing or the thrust line or the aircraft symmetrical line in a way of speaking , all manufacturing companies due a significant amount of effort during the design phase to get the over all CG to a desired point where the aircraft moment of inertia in X,Y, Z will require as less as possible energy interence to guide the aircraft .

Chris Nicastro

My Feedback: (3)

The thrust of the engine pushes on the mounts granted however the high velocity exhaust stream is guided by the pipe or tailcone when there's no pipe. The pipe must be concentric to the tailcone and in alignment longitudinally. If the pipe is not aligned to the turbine it will effect the thrust vector. So either way, pipe or no pipe, the alignment of the turbine is critical.
This is why 3D nozzles work at the end of the pipe, the nozzel is deflecting the high velocity exhaust. That force is transferred to the aft bulkhead and so on.
So again in this case with the engines so close together it doesn't make sense to me unless the engines were way out of alignment or moved somehow.
I guess that will never be clear but I'm sure when I get around to getting one this will be more sorted out.

lavi rider

My Feedback: (6)

+1

gunradd

My Feedback: (9)

These are the facts.
First flight I used stock factory alignment. Angle was to much and airplane almost crashed but I was able to save it. The plane flew great on that flight until the engine problem. To make matters worse after the flight trying to figure out why the engine shut down the ECU was telling me it did not shut down.... Now I know that was because it was reading the RPM of the other engine but at the time I had no clue. Factory alignment was probably about 5 degrees angle out.

I realigned the engines after that flight doing the best I could to align them pointing at the aircraft CG. They might have had about 1 degrees out thrust. Not an easy task the way the engines are mounted. I then did 2 good flights and the plane flew wonderful. Everything was getting dialed in and was even doing some inverted flight and 4 point rolls on the 3rd flight.

Flight number 4.....
This is the one on video and the crash.

Now LavI I have flown an A10 and I also successfully flew my old B25 several times single engine... Made it back every time. I also work with twins in fullscale aviation and very much know how to handle an engine out. I lost the right engine and you can see in the video I always turned to the good engine. I was trying to give it just enough power to fly. The roll/yaw was very strong and to much power would easily overcome the flight controls.

From pictures I have seen all the flight controls where working perfect. From downloaded data from the first flight I can see the RPM sensors where playing tricks on the ECU so I know I was having engine thrust problems.

As for the engine moving well.. Its possible but after the first flight they where still mounted in the same spot and the airplane flew worse then what you saw in that video. I was just lucky and go ti t back.

So I know A symmetric thrust was the issue. Now with the engines being so close it makes no sense for the plane to behave so bad. It was the hardest plane to fly that I have ever flown. I have done many maidens for people where all kinds of things are wrong and still make it back. This was my first crash with a jet.

The only reason I can come up with is the tail mounting. If you have a yaw effect from the engine near the middle of the aircraft it will have less effect then a yaw effect at the rear of the aircraft. The same as adding a pound of weight in the middle does nothing but a pound in the tail might make you crash.

I have talked to others that have flown planes on 1 engine and they said it just feels like less power and they don't report much else. F14 pilot said it just flew slower. Yellow F18 pilot same just flew slower. Mig pilot same you felt power reduction is all. Talked to Scott after his flameout with his yak same thing flew fine just less power. Some of those planes have a wider split then my hornet with no bad roll and yaw like mine had.

gunradd

My Feedback: (9)

Engine alignment is extremely critical with these tail mounted engines. It can be made to work I am sure but the alignment needs to be 100% perfect.

When I did mine the second time I taped a stick to the can of the engine that hung out the back. I tried using this to align the engines.

In order to eliminate all the problems I would just go with mid mount engines and have fun flying . Its a great flying bird. I am very much looking forward to the testing Anton and Likia do later this month. That will be the answers needed.

JJP

My Feedback: (1)

Anton I have an issue with the nose tire on my 104 and have told John about this. It has a large crack in the side wall and I am concerned that it will blow out on a landing and do some real damage to the plane. It has not flown yet. I need a replacement soon. Can you arrange please.