Center line Thrust

[joeheli]

Theres always a discouse about Center Line Thrust.
This is a very good info on this link that I got from Mike PRA Chapter 20:

CLT info

[KenSandyEggo]

That's from AAI's website.

[mceagle]

From the said article -

"In the event of the loss of power in a non-centerline thrust gyroplane, the aircraft goes into an immediate nose-up attitude. The normal pilot reaction is to drop the nose of the aircraft, which can cause a "Bunting Over." This creates airflow through the top of the rotor resulting in loss of lift or as it is commonly referred to as "unloading the rotor."

There must be other design considerations that come into the equation. I have only flown one gyro that "elevated" on power chop, and even then it did not pitch nose up (possibly because it did not have a stabilizer). It had an extreme thrust line offset.
For the rest, (guestimating moderate offsets of 5" to 7"), the response was mainly torque reaction with no significient pitch reaction. CLT on the other hand does have a significient pitch reaction to power chop. This is not necessatily a bad thing, it all depends on your point of view. The pitch reaction of CLT machines proves that there are other forces in the equation that we should be taking into consideration.

[quadrirotor]

Mceagle, CoG is aimed in case of lost of the lift of the rotor, but no one speaks of the center of drag in case of lost of thrust...

[Udi]

I think you are correct, Tim. My CLT* Aircommand is reacting to sudden power chop in a way I can't explain. When I go from cruise power to full power, I have to keep forward pressure on the stick or else the gyro would slow down about 10 mph from it's cruise trim airspeed. This tells me that the net moment change about the CG with the addition of power is nose-up (i.e. the result of a low thrust line). Something else is going on when I chop power. When I chop the power quickly from cruise setting, the nose goes up slightly.

Udi

* I have not confirmed the location of the thrust line in relation to the CG.

[Doug Riley]

Udi: You mention the movement of the nose on throttle chop, but what about stick pressure, and what about airspeed?

Can you determine whether your HS is up-lifting or down-lifting?

[Udi]

Doug – I set my stock AC stab with a negative 1 degree to the engine thrust line. I have not measured the stab angle in flight. The reaction I described was with a floating stick. The nose lifted for a second and the airspeed started dropping quickly. Since I was in the pattern for landing I didn't wait to see what would happen next, I just lowered the nose for approach airspeed. I can't recall whether I have to maintain a forward or backward pressure on the stick during descent. The stick is so light that almost any position feels neutral (unless I consciously let go of the stick).

Udi

[Aussie_Paul]

... remember my tests with 3 different brands of rotor blades?

That was after I had the airframe stable, ie within an 1" or so of CLT and an effective h/stab.
I did not realise that there was so much difference between blade design and stability.

Early 2004

I thought that it was about time for a little update about the rotor tests that I have conducted during the last week.
I have flown 3 brands of 30' rotors at the same gross weight and airspeed.
These tests were carried out with Hybrid, which is very close to CLT at the testing weight.


The rotor head has a torque tube offset of 5/8". The offset was not suitable for all the blades to have a reasonable trim spring pressure but I wanted to compare the 3 sets without changing anything.

I used my adjustable stick fixed testing bar to set each set of blades at 60mph IAS.

1)Oz extruded alloy.
Needed an uncomfortable amount of trim spring pressure to trim hands off. The control stick to dash distance was 330 mm. Extremely smooth right up to 70 kts/80.6 mph. Not stick fixed stable at all.

2)Raf blades 3 years old.
Much less/almost no trim spring required, and the distance from stick to dash was 302 mm. These blades have not been tweaked and had a slight stick shake at all speeds up to 70 kts/80.6 mph. Reasonably stick fixed stable but would not meet the suggested stick fixed pitch stability standards for the LSA.

3)Oz complete composite blades. The last 3' are the VR7 airfoil with an extremely small amount of reflex added by lifting the trailing edge approx 1 mm.
A little more trim spring required than the Oz alloy, and the stick to dash distance was 325 mm. These blades are basically shake free at 60 mph but cabin shake started to appear the faster I went. Tested to 70 kts/80.6 mph. Better in stick fixed stability than the Oz alloy but not as good as the Raf.

