FS2004 Chop and Drop ???

[scottb613]

Hi Folks,

RE: FS9

Been working my way through my first foray into FDE developement with with "Air Wrench" - what an amazing little program that you don't have to be an aeronautical engineer to use - and - Jerry the developer has been extremely helpful answering tons of questions for me... Certainly worth the $20.00 cost of admission if you've ever had FDE's you were unhappy with... Anyway - armed with Air Wrench - my experience as a real world GA pilot - and - the actual planes POH in hand - I believe I've been pretty successful in capturing both the performance numbers and sensations associated with flying this plane... I'm pretty happy with the results - so far - and I'm convinced it's a much better FDE than was actually sold with the plane... My biggest gripe about most FDE's in FS9 is that they make the plane too darn easy to land... In real life - the flare to landing takes almost constant input from the pilot to keep the plane in the correct attitude for landing - while all but the best FDE's in FS9 - you just pull back on the yoke a little and the plane plops down...

Air Wrench http://mudpond.org/ (No affiliation - just happy customer)

So anyway - while I've adjusted most of the aerodynamic qualities of the - wing - flaps - and - control surfaces - the one thing I've been unable to capture is the is the "Chop and Drop" sensation on short final... When in a light GA plane on final and you pull the throttle back to idle – there should be an additional element of drag and an increased pitch movement… I can’t seem to introduce this into the FDE I’m working on with the tools I have available… This happens with both fixed pitch and constant speed props… I made a cheat – by adding an unrealistic spoiler that I have to activate manually to emulate this sensation... It works fairly well – but – obviously – we can’t have something that we activate manually…

Would anyone know of what characteristics I could manipulate in the FDE to do this ??? From my limited experience - I don't think there is anything in the FDE and I’m assuming I’d probably need to create a custom gauge (never done before) to automatically and progressively activate a spoiler function as you pull back on the throttle ??? Is a custom gauge the only way to do this or is there an alternative ???

If I need a custom gauge - would this be hard to make and suitable for a first time project ??? I haven't programmed anything with C in many years...

Any help would be most appreciated...

Regards,
Scott

 

[jx_]

I think you may be taking the wrong approach here. It is common for FDE designers to try to emulate some characteristic by overcompensating or faking something else. This in turn puts bad coefficients into the aircraft which will lead to something else bring ruined.


I think the question I would ask is, from an aerodynamics standpoint, what is causing the "sensation" you experience.

Let's think only of a real aircraft for a moment.

...Either

• - a) that drag effect is always present at all speeds and altitudes when RPM is low
• - b) or the effect is due to the wing being that close to stall speed with high lift employed (flaps)

This would lead me to the conclusion that either the engine thrust curve is wrong or the lift/induced drag curve is wrong. There are other possibilities of course, but these would be where I would start.


The point of all this is each coefficient has a purpose and the sim contains all the coefficients that the real airplane does. If those coefficients are correctly matched, the performance in the sim will translate. If you are having problems somewhere, the coefficients are wrong. I would focus on finding and correcting those that are wrong and not use any falsified data.

 

[scottb613]

Hi Jx,

Thanks for responding - and - good food for thought... In a small GA plane - I'm assuming the reasons the nose drops when you pull the throttle to idle on final are - the prop transitions from providing thrust to just another source of drag - you also lose the prop wash over the control surfaces making them less effective -and - finally you are slowing down and so the wing is produces less lift... This is my first attempt at creating an FDE so I'm really not sure how to introduce these values... I did read the other thread on here on the boards and it suggested the spoiler gauge...

The other issue is I can't seem to get the aircraft to rotate at the appropriate airspeed - I can't lift the nose until I'm 7 to 10 knots faster than it should be... It's strange as most of all the other metrics I've tested seem right in line with what they should be... The weight and balance seems good now...

I am using "Air Wrench" which does most of the calculations in the background - but - I'm not adverse to digging deeper if necessary...

I'll poke around some more and see if I can come up with a solution... Appreciate the input...

Regards,
Scott

 

[Roy Holmes]

Propeller slipstream has a large effect on lift with a prop airplane. This is simply due to the increased velocity of the slipstream air over the wings compared to free stream air. It also has the effect of reducing the stall speed.

A short-field approach can be flown with power on and flaps quite a bit slower than a glide approach, for the reasons given above. However, if you chop the power, you effectively raise the stalling speed as well as creating a bunch of drag with the freewheeling prop. The plane will drop.

