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View Full Version : A couple of quick aviation questions (tail units and cruise speed).



Hunde_3.JG51
10-26-2004, 03:11 PM
Hopefully these questions are not overly stupid:

1. What are the benefits and disadvantages of a plane with dual stabalizer tail (B-25, B-24, Bf-110, etc.) as opposed to the more common single stabalizer type (B-26, B-17, Mosquito. etc.)? I would guess the dual unit offers more stabilization at the cost of weight and drag, but maybe I am wrong.

2. In a book I have it gives a "cruise speed" for most planes but sometimes it says "normal cruise = ***mph", sometimes it says "range cruise = ***mph", and other times it says "max cruise = ***mph", yet it rarely gives more than one for each aircraft. What are the differences between the three if any?

Thanks in advance for any information that can be provided.

El Turo
10-26-2004, 03:18 PM
Split tail designs can offer more stability, and also alter how prop-wash and "dirty" air are passed over the control surfaces. Also allows for dorsal gunners to have a direct-six firing lane.

"Cruise" is more or less a description of anything lower than full military power that will allow for stable and continuous level flight. You can get anything from "economy" cruise for maximum fuel efficiency to "max" cruise which will generally give you the maximum sustained airspeed without overheat.

Granted, these are all greatly simplified.. but it's the general gist of things.

diomedes33
10-26-2004, 03:18 PM
I'm not positive this is the reason, but it happens too much for it to be a coincidence. If you notice only multi-engined planes have the split tail.

If you look at this shot of a b-25.
http://www.carnut.com/show/03/osh/osh173.jpg

You can see how the prop-wash goes right over the vertical stabilizer, giving the rudder more effect when the plane is at low speeds or on the ground.

BinaryFalcon
10-26-2004, 03:21 PM
1. In part, it lets you get away with more stabilization for a smaller sized tail. For example, you could have two half-height tails instead of one full height one. It should also help at higher AOA where a regular centerline tail is more likely to be masked from the relative wind by the fuselage.

2. Cruise speed will depend on power settings, and power settings will play a large part in fuel consumption. "Normal" cruise is usually a power setting that will provide a good balance between range, speed and engine wear. However, if you want to get maximum range you'll use a different setting, which will typically result in a slower speed but also a lower fuel consumption, which will allow you to stay in the air longer or cover more distance.

The specifics will vary for each aircraft, and you'll generally choose the setting that best suits what you're trying to do.

Korolov
10-26-2004, 03:25 PM
IIRC, the reason for dual verticle stabalizers is to increase verticle area without making the stabalizers larger.

Cruise differences would be represented in the best cruise for each plane - a plane might perform better on economy cruise, while another might perform better on max cruise.

Essentially, normal cruise = average cruise speed, range cruise, or economy cruise = best speed, using least amount of fuel, and max cruise = maximum speed while retaining cruise fuel consumption.

I think I got it right. http://forums.ubi.com/images/smilies/blink.gif

berg417448
10-26-2004, 04:35 PM
And sometimes the twin tail fins are not as good as a single larger fin:



"As early as 1942, the Army had concluded that the B-24 Liberator would have better aerodynamic stability if it had a single fin and rudder. However, the Liberator was destined to go through almost its entire career with its original twin fin-and-rudder assembly.

In early 1943, Ford/Willow Run decided to test this assumption of better stability with a single fin and rudder. They modified a B-24D airframe to accommodate a single vertical tail unit taken from a Douglas B-23 Dragon. This aircraft was initially known as B-24ST (where the ST stood for *Single Tail*), and made its first flight on March 6, 1943. Following a change to a C-54 tailplane and a new rudder, the new fuselage was attached to another, later production B-24D airframe (B-24D-40-CO 42-40234). At the same time, it was fitted with more powerful R-1830-75 engines, each developing 1350 hp for takeoff. This airframe was also fitted with the power-operated nose turret that had been installed on later Liberators, while retaining the Consolidated tail turret.

This highly modified aircraft, designated XB-24K, flew on September 9, 1943. Tests revealed that the new tail configuration did indeed greatly improve the stability and handling of the Liberator. An additional benefit was an improvement in the field of fire for the tail gun. As a result of its additional engine power, the XB-24K was 11 mph faster than previous Liberators and had a much improved climb rate.

The results were so encouraging that in April 1944 the Army recommended that all future Liberators be manufactured with single tails. It was planned that the single-tailed Liberator would first appear on the production line with the B-24N version, but the approaching end of the war led to the cancellation of the B-24N contract after only 8 examples had been built. However, the single-tail configuration was later adopted for the PB4Y-2 Privateer"

WTE_Galway
10-26-2004, 05:48 PM
i am not sure on this one but i think i read somewhere the twin tail designs also require a lot less pressure on the rudder bar


as far as cruise speed there are a number of "optimum" speeds all of which are different
- range (maximum distance travelled)
- max duration (maxiumum time in air, useful for recon missions)
- min duration for range (useful to reduce engine hourly scheduled maintenance, important for airlines)
- max sustainable power
- cruise climb speed

Chuck_Older
10-27-2004, 05:44 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by diomedes33:
I'm not positive this is the reason, but it happens too much for it to be a coincidence. If you notice only multi-engined planes have the split tail.

