190D9 elevator authority

[straffo]

Mandoble, i think the problem here is not that the D9 locks up, but that the spit and tiffie dont do it.

tss tss ...

The best dive done by a prop aircraft was done by a Spit if I remember right (I know that the wreck was scrapped after )

Found on the tempest page

n March of 1940, Hawker initiated a number of design studies aimed at improving the Typhoon. Among these studies were ways of improving the Typhoon´s high altitude performance.
These involved the use of a new wing design that featured a thinner wing section and a reduced wing area.
The new wing had a eliptical planform and showed a great potential for increasing performance at altitude while reducing the tendency of the original Typhoon wing to buffet at speeds around 500 mph .

And you said :

If i remember right, the critical speed for the D9 was something around 460-470 mph IAS.

[MANDOBLE]

straffo, the hi diving speed of the Spit does not mean hi elevator control, and the comments about the wing redesing of the Thyph seem referred to the Tempest, also, does not mean hi elevator control.

[Naudet]

Than we would have the answer that the tiffie compresses later.


Also i remembered a few things i read about compression and dive, that is related to wing profile.

1st the spit, it has a very small wing, that will suit to very high dive speeds, but on the opposite a plane with a small wing will compress earlier (dont ask why i just copy what i read). The spit therefor could reach extreme dive speeds (one reached mach 0.9 or 0.92) but would compress very early. The spit that did mach 0.9 lost its complete engine in the dive, it was riped off.

2nd the FW190 had a typical 1941 fighter type wing, that was thinker than the one of the spit. So it compressed later and keeped it's control longer.

Now we must know about the tiffie, did i feature an even thinker wing? If yes, it is possible that it compresses later.


But beside the "when does a plane compress", there is the structural question, if the plane can withstand the high forces in a dive.

What i definitivly know from the FW190 is, that i could withstand extreme speeds. Not seldom, pilots flew it in exess of the 750km/h max on the speedometer (it reads IAS) in a dive and would recover safety.


Now we would need some comparable data for the tiffie and spit.


and Straffo btw, did the tempest page mention if the 500 mph were IAS or TAS?

Edit: I have done a fast test myself, using FW190A8&D9, Tiffie and spit. And now i understand why Mandoble is curious about the performances.
The FW190A8 starts to compress at about 500-510 mph IAS, the elevator is somewhat heavy but you can still recover from speeds of about 550 IAS using full elevator deflection.
The D9 starts to compress at about 475-480 IAS, and above 530 IAS the elevator is so heavy it is useless.
The spit, i was not able to fly the spit into compression, the speedmeter is limited to about 480 IAS, and i flew it far in exess, i guess from the time after i hit the speedometer max limit (not indicated but around 500 IAS) i was doing a max dive of 530-540, the spit never compressed and the elevator was about the same as at 400 mph IAS. Easy to recover, also it still rolled with ease.
The tiffie. Same as with the Spit. Not able to compress it, cause i choose a higher starting alt, i guess i reached a max of about 550-560mph. Elevator was extremly light for that speed. A minimal pull on the stick would recover you from the dive, also ailerons never locked up, they stayed about the same from 400-max.

Now i really think there is a problem with the spit and tiffie model. I think from an alt of 30K you can get to mach 1 with both in AH.

Ups i forgot to mention, that both even dont show any airframe stress sound. Whiles both FW190 will really make some overstress noices from about the point were they compress and even before. Neither the spit nor the tiffie will show any indication that they are flown far far of there construction limitations.

[Seeker]

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[MANDOBLE]

Originally posted by Naudet
Mandoble, i think the problem here is not that the D9 locks up, but that the spit and tiffie dont do it.

The principal problem is not with the total lockup itself, but with the gradual loose of elevator authority above 300 mph. Aside the speed where u get a compression, the fact is that much earlier u have a real maeouver handicap against spits and typhs (as many others) while being into a speed range where (supposedly) D9 should shine over planes with much better control at slow speeds.

D9 is a brick near stall speeds, but is also a brick at hi speeds

I've read somewhere (poor proof, I know) that D9, as well as A family, was very good at gaining hi AOA really fast. I know this is no really an inmediate change of course, but in any case, this implies a good elevator authority.

I did more tests with D9 and Typh. Once D9 get into real compression, it tends also to roll while, in the same situation, Typh keeps compressed but into a stable flight path. Torque? Is the enormous Typh torque only noticeable at takeoff time?

[straffo]

@naudet

I don't know if it TAS or IAS ...
Will search a bit but I'm pretty poor for documentation

@mandoble
concerning the rool at high speed the typhoon was know as a stable plateform (dunno if it's related ...)

Concerning the enormous torque you got it when you firewall the throttle but in AH you cannot do that.

Just look at the indicator even if you firewall the throttle the indicator don't react according to your input.

[Naudet]

Mandoble u managed to compress a tiffie? what was ur starting altittude?

I tried it 3 times. 2 tries from 15k one from 17k, i could not bring the tiffie to compress. And i am sure i was way above 500 mph IAS.


@seeker: a nice diagramm, but not of any use in this discussion. The graph is for a Spit XIX, as i know that had a completely overworked wing. And that wing had not much too do with the spit V,IX or spit XIV wing. The wing of the XIX was changed due to the high speed control probs the old wing suffered from.

A Spit IX would never reach such speeds.

