Menu Sign In Contact FAQ
Banner
Welcome to our forums

More Speed in a TB20

Hi there,

Can you help.

I am flying a TB20 (SN2008) and do not get close to the POH figures for Cruise performance. It is 8 to 10 knots slower than I would expect. Others have mentioned that they achieve 138 knots at 23/23. Other TB’s I have flown are also noticeably quicker.

I know we are not getting 138 knots at 23/23?
My average ground speed last weekend over a trip from Guernsey to Blackbushe with a slight tailwind (and joining downwind, no dead side join) was 137 knots. Power setting 24/24.
Going to Guernsey with a slight headwind and again joining straight onto finals, was 119 knots at 23/23
The weather was virtually unchanged on each leg and one could reasonably assume the headwind/tailwind was about 10 knots.
There are some knots missing somewhere compared to other similar aircraft. My numbers are derived from my SkyDemon GPS data.

I have the original 2 bladed prop from the factory, which has just been overhauled. The CSU has been overhauled as well.
Any ideas how we can improve the speed?
I have tried using the POH factory Power Settings but again get no improvement in speed.

Anybody got any ideas.

Colin

EGLK, United Kingdom

Rigging?

EDDS, Germany

I suppose a good place to start is checking the accuracy of the MP gauge, the RPM gauge and the airspeed indicator.

Anyone of them could make a significant difference to your readings.

EIWT Weston

dublinpilot wrote:

I suppose a good place to start is checking the accuracy of the MP gauge, the RPM gauge and the airspeed indicator.

Anyone of them could make a significant difference to your readings.

I was thinking along the same lines, but before doing that you set power to a known power setting (65 or 75%), lean for best power or best economy according to the POH and compare the actual fuel flow (and speed) with book values. If you find a big discrepancy in fuel flow, the engine is probably not producing the power, hence speed, you expect.

Last Edited by Aviathor at 16 Jul 06:49
LFPT, LFPN

I would check the ASI, as suggested above, using the three GPS groundspeeds method. On a calm clear no-clouds day, or wind under 10kt, say at 5000ft, fly three headings 120 deg apart and average them. The result is the TAS which you can convert to IAS using a Jepp slide rule, or using any of a number of online calculators. You will need the temperature aloft also.

The flaps must be fully UP during flight, not drooping by say 2mm.

Straighten any bent aileron trim tabs.

In flight, with balanced (or both full) tanks and two same weight people in the front (within say 10kg) the ailerons should match in angle and the ball should be in the middle when rudder-trimmed. There should not be any excessive rudder trim needed.

Further work would be checking, on jacks, that the gear doors close just-right. They should just exactly come up and touch the airframe firmly, no more and no less. Note that they will (and must) be loose when on the ground; there is supposed to be some play in the linkage; 5-10mm.

At 23" 2400rpm and 11.5 USG/hr mine does 138kt IAS low level (say 3000ft) and so do a couple of others I have been in, one of which was a very early model with a 2 blade prop. Incidentally the SR22, a Cessna 400 and a DA42 did that same speed when I tried it, at 11.5 USG/hr and peak EGT on the avgas engines – no free lunch anywhere.

11.5USG/hr is slightly LOP.

Do you have a fuel totaliser like a JPI or Shadin?

The engine will produce the rated power, or very close, unless it is completely shagged e.g. a large % of the camshaft sitting in the oil filter.

Administrator
Shoreham EGKA, United Kingdom

Once you’ve checked everything that Peter’s suggested, then you need to check the rigging. The simplest yet most often overlooked item is the TC ball : level the plane in the hanagr with a good spirit level, now look at the ball – is it perfectly centered ? If not, adjust so that it is and go flying !

FAA A&P/IA
LFPN

Peter wrote:

At 23" 2400rpm and 11.5 USG/hr mine does 138kt IAS low level (say 3000ft) and so do a couple of others I have been in, one of which was a very early model with a 2 blade prop. Incidentally the SR22, a Cessna 400 and a DA42 did that same speed when I tried it, at 11.5 USG/hr and peak EGT on the avgas engines – no free lunch anywhere.

My Lancair Columbia 300 trues a solid 160K TAS (‘bout 155K indicated) @ 3000’ on precisely 11.5Gal/h .

That said, who would want to cruise @ 3000’ ?

My sweet spot is 170K TAS @ 8000’, gear down, on 12 gal.h, LOP biensur !

FAA A&P/IA
LFPN

Thanks everyone for the info.

I should add that all the equipment is original from the factory.

Rigging is a possibility, although last weekend in smooth air I had the aircraft trimmed perfectly and was flying straight and level (hand off) without the Autopilot on. It seems to me that we are losing a lot of speed for just a ‘rigging issue’, unless a TB20 is susceptible to this.

The RPM gauge could need calibrating but they seem to be pretty reliable. It might be worth checking against a Strobe on the Prop and compare it to the RPM gauge reading.

There is a fuel computer (original from factory) and I am running rich of peak when checking my speed, to avoid over leaning, as there is no Engine Analyser yet. I am fitting the EI CGR-30P this winter. The Fuel Flow reading against the POH data settings is definitely worth a look.

Gear Doors have been checked and rigged on jacks and fit snugly onto the wing when closed. I was kind of hoping that this was the problem as it is a relatively easy fix.

I know my Airspeed Indicator under reads by 4 knots as this has recently been checked.

The TC ball is a good idea and not one that I had thought of. I’ll get that checked.

Most of the camshaft residing in the oil filter is a possibility, although it was checked at the annual (40 hours ago) and found to be OK. I’ll get that checked. The performance is unchanged in the last 40 hours.

I have only been operating this aircraft since April, so I not completely familiar with all its peculiarities yet, but the performance is definitely down on other TB20’s I have flown.

EGLK, United Kingdom

On the TB20 there is a “level reference” – it is in the MM (I have the Feb 2006 CD off Ebay… current since the last change by Socata is 2005 ) and it involves putting a spirit level across the two door sills. But it has to be on standoffs, not sitting on the window seal which is obviously uneven. Hard to explain in words. With the aircraft thus levelled (may need jacks though usually you can do it on the gear, possibly with somebody sitting on one of the wings) the TC ball must be accurately centred. This check takes only minutes.

Note that the TC itself will always show “level” – it is spring loaded to always do that when there is no rate of turn

And the KI256 vacuum AI must show level flight when the aircraft is levelled, but that is another job and will need an electric motor driven vacuum pump. I have one of those and every avionics shop must have one too.

It is possible for a plane which is flying skewed to fly semingly OK with hands off the yoke but you will lose a few kt.

It is possible to check the ASI on the ground – see here – but nothing beats the flight test.

Your Shadin totaliser might be well off. Socata’s “gallic shrug” attitude to this was absolutely outrageous. Your GT probably has the transducer installed in the wrong place… I would get that sorted too, though the FAA STC can be applied only to an N-reg. EASA-regs are stuck with a barely usable flowmeter unless done off the books.

Administrator
Shoreham EGKA, United Kingdom

I have the MM (most of it) P/N Z00. 18010320E0R19 but I have not found anything related to the TC. Which section is it in? Might be section 08 which I do not have.

The KI256 can be checked when the Avionics man is up at the airfield as he has the equipment. Similarly the ASI has been checked on a pump which is why I know it is under reading by 4 knots.

The Shadin Transducer is mounted in the wrong place (with the Fuel Selector under the cabin floor). Again that will be moved, as per the STC, as the aircraft is N reg now. The readings seem to be OK but it is likely to give problems from what I have read.

Thanks.

EGLK, United Kingdom
23 Posts
Sign in to add your message

Back to Top