However, FS does not have any provisions for generating lift from propeller slipstream. Consequently, any chop and drop will have to be fudged.

Roy

 

[jx_]

Thanks for responding - and - good food for thought... In a small GA plane - I'm assuming the reasons the nose drops when you pull the throttle to idle on final are - the prop transitions from providing thrust to just another source of drag - you also lose the prop wash over the control surfaces making them less effective -and - finally you are slowing down and so the wing is produces less lift...

The effect of prop wash on controls is in the airfile. Look for moments with a suffixed (T). This would be where you add the lowering of the nose.


The effect of prop drag can be correctly set in the power tables. Prop drag is no different than ram drag. They are caused for different reasons, but they affect thrust in the same way. If your power table accounts for net thrust correctly, the sim would replicate the drag correctly.


I know it may sound silly to add drag via a thrust table, but as I have tried to explain in this forum before, thrust, lift, and drag are all identical --- they are just force of air --- and when a force vector points in a different direction we change it's name. Hence, by reducing thrust you are actually adding drag. Our reference is always static. If idle on the ground produces enough airflow to blow off your hat, it is producing positive thrust, but that same thrust at 80 knots velocity is less than positive, causing a perceived increase in drag.


Once these entries are set correctly, cutting power will lower the nose and add drag simultaneously, causing an increase in sink and decrease in speed.


Roy is probably correct about the prop wash effects on stall speed, I have not looked into it, but FS does compute a higher Lift Coefficient with 'power on' correctly for the jets, so mathematically speaking, if the power table is correct, it should increase the CL for props as well. It would not have the effect of keeping the airflow attached to the upper surface, but FS doesn't model flow conditions anyway.


It's been years since I've worked on prop airfiles, but I do remember an adjustable 'propwash' variable in the lift section. I don't remember it's effect or how it was formatted or if it works for that matter. Maybe something worth researching.



And finally, if all else fails and you want to make a spoiler gauge to fake it, that would be fairly simple to do in XML or C (unless your aircraft has animated spoilers). Just be sure to add a protection mechanism just in case the user accidentally extends them manually.

The drag and lift reduction coefficients are in the airfile, per radian.

 

[jx_]

The other issue is I can't seem to get the aircraft to rotate at the appropriate airspeed - I can't lift the nose until I'm 7 to 10 knots faster than it should be... It's strange as most of all the other metrics I've tested seem right in line with what they should be... The weight and balance seems good now...

Scott,

Almost forgot...

If you're sure it is control effectiveness, that is in the same section as the prop wash effect on controls. All aircraft have a speed called minimum unstick speed. It is the speed where the elevator is able to lift the nose to rotation attitude and hold it there. This speed is usually well below rotation. The aircraft will hold that attitude until lift off at it's VLOF.

Try elevator lift or elevator effectiveness. Real coefficients will translate so long as the aircraft geometry and weight/balance is correct.

I've found that in most airfiles, the effects of table 1534, Wing Center of Lift, are often neglected in calculating total pitch moment. The Wing CoL usually adds nose down to the airplane. Be sure to check that.

 

[mgh]

- the prop transitions from providing thrust to just another source of drag -

That's modelled. Table 512h in the airfile provides the propeller power coefficient (Cp) against Advance Ratio (J) where J = Velocity (Ft/sec) / (Diameter (Ft) * Rev/Sec).

Cp becomes negative (ie the propeller creates drag rather than thrust) at larger values of J when velocity is high or engine speed is low, or both.

The chart shows the variation for the FSX default C172.

 

[Roy Holmes]

Here are some values of the effect of slipstream on approach speed. They are for the Piston Provost. For those unfamiliar with that beast, it was a low-wing monoplane with a 9-cylinder radial Leonides rated at 550 BHP.
Speeds are for the "round-out" , transition from steady approach to flare and 3-point landing.
Normal engine on approach with flap 75 kts
Short field approach, engine on and flaps 65 kts

We were taught to do short field approaches at 60 knots, which was a bit scary at first since all stall margin came from engine effects.

Stall, engine off, flaps down 60 kts.
Stall, engine on, flaps down 50 kts

Some of the speed reduction came from being nose high so the prop thrust had a vertical component, that contributed to overall lift.

FS does compute a higher Lift Coefficient with 'power on' correctly for the jets, so mathematically speaking, if the power table is correct, it should increase the CL for props as well

FS computes a lift component for power on with jets, but it is not a lift coefficient. There is no CL that comes from jet or prop thrust in FS.

This fact also causes difficulty in modeling an airplane like the F-4 that had boundary layer control from engine air bleed. The effect here was to increase CL by allowing higher AOA before the boundary layer separated. Full flap reduced the approach speed by 25 kts compared to flaps-up. An early power chop in the F-4 really produced a big drop unless the airplane was at low weight.

There are four "slipstream" records in the air file. Two are in the pitch section and two are in the yaw section. The pitch records are negative which means they give a nose-up moment with power on and nose down when power reduced.
The yaw records give a moment that varies with power and requires rudder trim to zero out, plus an increase in rudder effectiveness, useful when taxying a tail dragger without wheel brakes.

Roy

 

[jx_]

FS computes a lift component for power on with jets, but it is not a lift coefficient. There is no CL that comes from jet or prop thrust in FS.

Yes component is the correct "word".


You are forgetting the prop wash record. Would someone look into the prop wash record and find out what it does?


Did you get my message Roy?

 

[jx_]

That's modelled. Table 512h in the airfile provides the propeller power coefficient (Cp) against Advance Ratio (J) where J = Velocity (Ft/sec) / (Diameter (Ft) * Rev/Sec).

Cp becomes negative (ie the propeller creates drag rather than thrust) at larger values of J when velocity is high or engine speed is low, or both.

The chart shows the variation for the FSX default C172.

mgh is correct, and it may be useful to add, since this is your first time doing this Scott, Power is equal to Thrust lbs * Ambient Velocity in feet per sec. The conversion from knots to feet per sec is 1 knot = 1.6878 feet per sec

Your work is going to be in finding the velocity range you want to change for Advance Ratio using the equation mgh provided, then applying the reduction in thrust you want to the appropriate Power entries for the appropriate blade pitch. Don't let the graphic mgh posted confuse you, that is a Power by RPM at Speed chart; it's supposed to look different than what you see in the airfile.

Good luck.

 

[mgh]

Propulsive Power = Thrust * Aircraft Velocity

Propulsive Power = Engine Power * Efficiency

where Efficiency is given in Table 511h against Advance Ratio so

Thrust is equal to (Engine Power / Aircraft Velocity) * Efficiency

Power is foot lbs/sec
Thrust is lb
Aircraft Velocity is feet/sec

EDIT

The 4 scalar entries in the ESP propeller aircraft .asm sample are

CMde due to propwash

CMq due to propwash

CnR due to propwash

Cn Delta Rudder due to propwash


They appear in the compiled .air file and are recognised by Aircraft Airfile Manager but I've no idea how to determine the relevant value of propwash!

 

[scottb613]

Hi Folks,

Thanks for all the additional information - let me digest this for a bit and see what I can come up with...


Regards,
Scott

 

[jx_]

but I've no idea how to determine the relevant value of propwash!

The same as other moments. Elevator, wing, etc.... distance from CG times force.

Moment / qSC or (Torque Force * Distance) / qSC


If you know how many degrees of change from equilibrium for a given q, you can easily work out the math.

 

[Roy Holmes]

There is a good explanation of the prop wash, slipstream math in http://library.avsim.net/download.php?DLID=170811

This is Yves Guillaume's excellent document on FDE


Roy

 

[mgh]

That is true. Unfortunately, as Yves Guillamme accepts, many of them for propwash don't seem to work.

 

[jx_]

That is true. Unfortunately, as Yves Guillamme accepts, many of them for propwash don't seem to work.

Far as I can tell, they only work when you're in motion which is not realistically correct for an aircraft on the ground...but at least they work in flight.

As power is changed the moment is applied. I suspect what microsoft did wrong here is tied the coefficients to ambient dynamic pressure instead of ambient dynamic pressure plus the dynamic pressure of the prop wash.

For jets, this is usually okay because the moment is usually due to thrust torque on the airframe. Accurate placement of the engines and CG in the aircraft config are usually sufficient, and the prop wash variables can be used to tune it.

In FS10+ can you manipulate Lift, Drag, and moments via Simconnect?

 

[scottb613]

Hi Folks,

If interested - here is what I have so far - again - this is my first time through attempting an FDE - so ANY feedback welcome... I've got the nose pitching up smoother - I'm still working on the "chop and drop" aspects... Appreciate all the input...


Regards,
Scott