Why so slow?

Can anyone with deeper knowledge about the IRL aircraft explain why the Hog is so slow? As I understand it the exact same engines was used by the S-3B Viking - a considerably faster bird.

Also, the civilian version of the engine - the CF34 - are used by both Bombardier CRJ and Embraer E-jets series. Those, of course, aren't the exact same engines but we are talking much higher speeds here.

Is it a weight/technical thing or is the Hog simply made to fly slower for tactical reasons?

Look at the wings.

Straight shape is against speed.

I assume there must be some advantage with the lower speed, that warranted this design? Higher precision in A/G engagement for example?

Payload, take off distance, fuel economy.

Aircraft designs are compromises built on specific performances desired. The wings are the primary aerodynamic determining factor relating to the speed of the aircraft. The CRJ and E-jets are designed for long range fast cruise performance and have relatively thin wings compared to the S-3 and A-10. The S-3 Viking was designed for long range, long loiter times at low to medium altitudes to engage enemy surface and subsurface threats. It has a large, high lift wing with a mild leading edge sweep. The A-10 was designed for Close Air Support (CAS) where low speed and high maneuverability was desirable in order to maintain visual contact with targets on the ground. Having a large, straight, high lift (draggy) wing is perfect for the CAS mission.

You should also compare the turning performance between the S-3 and the A-10. The A-10, due to its slower speed and straight, thick wings can practically pivot on its wings and reverse direction. That helps with not only being able to quickly get back on a target, but also with making it harder for the people on the ground who are trying to shoot you down.

big wings, big nacelles, big body- lots of armor, and with payload that's even more weight and drag (especially with TER)

upside: lots of lift, relatively resilient, able to carry a lot of payload, lots of gas and long station time

downside: slow, high RCS, relatively low AoA tolerance because of big board wings

Well a lot of those commercial CF-34s have twice the rated trust, so that is a thing too

the straight wings are a factor, but having more umph in the engines, would definitely be of help. It is not like it is reaching anywhere close to its overspeed limits. Better acceleration, and higher cruising speeds would have been nice.

I don't know if the engines are straight up untouched since the A-10 entered productions, but newer engines, would give it better characteristics for the same fuel economy for sure..

But you know what they say, if it ain't broke, don't fix it, I guess.

The A-10 is designed to be fuel efficient in order to have a good time on station. The A-10 is also designed to be able to carry a lot of weapons (a lot of pylons too, which produce drag) and fly really well at low altitudes and speeds. So there are a series of technical reasons as to why the A-10 is slow, one of them being the A-10 is slow because it has to. If it was designed to fly faster, it probably wouldn't be as good as it is at low/slow.

It has big, thick and straight wings which allow the airframe to produce a good amount of lift at slower speeds, big horizontal and vertical stabs also help the nose and rudder authority (survivability as well), big engines, and a not so aerodynamic fuselage. All these things produce a lot of drag, and when combined with not-so-powerful engines, it'll be pretty slow, but the fact that the A-10 is slow does not mean it's a bad thing, it's completely the opposite, it flies extremely well at low/slow, the stall speeds are pretty good, handling, maneuverability and so on. It's supposed to be that way.

The most important thing when talking about this though is that, the A-10 is NOT trying to be fast. It's trying to be efficient and stay in the air for a longer time, flying at low altitudes and slower speeds, which is a good thing when giving support to troops on the ground. If it was trying to be fast, it would probably lose all those low speeds characteristics, which is what makes the A-10 a good CAS platform.

One reason as to why the A-10 has big engines (which also contribute to drag in some ways - frontal area) is because of propulsive efficiency. Propulsive efficiency is the external efficiency of an engine and is measured in percentage, and this percentage indicates how efficient an engine is at converting the kinetic energy (high speed gases coming out of the exhaust) into making the aircraft flying forward.