22/4/04

4) I now have the third generation Patroney blades. These are the same as 3) but have the same airfoil all the way along. We seem to have our positive pitch stability back and are still 200 engine rpm better off than the original Patroney generation blades. These blades are the most efficient blades, with hopefully enough stability to meet the sport gyroplane stability standards, when they are accepted, that I have flown.

BUT, we have not finished yet. The rest of the testing will be conducted on Firebird to match the disc loading, blade loading, torque tube offset, trim spring pressure etc to the Firebird that we will be marketing.

Aussie Paul.

[Aussie_Paul]

....I believe, whether rightly or wrongly, that all factors have to be addressed to achieve the ultimate result. Less than ultimate does not mean danger though.

The coupling of all the forces should be no different than that used to design fixed wing aircraft. When the engine is cut the drag, weight, lift forces that are still acting, should automatically lower the nose to maintain airspeed. The opposite with an increase in power.

I believe that Magni are the only ones that have achieved this in a kit gyroplane. I am sure that a lot of individuals have achieved the results by adapting torque tube offsets to rotor design and airframe geometry stabs etc.

Aussie Paul.

[mceagle]

Paul said
"should be no different than that used to design fixed wing aircraft. When the engine is cut the drag, weight, lift forces that are still acting, should automatically lower the nose to maintain airspeed. The opposite with an increase in power"

This is a point that I would disagree with (my personal opinion). My fixed wing experience is limited mainly to Victa Airtourer and 180 Cherakee. In both cases there was not a significient pitch reaction to sudden changes in power. The changes in attitude of the aircraft were more directly related to airspeed, not throttle settings. ie when the throttle was cut, firstly the airspeed decreased and only then did the nose drop to keep the airspeed constant. Similarly when the power was increased, firstly the airspeed increased, which caused the nose to rise and the aircraft to climb.
Personally I believe this to be the correct way that an aircraft should respond.

Udi, your gyro's reactions are interesting. Possibly the engine has a downward thrust component in cruise. It would be interesting to know.

[Aussie_Paul]

Tim, you are saying exactly what I said. Just a little timing variation. I am surprised that you can remember in such detail your fixed wing flying. I can't remember mine from 1970!!!!

I think that you are getting a little picky here. I believe that we are talking about the same thing, don't you?

Aussie Paul.

[mceagle]

Not being picky with you Paul, just about the popular belief that Gyros should pitch instantly nose up or nose down with throttle changes.
I believe that airspeed changes should effect the gyro attitude rather than throttle changes. As I said, this is only my personal belief, derived firstly from the fact that it is easier for a beginner to absorb one thing at a time, rather than cope with a sudden pitch variation coincidental with a sudden variation in power, and secondly, because I believe that most popular GA fixed wings behave the same way.

[Aussie_Paul]

... that we are on the same page. You said, "just about the popular belief that Gyros should pitch instantly nose up or nose down with throttle changes." Where did you find that info?

That is definately not my definition of how a stable easy to fly, and train in, gyro should behave. I have always believed, and posted with my stable machines, that as you add power whether it be gradual or severe that the gyro(well all stable aircraft) will increase airspeed by a couple of knots and then the nose will rise sufficiently to maintain the original trimed air speed. The gyro then climbs at the whatever rate for that particular increase in power.

The same with a reduction of power. I certainly do not believe that the attitude (nose up or down) should be the result of high or low thrust line in relation to the CoM.

Is that where I have had you a little confused with my postings?

Chuck B. posted an excellent post in another thread. I will search it out and edit later.

Here it is, Chuck B said, "I guess it would be fair to say the stick controls attitude.

If you want to either climb or to go faster, add power and manipulate the stick accordingly.

If you want to either go slower or descend, reduce power and manipulate the stick accordingly.

A properly designed machine, with the stick locked, will automatically nose up and climb with the addition of power and automatically nose down and descend with a power reduction; maintaining trim airspeed all the while.

But there are few machines that meet that standard and some stick manipulation will be necessary.

Aussie Paul.

[Chopper Reid]

Is the idea to make CLT complusory for all new machines ??
What happens to the "CLT " gyro when the engine stops ??

[birdy]

You **** stirrer.Thats my job.