[Seeker]

"The graph is for a Spit XIX"  

That'll teach me to proof read

This, howerver, does relate to the IX:

"In 1944 at Farnborough a PRXI (Photo recce version of the IX) EN409 was
used for high speed diving trials.   On April 27, 1944 S/L Marty Martindale
reached a true airspeed of 606 mph, or .89 mach.  He started his dive from
40, 000 feet with a speed of 340 mph, gradually bunting over into a dive.
In the course of the dive the reduction gear of his propeller failed,
tearing the prop away.  He managed to dead stick the Spit in for a landing
so he could save the film and the recording instruments.

The thin wing of the Spit was what allowed it to have the highest limiting
mach number of the prop fighters."

[MANDOBLE]

Naudet, I managed to kill elevator authority of a typh in a 20k dive, no really a compression cause I still had full control with trims. What I mean with full control is that minimal nose up trim had a very very quick response and I got elevator authority again.

[straffo]

I've a little question (no flame intended) as I'm an FM n00b

What are the definitions of :

* compression

* elevator authority

[Seeker]

What's compression?

As you near the spead of sound, you're pushing an enormous shockwave through the air in front of you.

When you pass the speed of sound, that shock wave is behind you.

In between, in the so-called "trans-sonic" region, the shock wave appears around the aeroplane at different points, as the speed of the air differs at different points (fast at the wing roots, slower over the canopy, for example). This shock wave greatly interferes with the airflow around the control surfaces, sometimes locking them in place, sometimes making them useless, sometimes reversing their effect. The net result is a joystick set in concrete - an unresponsive plane - and it's why in some planes you can't pull out of a high speed dive. The compression speed is different for every plane, and is known as it's "critical mach number"; the proportion of the speed of sound (mach number) at which the pilot is no longer in controlled flight. To make matters worse, other important factors, such as the centre of aerodynamic lift, move around a bit at these speeds too - A famous example is the P38 developing a strong nose down tendancy at about the same time the elevator stops working (you can imagine the result...)


It's important to realise that the speed of sound changes with altitude (air pressure); and so, as you dive lower, you're entering air where the compression threshold is higher. This is why you may lock up the controls on a P38 ar around 380 IAS at 20K; but 10K lower, if you're lucky, you might get control back at 400 IAS - the lower you are, the faster you can go without compressing.

Another famous example is early Typhoons - they could dive so fast they'd start to compress, get into lower, thicker air where the elevator would suddenly regain it's authority (it'd work again); and you suddenly had a pilot doing 500 IAS and yanking back hard on a stick which would suddenly work - he didn't just black out, he'd snap the tail right off, a weakness of the early Typhoons.

Think of all the Kamikazees you've seen in film of the pacific war *just* missing thier target. They're not dead (probably); they're compressed - they power dived a Zeke, which has a low critical mach number (means it compresses - locks up - at a relatively low airspeed), compressed - and now they can't "aim" the plane - so they auger right next to it......

Control authority (elevator, rudder etc) is just a formal way of saying the control works. If I understand Mandoble's reasoning here, he's asking two questions:

At what speed should the various airframes compress? (in otherwords, are the critical mach numbers for each airframe correctly applied)

At which speed thresholds should the symptoms of compression set in for each aircraft? (compression rarely happens all at once, the controls get heavy and the plane may start to shake and grumble) - If I understand him properly he's questioning at what speed should the Dora's stick get stiff (if at all).

[MANDOBLE]

Seeker, excelent explanation. And u are right about my worries, specially in the last part.

There is also another point here related to stick pressures:

Plane A running at speed X is applying Y lbs pressure back on the stick and getting Z1 degrees nose up in T seconds.

Plane B running at speed X is applying Y lbs pressure back on the stick and getting Z2 degrees nose up in T seconds.

[hitech]

Seeker wrote.
It's important to realise that the speed of sound changes with altitude (air pressure).

Close. It's not a function of pressure, it's a function of temperature.

[Naudet]

Seeker, i never doubted that a spit thin wing profile would make it an extremly fast diver.

And the report you quoted is the one i know too.

The prob i have, is that i also read that the thin wing, was prone to compression more than a thicker wing. And so the spit had high speed control probs, cause it would compress relativly easy.

A thick wing on the other hand, doesnt allow for such high diving speeds, but has less problems with compression and so planes like the FW190 and P51 had better high speed handling.

In AH this is inverted. You barrely can fly a Spit into compression and the controls in it feel pretty light and easy to use even far beyond 500 mph IAS.

This is also one point about the dive of S/L Marty Martindale. He started at 40,000 feet and reached a speed of 606 TAS. i tend to say he reached that speed somewhere between 26-31K in the dive. So IAS would be closer to 480-500 mph. And i am sure it was not the calm and easy ride we have in the AH Spit or Tiffie.

Also the rule of the "regain control at low altittude" is not plane specific. In the books i read, the german pilots also recall that they had to wait till they reached dense air (3000 Meters or lower) to recover from emergency dives.
Cause before that they could not gain control authority back.

But in AH the D9 will not gain any better control authority once you get lower. The only way to recover is using the trim tab, otherwise you will just fly straight into the deck.

[Seeker]

"A thick wing on the other hand, doesnt allow for such high diving speeds, but has less problems with compression and so planes like the FW190 and P51 had better high speed handling."

I don't know enough about the subject to argue with any authority; but I thought the critical mach number was more a function of profile than thickness. The Spit has, for example a higher critical mach number than the 262, I believe; wierd as that may sound. Remember, critical mach is not quite the same as maximum speed: Almost by definition all supersonic flight must pass through compression (the period of disturbed transsonic airflow) to go supersonic. it's the elegance with which they do so that dictates survival.

( (At this point some one who knows should step in and save me from further embarrasment:))

I also thought the Mustangs laminar flow wing was a thin wing design?

To Hitech: Temperature is but a symptom of pressure, neener!