If the A-10 had to fly fast, you would need an engine that is capable of producing very high speed gases, and to accomplish that you need temperature, and temperature is obtained by an increase in fuel flow (see where I'm getting at?) and ANY time you increase the exhaust gas velocity, the propulsive efficiency is decreased, therefore the engine is less efficient at converting the high speed gases (kinetic energy) into speed. So the engine not only is less efficient, but it's also using more fuel. Since external efficiency is directly related to fuel consumption, hence the big engines on airliners...

A high bypass engine is preferable over a low bypass one (that would allow the A-10 to fly faster) because it works with a higher amount of mass flow and a low exhaust gas velocity which helps the propulsive efficiency. In order to fly fast, the exhaust gas velocity must be higher than the speed you want to fly at, and the A-10 wants efficiency instead of speed.

As you can see in this chart, a high bypass turbofan engine like the TF34 reaches very good propulsive efficiencies at much, much lower speeds. If you had an afterburning engine, you would need to be supersonic in order to reach these propulsive efficiencies. And that would use tons of fuel... again not exactly what the A-10 is looking for.

Turbojets/low bypass turbofans aren't good at slower speeds, because their external efficiency is terrible.

So conclusion; the A-10 is slow because of its aerodynamic design which makes it a good low speed/altitude/CAS platform, this aerodynamic design is optimized to be flown at low/slow, and it produces a lot of drag (result from thick and straight wings for example), and the engines are meant to be fuel efficient, so they don't produce enough thrust to overcome that huge amount of drag.

Any time you fly faster you waste fuel, which reduces your time on station.

Cool fact: 85% of the total thrust produced by the TF34 engine comes from the fan, and not the exhaust. That's why its propulsive efficiency is good. The fan moves a huge amount of mass flow, at a slower speed.

The fuel efficiency (S.F.C - internal efficiency, not external) is also very impressive, it's as good as some airliner engines such as the CFM-56, with an S.F.C of 0.371 lbs/lbf/h! It's really good.

EDIT: As I preview this I see Vitormouraa has sniped me but I think our points are different enough (he has focused on the economy issue) that I will post also...

@Shadow, I take some disagreement here

the straight wings are a factor, but having more umph in the engines, would definitely be of help. It is not like it is reaching anywhere close to its overspeed limits.

Why is it not reaching overspeed limits? Its not just the wings. As has been correctly alluded to above, it is the drag. When your total drag curve meets your thrust available curve, that is your max speed in level flight.

-Total drag is the sum of induced drag and parasite drag. At high speed, parasite drag is the limiting factor and it increases with the square of your airspeed.

-Thrust available isn't quite constant with airspeed at a given density altitude for turbofans but we will say it is for our purpose, it is close enough.

Vertical axis is pounds, horizontal is knots airspeed. Where the red line (total drag) meets the blue line (available thrust), that is max speed. You can see at that point that the drag line is increasing very rapidly (since it is an exponential curve) with airspeed, so it takes much more thrust to continue to increase the speed. It has been a long long time for me, but I believe at the high speed end it takes an approximate 30% increase of thrust to increase speed 10%.

More power will help runway performance, climb rates, OEI performance, and sustained turning performance. But if you want to make an aircraft faster, you really are spending your time and money more efficiently by making it have less drag, not more power.

Nice!

If you read this post, you'll see that thrust in flight varies with speed. So this 'encounter' between the total drag curve and thrust could come much sooner depending on the engine, especially the TF34...

Increasing thrust would partially increase your max speed, but you would be wasting so much fuel, energy and money to do that. And on top of that, you'd be dealing with other issues such ass flow separation, coffin corner, compressibility and so on...

:thumbup:

@Vitormouraa

Thanks, I totally agree that there is a change in thrust over speed, but as I said, in this very specific case I don't find it playing enough of a role to consider.