If you look at this shot of a b-25.
http://www.carnut.com/show/03/osh/osh173.jpg

You can see how the prop-wash goes right over the vertical stabilizer, giving the rudder more effect when the plane is at low speeds or on the ground. <HR></BLOCKQUOTE>

Well, actually, we cannot 'see' where the prop wash might be.

We can take a guess, but without some wind tunnel time, you are assuming that the vertical stabilizers will be in the prop wash.

The path the air wants to take around the tail might not be what seems logical; high and low pressures might be in the oddest places. Even in a car, airflow is very strange.

Ever been in a convertible at around 80 kph/55 mph? I have one, and have done this many times and it's always strange.

With the top down, my hair blows forward, not backward. There's a huge low pressure system in the passenger compartment and the air spills over the windsheild and then goes toward the front of the car.

My point is, to 'see' where the disturbed air will go on that a/c, you'll need some pics of smoke going over a model in a wind tunnel

diomedes33
10-27-2004, 08:00 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Chuck_Older:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by diomedes33:
I'm not positive this is the reason, but it happens too much for it to be a coincidence. If you notice only multi-engined planes have the split tail.

If you look at this shot of a b-25.
http://www.carnut.com/show/03/osh/osh173.jpg

You can see how the prop-wash goes right over the vertical stabilizer, giving the rudder more effect when the plane is at low speeds or on the ground. <HR></BLOCKQUOTE>

Well, actually, we cannot 'see' where the prop wash might be.

We can take a guess, but without some wind tunnel time, you are assuming that the vertical stabilizers will be in the prop wash.

The path the air wants to take around the tail might not be what seems logical; high and low pressures might be in the oddest places. Even in a car, airflow is very strange.

Ever been in a convertible at around 80 kph/55 mph? I have one, and have done this many times and it's always strange.

With the top down, my hair blows forward, not backward. There's a huge low pressure system in the passenger compartment and the air spills over the windsheild and then goes toward the front of the car.

My point is, to 'see' where the disturbed air will go on that a/c, you'll need some pics of smoke going over a model in a wind tunnel <HR></BLOCKQUOTE>

I understand what you're saying Chuck. I was looking at some pics of the B-25 and Bf-110 and noticed that the vertical stabilizers were always directly behind the engine nacels.

On most single engined prop planes, the prop-wash helps turn the plane (especially in IL2)

I assumed that the prop wash would travel directly backwards. And of course assumption is the mother of all ****-ups.

If there was a pressure distribution like you said, would the rudders/stabilizers still be effective? Wouldn't you want to place the stabilizer in as much of a uniform airstream as possible?

It seems to me if they were placed in a wierd pressure distribution like that there would be yaw stability issues.

Chuck_Older
10-27-2004, 08:13 AM
Well, it could go either way, I couldn't say.

A control surface needs to be able to affect the flow of air around it...if there's a boundary layer of some sort, you'll get reduced effectiveness

ddsflyer
10-27-2004, 09:35 AM
The most important stability factor in a multiengine aircraft is single engine performance. In the case of the B-25 it was felt that having the propwash directed at the rudder would improve (read lower) the Vmc (minimum single engine control speed). Whether it did or not is questionable. It was found in other twin engine high performance aircraft such as the B-26 which had an early abysmal safety record, that other factors such as wing loading, power loading and most importantly pilot training and experience were more important. How do I know this? I routinely fly a twin engine aircraft and have to undergo rigorous and frequent proficiency training if I am to feel at all safe operating it.

Jungmann
10-27-2004, 10:46 AM
Twin tails were pretty much a design fad of mid 30s aeronautical design. Think of tail fins on cars in the 50's and 60's--car designers justified them with some mumbo-jumbo aerodynamics, but in fact, everybody just thought they looked cool. Same with twin tails--designers thought they were cool. The rationale was by splitting them, the overall frontal drag of a single vertical stabilizer would be reduced. Didn't prove true in wind tunnel tests or the field. The Brits even tried twin tails on an ill-conceived Gloster fighter prototype. The fad passed by the early '40s--as someone correctly noticed, Convair replaced the twin tails on the B-24 and got better performance. The straight-tailed 24 became the PB4Y, several examples of which are still flying.

Cheers,

IL2-chuter
10-27-2004, 02:57 PM
The biggest advantage of twin tailed multi-engine aircraft is ground handling (steering). The rudders were reasonably effective reducing braking action required for steering (think "jets"). Nose wheels weren't, as a rule, steerable "back in the day".

http://forums.ubi.com/images/smilies/16x16_smiley-happy.gif

ddsflyer
10-27-2004, 03:23 PM
IL2_chuter is right. The B-25, as well as the P-38 for that matter, has a free castering nosewheel. Either differential braking or differential throttle or both is needed to steer it on the ground.

FI-Aflak
10-27-2004, 07:11 PM
I always thought the dual-tail config was to get the verticle control surfaces in the prop wash, and thus increase their effectiveness at low/mid speeds . . .


But the H2-162 also has it . . . my guess about that is that the hot jet exhaust would be bad to have constantly blowing over the verticle stabilizer.

BlakJakOfSpades
10-27-2004, 09:41 PM
i suppose another benefit is you can get one tail shot off and still have control over your yaw, and yes they do look cool http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif