Power/Torque Problem

[Tiger-II]

Hi,

I have a whole day left of my trial of the AH-64, which sucks because my new rudder pedals arrived a day early (today) and so I have almost no time to get used to both.

I noticed it previously but didn't really "see" the problem as I was controlling pedals with the second throttle axis of my HOTAS.

First flight, the helicopter required a lot of RIGHT pedal to counter torque (uhh... seems rather implausible... anti-clock rotor; left pedal).

Second and subsequent flights, she requires a tiny bit of left pedal. HUH?

Third problem, is while she is nearly at max gross, she requires 130% Tq to get her airborne, and once off the ground, feels to lack power to climb. Yes, I'm aware of translational lift, yada yada. Throttles are fully forward to the FLY detent.

All my missions are configured for standard day and zero wind. Aircraft is at Beslan (I think 1500 ft altitude).

I'm used to flying the beast that is the Mi-8, so I don't consider myself a rookie.

Is there a known issue with power/torque, or am I being dumb?

I tried the UH-1H out of the same location and it flies fine.

Edited January 4, 2023 by Tiger-II

[NeedzWD40]

I rarely need pedal at all when initially taking off as the yaw SAS normally handles that well enough. Right pedal for nose to tail trim at speed.

Depending on conditions, you won't be able to hover near or at max gross weight. If both engines are operational, then 100% is your max torque; beyond that is a few second limit, then after that your transmission will be wrecked. This is not currently modeled, but it's not a good habit to be overtorquing the engines anyways. Your sea level restriction is about 20000lbs for standard conditions, give or take a couple hundred pounds and this does not give you much room to maneuver: you're pulling 100% to make it hover in that condition.

For 1500ft you're looking in the realm of 18500-19000lbs for max hover IGE.

[Tiger-II]

I need to hit the manuals PDQ to see if Yaw SAS is operational. Feels like it might be.

She's at 21500 lbs (8*Hellfire, 2*fuel tanks).

[Raptor9]

@Tiger-II, see this thread here: 

TL;DR: 0% (centered) in the rudder axis does not correspond with 0% tail rotor thrust.

Regarding power vs torque...all aircraft are limited by how much lift they can produce, whether it is a fixed-wing or rotary-wing aircraft. If the gross weight of the aircraft is so great that the lift cannot overcome it, flight will become very difficult. If you max out the collective in a helicopter when fully-loaded, that is like pulling back on the stick of a heavily-loaded airplane immediately after leaving the runway to go into a steep climb. You'll probably go right back to the ground very quickly after stalling out. If you want to maintain flight in a helicopter, you need to only pull in the amount of collective with which the engines can keep the rotors at the intended RPMs for flight. All the other DCS helicopters are the same way. If you load a UH-1 with 38 rockets, 4x miniguns, and full fuel, you will struggle to get off the ground without losing rotor RPMs when the collective is pulled too far.

Reduce the weapons load, reduce the fuel load, or both. But if you lose rotor RPMs, you need to lower the collective because you are pulling in too much.

It's the same thing with loading down aircraft like the A-10 with the maximum amount of bombs that is possible. Just because it can be done, doesn't mean it is wise to do so. And environmental conditions that result in higher density altitudes exacerbate it.

[Tiger-II]

@Raptor9Thanks for the link.

I'm aware of rotorcraft aerodynamics. That is not the problem.

It seems my ... expectation ... is that it has more power/lift capability than it actually does. As it is, I'm loaded 1500 lbs below max gross, flying on a standard day out of an airport at 500 ft AMSL (I checked). I thought it would be able to handle that with margin to spare, so you can perhaps understand my surprise when it took everything it had to get off the ground.

The load I'm flying with seems to put it right on the edge of the envelope, which quite honestly I wasn't expecting.

[Floyd1212]

Bradmick posted a great video on how to use the PERF page to calculate the performance of the aircraft at given weights/altitudes/temps.

 

[Swift.]

6 hours ago, Tiger-II said:

@Raptor9Thanks for the link.

I'm aware of rotorcraft aerodynamics. That is not the problem.