In your examples in the other post, static thrust v something around mach 1, the difference is much more pronounced (which I totally agree with). Here, even if we could somehow induce a 10% increase in speed, we are only talking about a difference of ~30 knots IAS. The engines go though a difference of that magnitude 5 times just taking off. Thats why I decided to assume thrust as a constant in this small window so drag alone could be considered. But just to make sure I am following, the loss of thrust is not exponential, correct? Just a geometric reduction against speed?

Cheers though, it was a good discussion on efficiency. And like you said, so many other factors at play here; in addition to the ones you listed, controlability issues after losing an engine during takeoff would have to be addressed, probably with bigger horizontal stabs, adding weight and drag, ect ect ect haha! As you said, there is a lot more too it than bolting on new engines...

plus the lifecycle costs. even assuming you can find a reasonable way to just bolt on an F15/16 engine, you'll have to retrain techs, write new TTPs, adjust all your pilots to the new performance, and then pay money for all of these things. in addition, the plane's chief weakness (vulnerability to IR missiles) might be harmed by changing engines.

it's a "why rock the boat" proposition. the plane works fine, as long as it's in certain situations. new engines aren't going to solve the plane's top 20 problems in a modern engagement, and you'd have to spend a lot of money to "fix" the problem.

if they remain in the inventory you'll probably see lifecycle part replacements and stuff which are absolutely necessary to keep them flying safely- perhaps some avionics upgrades to handle new capabilities coming into the force in general, but trying to turn the plane into something it's not would be too expensive and fruitless to ever be seriously considered just imo.

plus the lifecycle costs. even assuming you can find a reasonable way to just bolt on an F15/16 engine, you'll have to retrain techs, write new TTPs, adjust all your pilots to the new performance, and then pay money for all of these things. in addition, the plane's chief weakness (vulnerability to IR missiles) might be harmed by changing engines.

 

it's a "why rock the boat" proposition. the plane works fine, as long as it's in certain situations. new engines aren't going to solve the plane's top 20 problems in a modern engagement, and you'd have to spend a lot of money to "fix" the problem.

 

if they remain in the inventory you'll probably see lifecycle part replacements and stuff which are absolutely necessary to keep them flying safely- perhaps some avionics upgrades to handle new capabilities coming into the force in general, but trying to turn the plane into something it's not would be too expensive and fruitless to ever be seriously considered just imo.

I think it was Snoopy that said you cannot bolt on larger engines without major strengthening on the mounts, which would not be worth the cost and speed gain advantages of the design.

The best thing to do (OP) no one has mentioned IRL weapon loadouts, if you are realistic with the loadout, she flies and performs much better.

Be interesting to hear what you normally load.....

Look up some air to air refueling to see some IRL loadouts etc.

 

The best thing to do (OP) no one has mentioned IRL weapon loadouts, if you are realistic with the loadout, she flies and performs much better.

 

Great point there, thanks for bringing it back to the OP!

@Vitormouraa

 

 

Thanks, I totally agree that there is a change in thrust over speed, but as I said, in this very specific case I don't find it playing enough of a role to consider.

 

In your examples in the other post, static thrust v something around mach 1, the difference is much more pronounced (which I totally agree with). Here, even if we could somehow induce a 10% increase in speed, we are only talking about a difference of ~30 knots IAS. The engines go though a difference of that magnitude 5 times just taking off. Thats why I decided to assume thrust as a constant in this small window so drag alone could be considered. But just to make sure I am following, the loss of thrust is not exponential, correct? Just a geometric reduction against speed?

 

Cheers though, it was a good discussion on efficiency. And like you said, so many other factors at play here; in addition to the ones you listed, controlability issues after losing an engine during takeoff would have to be addressed, probably with bigger horizontal stabs, adding weight and drag, ect ect ect haha! As you said, there is a lot more too it than bolting on new engines...

No problem at all!

The loss of thrust highly depends on the intake design. I said earlier it depends on the engine, that is not correct, but I was referring to the actual inlet design mounted on the TF34.