It seems my ... expectation ... is that it has more power/lift capability than it actually does. As it is, I'm loaded 1500 lbs below max gross, flying on a standard day out of an airport at 500 ft AMSL (I checked). I thought it would be able to handle that with margin to spare, so you can perhaps understand my surprise when it took everything it had to get off the ground.

The load I'm flying with seems to put it right on the edge of the envelope, which quite honestly I wasn't expecting.

To be fair, 2 fuel tanks on its own is a hefty load and not something you'd be lugging around into combat. Add another 1000lbs on top of that, and you can understand the issue. Do you have the FCR fitted too? Because that'll be another 700lbs ish.

If you want to be impressed by it's capabilities, note how you can load 16 hellfires and still takeoff fine and have enough fuel to endure for 3 hours or so.

[bradmick]

15 hours ago, Tiger-II said:

@Raptor9Thanks for the link.

I'm aware of rotorcraft aerodynamics. That is not the problem.

It seems my ... expectation ... is that it has more power/lift capability than it actually does. As it is, I'm loaded 1500 lbs below max gross, flying on a standard day out of an airport at 500 ft AMSL (I checked). I thought it would be able to handle that with margin to spare, so you can perhaps understand my surprise when it took everything it had to get off the ground.

The load I'm flying with seems to put it right on the edge of the envelope, which quite honestly I wasn't expecting.

I think this is something worth clarifying:

The max gross weight of the helicopter may be 23,000lbs, but that's for non-combat flying. The normal maximum gross weight for combat is 20,260lbs. The issue is that these numbers aren't so much theoretical, they're real numbers, but they're at very specific sets of conditions, and usually benchmarks to meet a requirement dicated by the DoD. The reality is, the performance capabilities of the aircraft are futher detailed in "the book". The only number that really matters for the Apache is the OGE Maximum Gross Weight, which is easily referenced via the PERF page. Keep the weight of the helicopter in DCS under this value and you'll do fine. Wikipedia does a great job of qouting max values, but doesn't have any of the context or subtext or nuance (pick your fancy word) that comes with operating the thing. 

[Tiger-II]

@Swift. and @bradmick Thanks for the posts.

Yes, I had removed the FCR from the loadout.

Quote

The max gross weight of the helicopter may be 23,000lbs, but that's for non-combat flying.

I realize that.

 

Quote

The only number that really matters for the Apache is the OGE Maximum Gross Weight

Exactly. I didn't get that far with it; struggling to lift off at all got my attention.

[Tiger-II]

To add: while 23000 lbs is MGW, it should still fly, correct?

What would be the correct way to take off in that situation?

I checked the mission: at Beslan, it reports the aircraft at 1700 ft on the ramp, SAT is +14.4 deg. C, pressure is 29.92 inHg, and there is no wind.

[Swift.]

2 minutes ago, Tiger-II said:

To add: while 23000 lbs is MGW, it should still fly, correct?

What would be the correct way to take off in that situation?

I checked the mission: at Beslan, it reports the aircraft at 1700 ft on the ramp, SAT is +14.4 deg. C, pressure is 29.92 inHg, and there is no wind.

With helicopter performance there is this concept of having a 'bucket speed' what that effectively means if that as you speed up from stationary the torque required to fly will reduce, until a certain airspeed at which point it will start to increase again. For apache you can estimate that certain airspeed (the bucket speed) as about 70 kts ish.

So if you are stuggling for lift, you need to get moving forwards to about 70 kts. If you are able to hover IGE then you can accelerate whilst still in IGE. If you cant even lift into IGE, you can roll along the runway until you hit that speed.

[mcfleck]

Operating at MGW, you have to set yourself up for a rolling takeoff /level acceleration within ground effect in order to get above ETL.
Once you have 30-40 kts, either aim for roughly 45-50kts which should be in the ball park for the best climb angle (steeper climb, but relatively slow) or around 65-70 kts for your best rate of climb (shallower climb, but will get you faster to the desired altitude)
You can do all of it practically without touching the collective even once after setting the required power for the takeoff (i. e. Max continuous TQ of about 100%) until you reach your cruising altitude.
You will observe, that your required total TQ will drop slightly above ETL as your vert. Stab is starting to work more and more efficiently, which reduces the power requirement for the tail rotor and thus reduces the total TQ, freeing up power that you could additionally use in your main rotor).
Best practice for landing in these kind of situations is performing a rolling landing.

[bradmick]

The vertical stab is not the primary reason for the reduction in power. The tail rotor accounts for a super small percentage of your total torque. The air flowing more horizontally over the rotor disc and the reduction of the induced flow velocity is where the reduction in torque is coming from. The rotor is becoming more efficient. This is also occurring on the tail rotor also.

[mcfleck]

Sorry Bradmick, but in this point I do not agree with you.

The more horizontal flow will give you more lift with the same angle of incidence as your effective angle of attack is increased (due to less induced flow as you rightly said). What you basically achieve is that the vortices at the blade tips get smaller and thus more of the air that is moved is used to create lift instead of drag. The total ammount of drag and thus torque does not change significantly due to more horizontal flow (always assuming you keep your collective position constant all the time).

The vert stab is definitely taking over a large portion of yaw stability at a certain speed unless the aircraft designers have screwed up. It is usually designed as an airfoil in order to create "sideways" lift additionally to the weather vane effect. As you certainly know lift increases quadratically with the increasing speed. Combine that with the rather large lever arm of your tail boom and there you go.
If it would not be like that (the vert stab compensating the main rotor torque), the designers would have just created an additional parasite drag area. Flying SAS OFF you will also notice that you need significantly less left pedal going with 70 kts.
Btw. That is exactly what you use in the EP for a Tail rotor drive failure or stuck pedals (at least in the 3 Helicopters I am rated in), that says you have to keep your airspeed above 70 kts and so on... It says that because you effectively barley need any tail rotor thrust above this speeds.

Not having flown the Apache, the intricacies of this particular AC might be special here and there, but it should still kind of follow basic aerodynamic concepts and logic. Sure the Apache's vert stab is way smaller than the ones I am referring to... Please correct me if I made wrong assumptions regarding the AH64 in this context. Would be an interesting chat and discussion I guess.

Having said all that the conventional tail rotor in general may require up to 15 % of your total torque. The Apache values of course may vary.
The flight school I went through had at least one Overtorque incident where the pretty heavy AC climbed out of a confined area, the student pilot was not aware of the wind direction, had to add sudden left pedal input in order to maintain heading and exceeded the TQ limit.

TLDR:
Whatever the explanation for the "freed up power/torque" you are willing to accept, the effect is there and also works the other way around while getting slower. So be aware of the possible increased power requirements during the descend/landing.

[bradmick]

The more accurate statement is:

A combination of the efficiency gained through increasing airspeed, combined with the vertical fin, contribute to the reduction of left pedal while accelerating the helicopter.

 

Somewhere around 90 or so knots the tail rotor being more or less "offloaded" and then 100 knots is the minimum speed in the event of a loss of tail rotor thrust. At lower airspeeds the fin doesn't have as much authority though, that swing that occurs is more a result of the tail rotor becoming more efficienct for the given angle of incidence. As speed increases the vertical fin becomes more effective, absolutely. But in that initial phase, it's all about the tail rotor becoming more efficient with the increasing airspeed.

Edited January 7, 2023 by bradmick

[Chic]

Thank you Gentlemen. 

[agamemnon_b5]

On 1/5/2023 at 1:26 AM, Swift. said:

To be fair, 2 fuel tanks on its own is a hefty load and not something you'd be lugging around into combat.

Unless you're in the Israeli Air Force, lol.

[Swift.]

On 1/11/2023 at 12:24 AM, agamemnon_b5 said:

Unless you're in the Israeli Air Force, lol.

Are those RCEFS thoug or just those monster tanks we have been in dcs? Because there are combat fuel tanks that are smaller and are actually crashworthy

WAH64s have been flying with them for a while. I think it's a relatively recent thing for the US Apaches though.

Edited January 12, 2023 by Swift.

[bradmick]

External tanks are lame. The Robbie accomplishes the same thing without all the drag and asymmetry. Also, flying for as long as those tanks allow is murder on the body. 

Edited January 11, 2023 by bradmick