It's difficult to say if it's one or another because that depends on the inlet as I said. The thrust curve will vary, it could decrease or increase. A complex inlet system will try to extract work from the gas at high speeds, with an inlet like the one installed on the A-10 it's really difficult to do the same, if take a look at the fan diameter, you'll see that the inlet is smaller in diameter than the fan itself, making a divergent duct/diffuser, the idea behind this is that, as your aircraft increases the forward speed, the Mass flow and jet velocity tend to increase due to ram effect, but so does the inlet velocity, so the difference between V1 and V2 tends to decrease, but if you decrease the velocity at which the air enters the engine, you can reduce this negative effect, which is called ram drag or inlet momentum drag, loss of thrust due to forward speed. By doing so, you can also recover some of that thrust which was lost. If you had no inlet, this loss would be exponentially high...

This is called diffuser (velocity decreases and pressure increases), but this effect is limited due to the extension of the inlet. In my other examples, I explained how supersonic jets will increase the thrust with their C-D inlet designs, and some airframes will increase the thrust beyond the gross thrust (static thrust - on the ground).

Again, any time you increase the V1 velocity (air entering the engine), loss of thrust is possible unless you can reduce that speed and extract work from the gas, i.e inlet is also compressing air, doing work for the compressor.

A few examples:

I like the old boy just the way he is. More time for us old farts to think. :)

Every once in awhile I'll do some valley cruisin' with no weapons and around 30% fuel. She'll scoot pretty good :thumbup:. She's a hog though..no doubt!

I'm sure i read somewhere on this forum that the DCS a-10 is a little under powered compared to the real thing..

I'm sure i read somewhere on this forum that the DCS a-10 is a little under powered compared to the real thing..

We've all read that but is it really the case though? I'm not sure...

IIRC it was a real hog pilot who said that in a Q&A on Hoggit.

Anyway the Hog is slow because it was designed to be slow. It gives you plenty of time to align, execute passes and also gives you enough endurance to just keep orbiting the AO for hours in case the guys in the ground need you.

Believe me, nothing is more satisfying that flying multi squadron missions and listen to everyone calling bingo and queueing the tanker while I'm still over half tank and going smoothly.

Doing searches to see if anyone has mentioned speed after having lost an engine and this thread looks closest. I lost an engine and found it incredibly hard to stay above trees. (It's alright, found the engine, it just fell between the couch cushions)

I had believed the Hog was more than capable of flying with 1 powerplant and the sim was a bit unrealistic in this sense. But reading here, I am thinking that my problem was that I was still fully loaded, didn't consider dropping my payload. Does this sound feasible?

It's reasonable in any emergency power loss to get rid of your stores, the sim isn't being "a bit unrealistic", it's being entirely the opposite.

If you were carry your wife on your back, and suddenly I told you that you could only "Hop", would you not consider putting her back down?

Doing searches to see if anyone has mentioned speed after having lost an engine and this thread looks closest. I lost an engine and found it incredibly hard to stay above trees. (It's alright, found the engine, it just fell between the couch cushions)

I had believed the Hog was more than capable of flying with 1 powerplant and the sim was a bit unrealistic in this sense. But reading here, I am thinking that my problem was that I was still fully loaded, didn't consider dropping my payload. Does this sound feasible?

DRAG

Doing searches to see if anyone has mentioned speed after having lost an engine and this thread looks closest. I lost an engine and found it incredibly hard to stay above trees. (It's alright, found the engine, it just fell between the couch cushions)

I had believed the Hog was more than capable of flying with 1 powerplant and the sim was a bit unrealistic in this sense. But reading here, I am thinking that my problem was that I was still fully loaded, didn't consider dropping my payload. Does this sound feasible?

That is definitely what you should be doing when you lose an engine. External stores in DCS have a very realistic effect on weight and drag. The A-10 will fly just fine on 1 engine, but that's only there to get you back home. It was never intended to make it so you could stay in the fight. ;)